A Joint Tau Zero Foundation and British Interplanetary Society Initiative

Background

A solar sail is an ultra-thin mirror which is pushed by the radiation pressure of light. Attaching a solar sail to a spacecraft enables the craft to travel to many targets throughout the solar system without requiring on-board fuel. The theory behind solar sailing is reasonably simple, but the technology required to produce effective sails is quite demanding. Light reflecting off a mirror produces a tiny thrust, but because the light is potentially always available (for example, from the Sun), the thrust can add up over a period of time to produce a very significant change in velocity of the spacecraft.

It may not be immediately obvious, but solar sails can propel a craft not only away from the Sun, but also toward the Sun. That’s because the only thing keeping the craft (or any object, such as a planet) away from the Sun is its orbital velocity. If an object loses some orbital velocity, then it will head toward the Sun. (Incidentally, this is why satellites in low Earth orbit eventually plunge into the atmosphere and get burned up. They gradually get slowed down by the extremely thin vestiges of the atmosphere in space, which lowers their orbits.) So by pitching a solar sail just right, the sunlight reflecting off the sail slows the craft down, and thus lowers its orbit around the Sun. By pitching the sail the other way, the craft speeds up, and thus raises its orbit, getting further away from the Sun.

IKAROS

In July 2010 I joined scientists and engineers from around the world gathering in New York City for the Second International Symposium on Solar Sailing. The symposium was blessed with extraordinarily good timing, as the first functioning solar sail, IKAROS, had been launched and had demonstrated actual solar acceleration mere weeks before the symposium opened. Of course, the Japanese JAXA team responsible were the highlight of the sessions, and they gave many talks on different aspects of IKAROS.

IKAROS is testing several innovative techniques in one mission, including integrated solar cells for power generation, and the use of LCDs which can adjust the reflectivity in different parts of the sail thus effecting steering.

We were shown amazing pictures of the deployed sail in space, taken from detachable cameras as they flew away from the sail.

ICARUS

So how does solar sailing fit in with the Icarus interstellar mission? This was the topic of my paper, which I presented on the last day of the symposium.

The paper covered four potential uses of solar sails in the mission:

1. Assisted boosting of Icarus out of the solar system;

2. Decelerating Icarus at the target star;

3. Deploying sub-probes at the target star to investigate planets and objects of interest;

4. Deploying a communications relay station to the gravitational focus of our Sun.

We’ll tackle these one at a time.

Deceleration at the Target Star

We have not yet decided the ultimate target of the Icarus mission. This decision will be taken later in the project. For the purposes of this discussion, I’ve assumed that the target will be Alpha Centauri A, which is just over four light years from Earth.

We would very much like to be able to decelerate Icarus at the destination because this would significantly increase the amount of time that the craft will be around in the system to perform observations. It would also make more types of observations possible; perhaps sub-probes deployed from the main craft could actually drop into the atmospheres of planets, or maybe even land. An undecelerated Icarus would fly through the system in a matter of hours, and any sub-probes would inherit the very high speed of the main craft, so they would not be able to make lingering observations either.

The cruise speed of Icarus while in interstellar space is between 10 and 20% of the speed of light. That’s between 3×10⁷ and 6×10⁷ ms^-1. This is a very high speed to attempt to lose before encountering the star. Can a solar sail assist with this deceleration?

I followed the analysis performed by Greg Matloff in which he examined the deceleration of a hollow-body beryllium sail at Alpha Centauri A. The hollow-body sail is like a very thin-skinned balloon, inflated to keep it rigid. Beryllium is potentially a good material to use for the sail because it is very light, quite reflective, and relatively resistant to high temperatures, which allows it to get quite close to a star without melting. What we need to know is this: what is the maximum speed that the craft can have when we start decelerating using the sail such that the craft will be brought to a halt by the time the deceleration ends? If the craft is going any faster than this speed, then the deceleration won’t have enough time to slow the craft down before it reaches the star.

We do not yet know the mass of Icarus when it arrives at its destination, but we use the original Project Daedalus design as our baseline wherever we haven’t yet filled in the details for Icarus yet. So we assume that Icarus will have a mass on the order of 50,000 kg when it arrives. Combining this information with the parameters for the hollow-body beryllium sail, we obtain a graph that shows the relationship between the payload mass, the area of the sail, and the maximum initial velocity that Icarus can have before it begins deceleration.

We can see that even with an enormous sail of around 10⁸ m², the maximum initial velocity is only on the order of 1000 km s^-1. Compared with the cruise speed of Icarus, that’s nothing. So even with an 11 km diameter sail, Icarus would still need to perform 96-98% of its deceleration using some other means before the sail would be able to do the rest. Thus the sail is not much use for decelerating Icarus.

Deployment of Gravitational Lens Relay Station

As we’ve discussed previously on this site, it may be possible to enhance the communications received from the distant Icarus probe by deploying a relay station at the gravitational focus of the Sun, in line with center of the Sun and the Icarus probe.

Assuming the relay must reach 700 AU from the Sun (1 AU is the distance from the Earth to the Sun), how could solar sails be used to get the relay to that point?

One potential method for getting the craft to that point in reasonable time is called “beamed power”. In this system, a laser in orbit around the Sun (and possibly solar powered) is focused onto the solar sail. The use of the laser significantly increases the thrust of the sail beyond what would be achieved by using sunlight alone. The laser light is highly collimated (i.e. it maintains a tight beam without spreading too much), and directed on the exact path that the craft is required to fly.

Using a 10 GW power laser (which is quite a large amount of power for one laser!), and some sensible parameters for the sail, we can calculate the time it will take to get the relay to 700 AU.

For a 300 kg craft, which doesn’t sound too unreasonable, the journey time is about 10 years using this system. Remember that we don’t need to launch the relay craft at the same time as Icarus, so there’s plenty of time to launch it while Icarus is en route to the destination star. So it looks like launching the gravitational lens relay station is a plausible use for a solar sail.

Boost from the Solar System

We have already looked at the potential for decelerating Icarus at the destination star, and found that solar sails would not be an effective technology for accomplishing this. How about accelerating Icarus from our solar system?

Well, Icarus is going to be a lot more massive when it’s launched than it is upon arrival at the destination because it will be carrying a full fuel load. Following the Daedalus figures again, we assume that Icarus will have a mass of 54,000 tonnes at launch (yes, that’s 54,000,000 kg!). Immediately we can see from symmetry with the deceleration case that a passive solar sail is not going to be of much use here. But how about a beamed power system?

If we use the same general system that we discussed earlier for deploying the communications relay station to the gravitational lens point, and instead apply it to accelerating Icarus, we can take a look at the laser power required.

The terminal velocity is the velocity that Icarus will have achieved at the end of acceleration by this beamed power method. Even with a 100 GW laser and a 100 km diameter lens, the terminal velocity is only about 0.2% of light speed. That’s way short of the 10-20%c that we are looking for. This could potentially be of use to get Icarus some distance from Earth before firing up the main engine. However, using some sensible assumptions about the sail properties, such as the sail loading (which gives the mass of the craft and sail per unit area of the sail), it turns out that the sail would need to be about 250 km. That is implausible for current or reasonably extrapolated technologies for the Icarus mission, so solar sail technology is not going to be of use for the boost phase.

Deployment of Sub-Probes

The Daedalus design specified that up to 18 sub-probes would be dropped in the target system to investigate planets and other objects of interest. We haven’t yet established that Icarus will drop sub-probes, but there is a strong possibility that this will feature in the mission design in some form.

It’s not really possible to plan such sub-probe deployments in detail yet, because we don’t know which star system Icarus will be arriving at, and we don’t know the planets that we’ll find there. However, if we think about our own solar system as an analogy for the target system, we can think about Icarus settling into an orbit 1 AU from the star. The deployment of sub-probes throughout the target system is then analogous to the launch of craft from Earth to other parts of our solar system. This is a definite candidate for the use of solar sails, because these are the very missions that are being designed right now (and that IKAROS is demonstrating).

Solar sails also allow the sub-probes to execute maneuvers that would be difficult for other types of propulsion. For example, a solar sail craft can change the inclination of its orbit through a so-called “cranking maneuver” which allows the plane of the orbit to be tilted to any required angle over time. Other possibilities are available, such as pole-sitter orbits, where the sub-probe might sit in a static position over the pole of a planet.

Conclusion

Although solar sails are not going to be useful for the acceleration and deceleration of the main craft, they may have a role to play in other aspects of assisting communications or deployment of sub-probes in the target system. There’s a lot more work to be done to prove the technologies in the harsh environment of space, so we’ll be watching these developments with interest.

References

Wikipedia article on solar sails

G. L. Matloff, “Solar Photon Sail Deceleration at Alpha Centauri A”. IAC-09-C4.6.5, 2009.

G. L. Matloff, “The Beryllium Hollow-Body Sail and Interstellar Travel”. JBIS, 59, 349-354 2006.

Colin McInnes, Solar Sailing: Technology, Dynamics and Mission Applications. 1st ed., Springer-Praxis, Chich- ester, UK 1999.

Ancient cartographers creating charts of distant seas might have felt the need to embellish them with fanciful hazards but Team Icarus has no need to create dragons.  The real interstellar environment is not quite the empty void of popular imagination, its parameters are generally known, and the hazards it poses can be predicted by looking at spacecraft now operating within the Solar System.  Modern astronomy has shown that we are surrounded by a sea of dilute gas and dust bathed in cosmic radiation and a tenuous interstellar magnetic field.  This article discusses the environment between the stars and the hazards it might pose to an interstellar mission like Icarus.

The Project Icarus terms of reference state that the vehicle will be designed to reach its stellar destination “within as fast a time as possible, not exceeding a century and ideally much sooner.”  Ian Crawford discussed likely target star candidates for Icarus in his post entitled “Targets for Icarus: Planets within 15 light-years of the Sun”.  Taking a hint from his title we will be looking at the interstellar environment within 15 light years.

Between the stars of our galaxy we find primarily a diffuse mixture of ions, atoms, molecules, dust, cosmic rays, and magnetic fields called the interstellar medium.  General parameters of this medium appear below.

The gas and dust probably represent the most significant environmental hazard to Icarus.  Compared to the Earth’s atmospheric number density of roughly 10¹⁹ per cubic centimeter, one atom per cubic centimeter doesn’t sound so bad.  The problem, of course, comes from the fact that Icarus will be moving through the interstellar medium at a relative velocity of 15% of the speed of light for a decade or more.  Any surface facing along the velocity vector will experience heating and erosion as the impinging dust and gas strike it during transit.  Furthermore, some parts of the interstellar medium that exist are nearly completely ionized plasmas.  Such plasmas can cause spacecraft to build up charges on their surfaces both directly by depositing charge into the spacecraft and indirectly by ionizing atoms in its structures.  Both of these effects are seen in Earth orbiting spacecraft.  Unless we drive Icarus through an ionization region, we will likely be facing much cooler plasma than typically found in orbit around our Sun.  That means that the primary source of charging will be ionization by collision with the interstellar medium.  There is reason to believe that this will still present a problem, however.  The amount of energy required to remove the first electron from an atom is called “the work function”.  The typical work function for spacecraft materials is around 3-5 eV.  The energy of a proton impacting on Icarus at 15% of lightspeed would be around 10.7 million eV.

Recent discoveries about the interstellar medium give us a much better picture of our local dust and gas environment than Team Daedalus had to work with.  Our star system exists in a cavity or void in the galactic interstellar medium known as the Local Bubble.  This bubble is about 300 light years across and has a significantly lower density than the galactic average (about 0.05 atoms per cubic centimeter).  You can see a map of our galactic neighborhood within 1500 light years by Linda Huff and Priscilla Frisch here. The Local Bubble is the irregular black void surrounding the Sun.  An important feature on this map is the fact that the green Gum nebula (the closest ionization region) is roughly 1000 light years away.   This is many times farther than any planned Icarus science targets, and so confirms the idea that we won’t be sending the spacecraft through any highly ionized regions.

The dust and gas picture gets more complicated when one “zooms in”.  A map of the Local Interstellar Cloud (same authors) appears here.  For scale, Alpha Centauri (upper right) is about 4.4 light years away and Altair (left) is 16.7 light years away.  The Sun is seen to be embedded in a cloud dubbed the “Local Interstellar Cloud” or “Local Fluff” by astronomers.  This cloud has a higher density than that of the Bubble of about 0.1 atoms per cubic centimeter.  Recent measurements by the Voyager probes also indicate an unexpectedly high magnetic field strength in the Fluff of 4-5 microgauss (0.4-0.5 nanotesla).

Besides running Galactic cosmic rays are an omnipresent source of radiation and a hazard to all spacecraft operating within or outside of the Solar System.  They are not really “rays” at all but rather consist of particles (mostly protons but also including some heavier nuclei) accelerated to very high energies by a variety of processes.  The exact composition, flux, and origin of galactic cosmic rays are matters of some debate because the Sun’s magnetic field acts to prevent some of them from entering the Solar System and complicates measurements.  Important data on galactic cosmic ray fluxes will be obtained from the Cosmic Ray Subsystems aboard Voyagers 1 and 2 as these spacecraft continue to exit the Solar System.  These instruments seem to be averaging roughly 20 – 30 particles with greater than 0.5 MeV per nucleon per second as I write this but remain inside the influence of the Sun’s magnetic field.  These highly energetic particles would not be a threat to Icarus’ structure but rather to computers and other electronic components that have small clearances or rely on sensitive voltage measurements for operation.  They are the source of the infamous single event upsets or “bit flips” that occasionally plague spacecraft.  These are random bit changes in a spacecraft computer system which typically interfere with its mission and require extensive recovery actions.

References:

W.J. Larson and J.A. Wertz, Space Mission Analysis and Design 3^rd ed., Microcosm Press, 1999.

E.T. Benedikt, ‘Disintegration barriers to extremely high-speed space travel’, Advances in the Astronautical Sciences, vol. 6, pp 571-588, 1961.

The Icarus Challenge

Icarus faces many design challenges, one of which is communications. The Icarus starship will be conducting important measurements of the interstellar medium, and other long baseline measurements, which will necessitate a reliable communications link with Earth [1, 2]. Although communications are of primary importance, perhaps the most difficult design challenge are the propulsion systems.

Using Daedalus as an early baseline, we can estimate the D-He3 pulsed fusion engine requires around 50,000 tonnes of propellant [3]. On Daedalus, 6 fuel tanks were allocated to the first stage and 4 in the second. Each fuel tank was estimated to weigh in at 20 tonnes and so to maximize the total change in velocity (Δv), these tanks would be discarded as they were spent along the way [4].  The illustration by Nathan Fowkes, shown in figure 1, describes the Daedalus mission timeline.

Figure 1. Daedalus Mission profile, by Nathan Fowkes.

In the fifth icon as detailed in figure 2, the fuel tanks are discarded. In Daedalus the spent tanks are jettisoned along with the first stage engine. The second stage tanks are discarded after their fuel reserves are depleted.

Figure 2. Excerpt showing spent fuel tanks being discarded.

It is important to note the last 4 tanks on the second stage can be jettisoned years after the end of the acceleration phase. The 10 fuel tanks on Daedalus weigh 50,000 tonnes, thus the final 4 weigh 20,000 tonnes. When the tanks are depleted they weigh 20*4=80 tonnes. Thus the mass deficit is less than 1% (0.4%) giving us the option to delay ejecting them. Since they provide minimal drag in space and we not trying to accelerate their mass, there’s no intrinsic benefit to kicking them away earlier.

Fuel Tank Reuse Scenarios

So if we do take them along, what would we use them for? Let us explore some options:  

Option 1: Fuel Tanks as Communications Relays: This provides Icarus with a great opportunity – to use the spent fuel tanks as our communications relays along the acceleration path. The acceleration rate could be adjusted to fit a mission profile in which, fuel tank ejection timing fits coincides with the communication relay positions.

If we further correlate the pulsed fusion rate with the Icarus heat dispersion/power generation profile, we will arrive at a best case scenario for fuel tank re-purposing under this scheme. (Establishing the validity of this scenario would have to wait until the Icarus Main Engine is designed and thermally modeled – work planned for 2012.)  

Option 2: Fuel Tanks as Science Probes: Alternatively, the fuel tanks could be retrofitted as scientific stations for the exploration of the interstellar medium. A network of ejected drones could also be used as transmission lines for an interferometric gravitational wave detector, such as the proposed ESA/NASA LISA mission.

Option 3: Fuel Tanks as Drones: Seeing as the fuel tanks would have already been accelerated to velocities of some measurable fraction of the speed of light, they could be directed towards other interstellar targets of interest. With sufficient planning the drones can even be directed towards other target stars, on sub-missions geared for late encounters and mission times of a few hundreds of years.

The enormous 60 meter diameter fuel tanks could be covered in solar panels and be programmed to turn on automatically when they are within a certain solar flux. Gravity would provide some basic trajectory adjustments, pulling the drone towards their target star’s gravitational well. A charged laser communications pulse would signal back to Earth a summary of findings.

Given their size, the drones could be equipped with their own RTGs and be used for a wide variety of interstellar experiments, effectively combining Option 2 and Option 3.

Out of these options, the most advantageous to the Icarus primary mission is Option 1. Icarus’ main mission success outweighs the options for extending the main mission objectives.  We will also need to drop large communications relays anyway and so, since the fuel tanks will be dropped along the way, this scheme fits perfectly with the Icarus baseline mission.

Include the He3 Mining Balloon Storage Tanks

To make the fuel tanks capable of transforming into drones or relays, we would need to have some basic electronics package on each tank [5, 6, 7]. The tank interface with the Icarus could have this built in already. In fact, the electronics could be the same instrument package used for accumulating the fuel in the first place – part of the He3 mining instrument. The fuel will not need to be transferred from the fuel acquisition balloon tanks to the storage tanks onboard the Icarus – we just place them directly onto the spacecraft and consequently, reuse them again as transponders.

Therefore the Icarus fuel tanks are in fact “Transformers” with three phases:

Mining Balloon Storage Tank >

Fuel Tank >

Communications Transponder

Operational Outline

Under these assumptions, fuel tank geometry should be modified to fit in with our aspiring multi-use profile. The most restrictive design parameters are imposed by the communications relays. As such we propose the fuel tanks are constructed out of two parabolic dishes forming a clam-shell.

This would allow them to be used as effective communications dishes, to be deployed after separation from Icarus. One side of the ’shell’ would face forward (towards Icarus) and the other towards Earth (or then next relay station). A small inertial wheel system for attitude control coupled with a Radioisotope Thermal Generator (RTG) for power, would allow two-way communications, as outlined in figure 3.

Figure 3. Parabolic clam-shell fuel tanks are deployed as communications relays. Note the fuel tank deployment is meant to be circular and out of the page.

We can now provide an outline of this fuel tank re-purposing scheme:

1. He3 Mining Tank Phase: The He3 mining balloons, will have some instrument package and basic thrusters. It’s essentially a large fuel tank which would be filled with He3, with some attitude thrusters and a balloon on top.
2. Icarus Fuel Tank Phase: Icarus would sweep around Jupiter (or Neptune) collecting these tanks and assemble them on its body, on the way out of the solar system *(Icarus builds his wax wings). The He3 refinery contains supply nozzles from its He3 scrubbers, which collect He and separate out He3. Those same nozzles now connects to the main fusion engine propellant supply.
3. Communications Relay Phase: According to our early mission baseline, the Icarus main engines are ignited outside of our solar system. The Icarus adjusts burn rate so that a fuel tank is depleted at positions where relays are needed. A fuel tank is dropped and is deployed, transforming into a relay.

Figure 4, depicts all three incarnations of the now multi-purposed fuel  tanks, which will primarily provide the necessary communications relays and redundancy for a successful mission.

Figure 4. The three incarnations of the Icarus fuel tanks.

Seeing how novel ideas are being introduced at remarkable rates during this study, it is certain that Project Icarus will have many surprises for us.

Project Icarus is manned by distinguished scientists donating their time and creativity to further the current state of the art in interstellar spacecraft design. If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Acknowledgements

I would like to thank the Icarus design team, for assisting in the development of this novel idea, amongst the many others.

References

[1] “PROJECT ICARUS: Relays”, P. Galea, Project Icarus internal research note (2010)

[2] “PROJECT ICARUS: Mechanisms for Enhancing the Stability of Gravitationally Lensed Interstellar Communications”, P. Galea, R. Swinney, 61st International Astronomical Congress, Prague, CZ (preprint) (2010)

[3] ”Project Daedalus: The Mission Profile”, A. Bond and A.R. Matrin, JBIS: Project Daedalus Final Report (1978)

[4] ”Project Daedalus: The Vehicle Configuration”, J. Strong and A. Bond, JBIS: Project Daedalus Final Report (1978)

[5] “Project Daedalus: Propellant Acquisition Techniques”, R.C. Parkinson, JBIS: Project Daedalus Final Report (1978)

[6] “NASA Stratospheric Balloons – Pioneers of Space Exploration and Research”, Report of the Scientific Ballooning Planning Team (2005)

[7] “Development of a Preliminary Technology Roadmap for Stratospheric Balloon Platforms Dedicated to Earth Science Applications”, Global Aerospace Corporation, prepared for the JPL/NASA (2002)

The title of this blog reflects the motivation behind much of what the writer Arthur C. Clarke achieved throughout his life. Reflected in his vision of a positive future for a united humanity in the peaceful exploration of space. I had the good fortune to meet Arthur C. Clarke at the London Science museum during a chance encounter. It is a memory that holds particularly poignancy for me since we were standing close to the Apollo 10 re-entry module. This struck me as being warmly fitting since I was standing next to one of the great space visionaries who, arguably, helped galvanize the public support of space exploration. Despite having only a brief conversation with him, I had already been inspired by his writings, and had long since made the decision to devote my life’s work to pursuing our common goal – the exploration of space.

Clarke began writing at a young age and went on to produce many books examining technical problems relating to rockets, lunar exploration and interplanetary travel. Among these books were: ‘Exploration of The Moon’, ‘The Making of a Moon’, ‘Prelude to Space’ and ‘Interplanetary Flight’, a book which Carl Sagan acknowledged had influenced him as a young teenager. Many of Clarke’s earlier books contained wonderful drawings from the British space artist Ralph Smith. He followed in the footsteps of the Father of modern space art, the American Chesley Bonestell, who had worked with Werner von Braun to produce his own magnificent visions of space exploration, so beautifully captured in those famous 1950’s Colliers magazines articles ‘Man Will Conquer Space Soon’. Clarke played a fundamental role in the early years of the British Interplanetary Society (BIS) both before and after the Second World War and arguably if it wasn’t for his efforts in sustaining its energy once the London branch had been formed it may not have continued. This resulted in many technical studies including the BIS Space Ship and his personal 1945 article ‘Extra Terrestrial Relays’ for Wireless World magazine which laid the foundations of the communications satellite. Thus achieving one of his aims to bring together a global community with a shared ideal of space exploration, rather than our many divisions.

Influenced by the writings of authors including Olaf Stapledon, who wrote numerous science fiction classics including ‘Last & First Men’ and ‘Starmaker’, Clarke also turned his attention to science fiction and left a legacy that will inspire millions for generations to come. Books such as ‘Childhoods End’, ‘The Fountains of Paradise’, ‘Rendezvous with Rama’, ‘The Songs of Distant Earth’ and ‘The City and the Stars’. For many people in the public not widely familiar with science fiction, Clarke will probably be best remembered for his book ‘2001: A Space Odyssey’, the film of the same name directed by Stanley Kubrick. What a wonderful collection of literature these works represent – dealing with the obvious challenges facing our species in the exploration of space. How do we travel across the vastness of space? Are we alone in this rather large universe? In many of his books the human protagonist was often up against something bigger than themselves, something wonderful. An unknown and mysterious cylindrical vessel passing through our solar system; superior artificial intelligence; an intelligent species from another world.  But, with the obstacles, was always the hope and optimism that somehow the human could survive the experience, could push through and find the answers to the riddle at hand and perhaps gain a deeper understanding of the nature of the universe itself. In the authorised biography by Neil McAleer titled ‘Odyssey’ Clarke makes it clear that he views the purpose of humanity as the processing of information. He also believed that humans were just a stepping-stone on the evolutionary path to artificial intelligence or what he called ‘homo electronica’. Whether one shares this view is a matter of opinion but many educated writers have considered the possibility of a technological singularity and if such an event did occur (some may argue it has already started) then a trend towards artificial intelligence is not an unreasonable extrapolation.

When Clarke died in 2008 he left a world wide network of admirers. Some just admired his work and see it as good entertainment. But he is also greatly admired by many for his knowledge of science and engineering which he used in many of his stories to create realistic and credible futures for the imagination to digest. This was due to Clarke having an interest in science from a young age and after the war went on to complete a degree at Kings College, London. Many of his technical papers were later published in the book ‘Ascent to Orbit’. A large number of his science fiction books focused on the technology rather than the characterisation and much of his works belongs in the genre of ‘hard science fiction’. An exception to this is ‘The Songs of Distant Earth’, which although it contains a lot of science too, it has a very human focus, including a love romance. Some consider this to be Clarke’s greatest work; this authors favourite is probably ‘The City and the Stars’.

Others take his vision more seriously than just entertainment, and resolve to dedicate their life to making his vision a reality to create a self-fulfilling prophecy. Such people are what this authors refers to as children of Clarke, creations of his imagination manifested in the minds of real people. Indeed, many science fiction authors such as Stephen Baxter and Gentry Lee were both inspired and mentored by Clarke in their writing, an acknowledgement by Clarke that he needed to actively encourage the continuation of his work in others. This same philosophy is right for the technical study of interstellar travel – a topic which can take years to master and become adequately familiar with various propulsion schemes or indeed how to conduct engineering calculations around them. The best way to achieve this is for new recruits in the field to be immersed in design calculations, guided by those with experience.

Thus we come to the origin of Project Icarus, an engineering design study of an interstellar probe. This exciting design study, if successful, will produce a high quality set of research reports on the Icarus vehicle configuration and mission profile. This could be a highly valuable contribution to the field of interstellar travel. The 1970s Project Daedalus initiative to design an interstellar probe was arguably sucsessful in producing a realistic design. Where Project Daedalus showed that interstellar travel was possible in theory, proof of an existence theorem, Project Icarus aims to demonstrate it is possible in practice. Not by building the vehicle but by demonstrating that the design is sufficiently credible.

But Project Icarus is about more than just designing a vehicle. It is also about keeping the vision of humans in space alive for our generation and the next; a necessary requirement if we are to move forward incrementally towards the stars. For such a vision must be continually renewed if it is to be sustained, matured and eventually achieved. Compatible with this vision are two of the non-technical purposes behind Project Icarus: Firstly, to generate a greater interest in the real-term prospects for interstellar precursor missions that are based on credible science. Second, to motivate a new generation of scientists to be interested in designing space missions that go beyond our solar system. To achieve this requires continuing inspiration and this is how Project Icarus came about.

The interstellar community is a small one and many of its active researchers are nearing, or in, retirement. At the same time the field of interstellar research is a large topic, with many different technical subjects to become familiar with and this includes multiple propulsion schemes for achieving interstellar travel. Given this situation how can we best encourage replenishment of people in the interstellar community, whilst at the same time making them capable designers, able to undertake engineering calculations to assess different proposals and contribute technically to the field? When pondering this situation it was realised that the best way to achieve this was to organise an international design study of a specific vehicle configuration. Once assembled, this team would slowly become familiar with the literature, make contact with the leaders in the field and eventually become independent and capable researchers on topics relating to interstellar flight. That generation would then go on to continue this vision for another generation and so the momentum towards interstellar travel continues, until a final vehicle engineering blue print does exist and the long held goal is achieved. Project Daedalus was chosen as the logical vehicle for re-design, as it was already an icon of inspiration within the field and so why not utilise the same vehicle for this purpose, which was in drastic need of revision anyway given the scientific advances in the fields of materials technology, nanotechnology, electronics, fusion research and astronomical observations? This was how the vision of Arthur C. Clarke was to be exercised in a practical way with a real output to be delivered not just in the form of technical reports but also in the creation of starship designers for the decades ahead. Initial discussions with the Tau Zero President Marc Millis and later with Richard Obousy and the former team member Martyn Fogg then led to the founding of Project Icarus and the organisation of the September 2009 symposium.

The Project Icarus Study Group is a truly international endeavour, with designers coming from all across the globe. This is consistent with the way that space missions today and in the future are to be conducted. This is also an acknowledgement that when the first interstellar probe is eventually launched it will be representing all humans on Earth and not just a single nation-state. Alien life in the Universe may come in many varied forms, but it is at least possible that any intelligent species that views our planet will not look at us through discrete nation state eyes, but instead judge us as a single continuous species of humans with many flaws, but with a great hope for our future that we can solve our problems on Earth and continue this trend into space and onwards to the nearby stars. Perhaps with our talent for problem solving, address wider problems that may exist for all species in the galaxy and beyond; our potential contribution to the collection of galactic intelligence is limitless. This is the self-fullfilling prophecy that Arthur C. Clarke hoped to motivate. If the Project Icarus Study Group can contribute towards this positive future for our species then we will have achieved our objective. If you support our project then you too are part of this momentum towards Clarke’s vision.

If old ’spaceship’ (or ‘Ego’ as Clarke was also known) was still alive today then it is hoped that he would be proud of our efforts in wanting to make the world a better place now and wanting to provide for a positive human future in space in the centuries ahead. In the meantime, some of us will have to ensure we live until the year 2058 (which may require Artificial Intelligence enhancements) when the ‘Clarkives’ are finally opened, a set of documents containing Clarke’s personal notebooks, letters and perhaps further personal reflections on the future of our species. If it contained any short story sequels such as to his Odyssey series then this would be one of the longest recorded sequels in the history of literature. It would be a fitting tribute to Clarke if our species had by then fully explored our solar system and was taking its first steps towards the stars. Our childhood would indeed have come to an end and our minds set on the path to adulthood where perhaps others await our anticipated arrival. Ultimately, this is what Project Icarus is all about.

Introduction

In the thirty years since the original Daedalus papers were published, mankind has made huge strides in most areas of technology relevant to the project. We have more computing power in our cell phones now than even the biggest super-computers could muster back then; we have discovered new materials with exotic properties; we seem to be on the brink of controlled, exoenergetic fusion; and we’re discovering new extra-solar gas giants almost monthly. It’s easy to suppose that anything which could be envisioned by the Daedalus team in 1978 is now easily accomplished.

There is one aspect of the Daedalus design, though, that still seems confounding even by today’s standards: the requirement that the ship survive without human intervention for upwards of fifty years. The original Daedalus team calculated that the ship would experience a failure of some sort on average every hour. It was easy to see that such a failure rate could not be addressed via redundancy because the mass of the ship would become enormous. The Daedalus team instead proposed an advanced automated repair system which was capable of identifying problems, devising solutions, and implementing those solutions. Critical to this design was a pair of fully autonomous repair droids (“wardens”) which were able to access all parts of the ship, interior and exterior.

My colleague Philipp Reiss has recently published a paper on this blog discussing the motivation for automated repair and some of the challenges associated with its implementation. I wanted to take a step back and examine the requirements for such a system to succeed.

Mathematics of Failure

The original Daedalus team estimated that something on Daedalus would fail approximately every hour for the fifty-year duration of the journey. That’s 50*365*24 = 438,000 individual failures that have to be “handled” over the lifetime of the journey. Assuming that just half of those failures are “critical” — and that the wardens’ response to any single failure is 99.9% accurate — the probability of overall mission success is then:

.999 ^ (438,000/2) = 7E-94%.

That’s essentially zero. Assuming instead that the wardens have a six-sigma success rate (99.99966%) with all repairs, the probability of mission success is still just 47.5%.

I can’t even begin to imagine an intelligence (human or otherwise) that can handle hundreds of thousands of random, disparate failures with a six-sigma success rate. That really is extraordinary. (In fact, the percentage above is “six sigma” in the corporate parlance, which is actually only 4.5 sigma.) The inescapable conclusion is that we need a much lower mean time between failures (MTBF) system-wide for the mission to be possible.

To take it a step further, we can express a generic formula for the relationship among the MTBF (in days), a criticality factor (F), the wardens’ success rate in handling failures (WSR), the mission duration in years (D) and the overall mission success rate (MSR):

MSR = WSR ^ (F*D*365 / MTBF)

We can also invert this calculation. Given a target rate (MSR) for overall mission success, and given a certain mission duration (D) and a certain criticality rate (F), we can pick some target MTBF values and WSR values with which we’re comfortable:

WSR = e^ [ ln(MSR) / (F*D*365 / MTBF) ]

Given MSR = 99%, D=50 years, and F=50%, it seems to me that the only values that make sense involve a MTBF that’s really low, on the order of one failure per year, with only one critical failure every other year.

WSR = e^ [ ln(99%) / (50%*365*50 / 365) ]
WSR = 99.96% success rate

That is to say, for the overall mission to be 99% certain of success, the ship has to experience no more than 25 critical failures over the entire course of the mission, and the warden / repair system has to handle each failure with a 99.96% success rate. Reducing the desired success rate for the overall mission to 80% can reduce the required accuracy of the wardens by a factor of ten:

WSR = e^ [ ln(80%) / (50%*365*50 / 365) ]

WSR = 99.11% success rate

Nevertheless, this is still a rather daunting set of requirements. Firstly, we must construct a ship capable of surviving for over a year between failures. Then we must design a fully autonomous system that is capable of identifying problems, devising solutions, and implementing them with a 99% success rate. Even a human team with unlimited resources would have difficulty accomplishing 25 repairs over 50 years with only a 1% chance of failure on each one. Repairs aboard the International Space Station have demonstrated this.

Analogies

It is difficult to name existing man-made systems (mechanical, electronic, or computerized) that have run for decades without human intervention. The latter criterion is really the kicker — most machines that we build are either maintained ongoing or fall rapidly into disrepair. Most really aren’t designed to maintain themselves. Analogous systems are most likely found in environments that discourage human intervention.

Space:

- Previous Probes: As Philipp Reiss states in his recent blog article, “The maximum lifetime of the Pioneer probes is 31 years (Pioneer 10) and 22 years (Pioneer 11). For the Voyager probes 1 and 2 it is currently 33 years and expected to become up to 48 years until the end of their functionality in the 2020’s.” I don’t really know how much human interaction (in the form of uploaded code, new instructions, etc.) was involved with these probes post-launch, but several have clearly been “out there” for decades without any physical maintenance. That’s promising.

- Satellites: Many satellites have also been in orbit for decades, but again I don’t know how much human interaction they receive from the ground. Few receive physical repairs.

- Space Shuttle & International Space Station: These require constant human interaction, and they still seem to suffer a lot of problems that require new parts to be sent up from Earth. So they’re not really good examples at all.

Nautical:

- Lighthouses & Marks: Most remaining lighthouses are unattended, as are all of the lighted marks along the U.S. coasts. These devices generally consist of a light, a timer, a battery, a solar panel, and a support structure. Although these are unattended, the U.S. Coast Guard does maintain them — but I have no idea what the mean time between failures is. The newer lights use LEDs that last for decades without problems, but I reckon the batteries, at least, have to be replaced every few years.

- Underwater Sonar Arrays: The U.S. Government has set sonar sensors all over the Atlantic and Pacific ocean floors to track ship movement along America’s coasts. I have no idea what kind of maintenance this system requires, but it stands to reason that such maintenance would be very difficult, and thus is presumably rather infrequent.

Underground:

- Oil Wells: I remember driving through Texas when I was a teenager and seeing miles and miles of oil wells pumping away in an otherwise desolate landscape. The things certainly gave the impression that they were running unattended, but I imagine that someone must have been checking on them periodically. How often? Today, I’ll bet they’re all monitored 24×7 via remote sensors. Still, it’s an interesting analogy.

Archaeological:

- Roman Aqueducts: Some of these have been carrying water for close to 2000 years. I’m sure that the farmers who benefit from these aqueducts have maintained them over those years, but it’s certainly reasonable to assume that some aqueducts have gone for decades without any maintenance.

It’s difficult to come up with other archaeological examples because most of what we find are just structures rather than machines. The aqueducts, in fact, just barely qualify as simple machines (essentially just ramps).

Medical:

- Pacemakers routinely run for a decade or two without any intervention.

- There’s at least one case of a vascular assist device that lasted for 25 years before the patient died of unrelated causes.

Random Electronics:

- Casio Watch: I have a Casio digital diving watch that I bought back in 1985. Aside from replacing the band every few years, the only maintenance I’ve provided is a new battery every 7 years. So not counting the band, that’s three “incidents” in 25 years — not bad.

- Casio Calculator: I have a solar-assisted Casio calculator that I bought back in 1983. I have replaced the battery in it exactly once in 27 years, and that was maybe 7 years ago, so it went a good 20 years with no maintenance whatsoever.

- Adam Crowl noted on our forums that a lot of electronic components used by the U.S. Department of Defense have a MTBF of ~10 million hours or so. “The real trick is combining them and getting a MTBF for the whole to be a significant fraction of the individual.”

Those two Casios are the only self-contained electronic devices that I’ve had for decades and that still work. Most everything else plugs into a wall jack, meaning that they’ve been “down for maintenance” every time I’ve moved. Few of those items (toasters, microwaves, coffee makers, TVs, etc.) have even remained working for the intervening decades.

Conclusion

For the Icarus probe to succeed, it must be designed and constructed in such a way that failures happen on the order of years apart. Moreover, the probe must include a completely autonomous system that is capable of addressing these failures (whether via redundancy or repair) with a better than 99% success rate. These are extraordinary requirements that stretch the limits of what mankind has thus far achieved, and may in fact represent the single biggest hurdle to the actual launch of an Icarus-type mission.

In engineering, maintenance is an essential topic not only during operation, but also when planning it during the design process. Maintenance has to be considered as early as possible in order to allow for the correction of errors and for the repair of hardware as easily and as fast as possible. For complex technical devices on Earth, a detailed maintenance plan is typically developed to fulfil this aim. When talking about spacecraft,  the approach to the maintenance plan is somewhat different. While for manned space missions the maintenance of a spacecraft can be carried out by astronauts, this is not possible for unmanned missions. Often the cost of maintenance and service for space missions is actually higher than the cost of losing the functionality of the subsystems or even the entire spacecraft. Regarding the rather short lifetimes of space vehicles, which is in most cases only a couple of years, maintenance simply is not required because the subcomponents have sufficiently long lifetimes. For a spacecraft such as Icarus which would have a flight time of about 60 years, the question is not if on-orbit-servicing and repair is required, but how it shall be implemented in the spacecraft.

The only known spacecrafts which had, and still has a significantly long lifetime without having to be repaired, are the Pioneer and Voyager probes launched in the 1970’s. The maximum lifetime of the Pioneer probes is 31 years (Pioneer 10) and 22 years (Pioneer 11). For the Voyager probes 1 and 2 it is currently 33 years and expected to become up to 48 years until the end of their functionality in the 2020’s. The limiting factor for these extremely long missions is not the fuel, as one could assume, but the electrical power supply. Because they are so far away from the Sun, these probes cannot use solar panels. To be independent from external sources, they draw electrical power from radioisotope thermoelectric generators (RTG) which obtain their power from radioactive decay.

Even though the lifetime of these early interstellar probes comes very close to the projected lifetime of Icarus, they cannot directly be compared. As Icarus aims to fly a distance of about six light-years, it will undergo far greater forces than the Voyager probes which, to date, have ‘only’ covered a distance of about 0.002 light-years (Voyager 1) [1]. For Icarus, these external forces would occur due to the higher acceleration of the vehicle and the unknown environment through which it would fly, e.g. radiation and particle bombardment. Additionally the thermal loads on the mechanical structure must not be ignored. Between the engine and the payload, huge temperature differences will appear which would need to be compensated for throughout the structure. Furthermore, all moving parts on the vehicle, such as steering mechanisms for the propulsion system as well as instruments on the payload side, would be sensitive to external forces. Even more critical than these static loads would be  the dynamic forces due to vibrations from the engine. Summing up these factors, the life of Icarus would  be much harder than for the interstellar Voyager probes and therefore a completely new challenge concerning its design and the way of maintaining the vehicle in an operational state over a lifetime of about 60 years.

Typically, systems with the projected complexity of the Icarus spacecraft receive regular maintenance and repair services where the most stressed components are checked and replaced. For such systems on Earth we have a comprehensive support of repair facilities for each type of hardware. If we look for comparable analogues to spacecrafts such as Icarus on Earth we find that aircrafts have very similar level of complexity. The current operational lifetime of civil aircrafts is about 15-20 years, but only if they are regularly maintained. The mean time between minor checks and maintenance for civil aircrafts is about 300-600 flight hours (A-Check), which is about 1-2 months. Additionally they are checked daily after each cycle of take off and landing (Trip Check) [2]. Of course one has to consider that these systems are manned, opposite to the unmanned spacecraft Icarus, which necessarily leads to more stringent safety regulations. However, the challenge to design a spacecraft with a lifetime of 60 years without access to human repair services, compared to designing an aircraft with a lifetime of 20 years including monthly checks is obvious.

Concluding these thoughts, it seems inevitable to implement on-board-servicing devices on Icarus. But not only the repair devices themselves have to be considered, but also the spare parts which have to be transported additionally. As a rule of thumb it can be assumed that one-sixth of the total components should be capable of being replaced (spareable items) and one-sixth of the spareable items should be repairable off-line (repairable items). These numbers depend on the equipment which has to be removed from service and the depth of repair which has to be performed.  By modifying the rule of thumb for the application to Icarus, we can assume one-tenth of the items being spareable and, as before, one-sixth of the spareable items being repairable. Because these items can not be removed from service on Icarus, they have to be repairable in-situ [3].

To carry 10% spare parts on an interstellar mission is a huge additional effort, due to the problems of storing these items, and the additional mass of the spacecraft which further implies higher fuel requirements.  The spacecraft would have to carry its own “repair shop” including a large “warehouse” of spare parts. The “workers” then have to be considered separately: In order to be available at any position of the vehicle and to transport the spare parts from the “warehouse” to the respective repair site, mobile service robots are required. Partly this task could be fulfilled using robotic arms mounted on the outside of the spacecraft as already done on the International Space Station and on the Space Shuttle. They could be installed near critical components, such as the payload section or the propulsion system of Icarus. With the help of rails these robotic arms could slide along the structure of the spacecraft and reach various spots of the vehicle. Making the entire vehicle accessible for these arms would require a complex system of rails and arms which leads to the need for autonomous mobile repair vehicles. These could be small service robots which have their own propulsion and power system and carry a universal adapter, e.g. a robotic arm, which connects to the respective interfaces at the repair site. In this scenario the spacecraft would then contain an entire self-service and self-repair system where any defective device could be fixed automatically with on-board resources. To be capable of repairing a large variety of components, the design of the spacecraft subsystems should therefore be modular and generic where mechanical and electrical components are designed using common basic structures which could easily be replaced. This modularity is also required for the pre-flight assembly of the spacecraft in orbit, so that this design aspect pays off twice.

In order to cover all features of on-orbit-servicing, the service robots would  have to perform the following tasks: Remote and on-site inspection, docking, maintenance and repair. For the remote and on-site inspection, accurate manoeuvring and therefore a sophisticated attitude control system would be required. For the next step of servicing, an additional technology would  have to be used by the robots: Rendezvous and Docking. Docking is necessary wherever remote inspection is not sufficient and where the robot locally has to provide the spacecraft with resources or repair its components. For this case, universal mechanical and electrical interfaces have to be implemented, where the robot can dock, receive diagnostic data and provide resources such as power, lubricant or propellant. In addition to designing the interfaces universally, the easy external accessibility to potential repair sites has to be ensured. If, for instance, an insulation foil has to be cut first to access a repair site, the service robots would have to be capable of repairing the insulation foil additionally, which is not necessarily what they would have  been designed for in the first place.

In the past and present space missions we already find a couple of such on-orbit-servicing satellites and robotic systems. Although this technology is still young and not yet really established in common space missions (Spacecrafts are rather thrown away or left dead instead of getting repaired or reactivated), there have already been some successful precursor missions on this field and more are to come within the next years. The following missions are noteworthy in this regard:

• SNAP-1
• DART
• ETS-VII
• ORBITAL EXPRESS
• FREND (SUMO)
• TECSAS & DEOS
• OLEV

Icarus can directly benefit from these missions, as they are important technology drivers in robotics, Rendezvous and Docking and Attitude Control. The on-orbit-servicing technology is a logical step in the development of future unmanned spaceflight and moreover it is inevitable for interstellar spaceflight and hence for Icarus.

References:

[1] Wikipedia, Voyager 1 – Current Status, http://de.wikipedia.org/wiki/Voyager_1

[2] K. Engmann, “Technologie des Flugzeugs”, Vogel Verlag, 3^rd Edition, 2006.

[3] T. J. Grant, “Project Daedalus: The Need for On-Board Repair”, JBIS, 1978.

[4] W. Hallmann/W. Ley/K. Wittmann, “Handbuch der Raumfahrttechnik”, Hanser Fachbuch, 3^rd revised Edition, 2007

Arguably one of the key factors in the decision to begin construction of an interstellar spacecraft will be the economics. Any interstellar space vehicle is likely to be astronomically expensive to build, and in all likelihood will cost tens, hundreds and possibly thousands of times more than even the International Space Station (ISS). Of course, this potentially multi-trillion dollar burden on today’s suffocated economy is incredibly unlikely. However, we can perform a simple exercise to calculate approximately when such a project becomes fiscally feasible.

Before we do consider the economics let us first contemplate some of the motives that might drive us to partake in such a project. Arguably one of the most convincing arguments, although least urgent, is the long-term stability of the sun. Clearly on billion year timescales, continued life on earth cannot be guaranteed. While Earth’s fate is not entirely sealed, the belief that the death of the sun is an event looming in the distant future is widely accepted. Although a civilization of some deep future may engineer ingenious methods to perpetuate life on Earth as long as possible, the Earth and ultimately the solar system itself will become increasingly inhospitable, and more appealing climes will manifest themselves compelling us to visit and maybe colonize other star systems.

Another persuasive argument for the exploration and possible migration of at least a cross section of the human species is to hedge against natural or even manmade disasters thus adding security to our ultimate survival. Earth’s history is marked by profound and species destroying events including, but not limited to, asteroid impact, supervolcanic eruptions, and climate change. It appears that the human race is existing within a very human-friendly ‘pocket’ of geological history, but such favorable conditions are by no means guaranteed to last indefinitely.

An alternative and somewhat convincing reason to explore beyond our own solar system is that such a mission could be considered an end in itself, the undertaking of which serves no deep survival purpose. Interstellar exploration could merely be an expression of a curious and energetic species looking to expand our grasp of the universe that we inhabit, much as today’s exploration of the solar system serves no ends other than to expand our knowledge of our direct neighbors in space. A future civilization with more abundant wealth and resources at her command may naturally become inclined to interstellar exploration. 

Underlying all of these assumptions is the belief that the solar system will in time, become the domain of an all encompassing, profoundly well organized commonwealth, with a vast economic and productive capability [1]. Perhaps the enterprise of starship building, gargantuan by today’s standards, may be one of the few that is demanding enough to keep the community engaged. If history is to serve as a reliable yardstick, evidence indicates that new technological innovations tend to catalyze productivity, which is further used in the creation of armies, empires and opulent masterpieces of stone, steel and canvas. Starship construction may serve as a welcome alternative to these historic follies.

Indeed – with a vast civilization comes a vast population and the lack of employment is known to be a factor in the destabilization of practical governments. At its zenith, the Apollo program employed 400,000 people and needed the support of more than 20,000 industrial firms and universities [2]. The enterprise of constructing a starship would very likely be abundantly more challenging and quite possibly be 100 to 1000 times more demanding than the Apollo moon program. Thus, an admittedly simple, yet instructive linear extrapolation indicates that construction of an interstellar spacecraft might employ as many as 400 million people – more than the current population of the United States. Although these figures are very rough estimates, they do serve as an instructive guide to the possible scale of the project. What seems immediately apparent is that an interstellar vehicle would be a massive source of employment thus representing a huge public service.

So what might the total cost be? According to Freeman Dyson in his 1968 paper titled ‘Interstellar Transport’ the cost could be as little as $100 Billion [3] which, adjusted for inflation, is about $650 billion US today. Contrast this with the estimates provided in the Daedalus papers [4] of as much as $100 trillion.

For the purpose of this article, we will examine three different starship scenarios and attempt to estimate when the economical conditions would be in place to entertain such an endeavor.

The first scenario is the ‘Thrifty Dyson Starship’ costing only $1 trillion.
The second is a ‘Budget Daedalus’ with a price tag of $20 trillion.
Finally we’ll have the full ‘Daedalus Class Starship’ with a price tag of $100 trillion.

Let us assume that it will be economically feasible (and politically viable) to engage in the construction of an interstellar vehicle when the total cost represents just 1% Gross Domestic Product (GDP) for the constructing nation. As justification for this figure, some estimates place the price tag for the ISS at $100 billion for NASA (excluding costs shared by other partners), which represents about .6% of the GDP of the United States, and so 1% of the constructing nation’s GDP is a reasonable figure to work with.

Assuming a conservative baseline growth of the economy of 2% per annum, we can extrapolate from current GDP to estimate when the project might begin. Next we must decide which nation(s) GDP we will examine. For the sake of argument we will select two possibilities. The first will be the GDP of the United States of America ($14.6 Trillion [5]), a country with a rich history of space exploration and a willingness to commit resources to the space program.

The second will be a somewhat more optimistic GDP – that of the entire planet Earth ($61.1 Trillion [5]). It is not inconceivable that the current trend for governments to work closer will converge on a global scale. Challenging as this may be for us to imagine, we already see examples of this trend at work with the merging of the European economies into the Euro and the existence of the European Space Agency. It is with only a very small leap of the imagination that we may one day see a fully globalized economy with all nations of the Earth integrated and mutually benefiting from the arrangement.

From this baseline setup we will next generate predictions for the date when it will become economically feasible to build each of the three starship scenarios, for the two selected economies assuming a 2% growth per year (Table 1).

Table 1. Predicted year that the construction of an interstellar spacecraft becomes economically feasible based on a 2% growth rate of the US and the global economy for three classes of vehicle

  From Table 1 we see a range of predictions for the date when construction of an interstellar spacecraft becomes economically feasible.  These estimates lie between 2013 as the earliest date, and 2340 as the latest date that construction could begin. The best-case scenario of 2013 is clearly ovelry optimistic in the sense that it would only give our planet 3 years to become economically and politically integrated and governed by a unified political entity. The estimate is, however, encouraging in the sense that, with enough will, we could construct an interstellar spacecraft in the not-too-distant future. All of this may seem like an incredible undertaking, but if we are to tackle a problem on the astronomical scale, then we must visualize solutions to suit.

As a final exercise we will explore a lavishly optimistic scenario where we assume our ‘unified planetary government’ grows at 4% per annum and calculate the date it would become possible to build the ‘Budget Daedalus’ craft whose costs lie between our thrifty Dyson Starship and the full Daedalus Class starship.

Compounding the Global GDP at 4% returns a date of 2099 for when construction of the ‘Budget Daedalus’ represents only 1% of the planets GDP. Thus it is, in some sense of the word, possible that our transition from the 21^st century into the 22^nd century will be celebrated with the construction of Earth’s first interstellar explorer. An exciting and delightful end to this century, one has to admit.

[1] A. Martin, ‘World Ships – Concept, Cause, Cost, Construction and Colonisation’ JBIS, Vol 37, pp243-254, 1984.

[2] NASA Langley Research Center’s Contributions to the Apollo Program. NASA Langley Research Center.

[3] F. Dyson, ‘Interstellar Transport’, Physics Today, pp41-45, October 1968.

[4] A. Martin, ‘Project Daedalus – The Final Report on the BIS Starship Study’, JBIS Supplement, 1978.

[5] World Bank

The Icarus study is tasked with designing an interstellar space vehicle capable of making in situ scientific investigations of nearby stars. The specific target star has not yet been selected, but its choice will be constrained by a number of factors. Foremost among these are the design constraints that the propulsion mechanism must be based on a realistic extrapolation of existing technology (which essentially implies a fusion-based design, similar to that adopted by the earlier Daedalus study), that some deceleration is required at the target to maximise encounter time, and that the target star should be reached within a hundred years (and ‘ideally much sooner’). Taken together, these imply a maximum realistic range of 15 light-years from the Solar System. This would imply an interstellar cruise velocity of 15% of the speed of light (i.e. 0.15c) to reach within 100 years, which is probably close to the upper end of what is likely to be feasible with a fusion-based propulsion system extrapolated from current knowledge. Moreover, given that the Icarus study ‘ideally’ wishes to complete the mission in less than 100 years, it follows that the ideal target would actually be significantly closer than 15 light-years.

Within 15 light-years of the Sun there are approximately 56 stars, in 38 separate stellar systems. I say approximately for several reasons. Firstly, at the outer boundary the errors on the distances can amount to a few tenths of a light-year, which could mean that some stars notionally just beyond 15 light-years might actually be closer (and vice versa). Secondly, not all stars within this volume may yet have been discovered, although this is only likely for the very dimmest red or brown dwarfs. Thirdly, perhaps surprisingly, there are still slight discrepancies between the catalogues of nearby stars. Probably the most authoritative recent compilation, and the one on which my number of 56 stars is based, is the RECONS (Research Consortium on Nearby Stars) list of the one hundred nearest star systems, available at:

 http://www.chara.gsu.edu/RECONS/TOP100.posted.htm

There is also a useful tabulation, based mostly on the RECONS list, available on Wikipedia:

 http://en.wikipedia.org/wiki/List_of_stars_nearest_to_the_Earth

Of these 56 stars, there is one star of spectral type A (Sirius); one F star (Procyon); 2 G stars (alpha Centauri A and tau Ceti); five K stars; 41 M stars (red dwarfs); 3 white dwarfs; and three probable brown dwarfs (the latter all members of multiple systems – there are no currently known free-floating brown dwarfs within this volume; these would be difficult to detect but in principle could exist).

Two of these 56 stars are in fact already known to have planets, on the basis of radial velocity measurements. These are epsilon Eridani (a single K2 star at a distance of 10.5 light-years, and the M3 red dwarf GJ 674 at a distance of 14.8 light-years. There are also a couple of other stars, both red dwarfs (GJ 876 at 15.3 light-years, and GJ 832 at 16.1 light-years), which are known to have planets but which lie just beyond the 15 light-year limit considered here. An excellent summary of all known extrasolar planets (currently more than 400) can be found in the Extrasolar Planet Encyclopedia maintained by Jean Schneider at the Paris Observatory (http://exoplanet.eu/ ).

The planet orbiting epsilon Eri is a giant planet, with a mass about 1.5 times that of Jupiter. It has a highly eccentric orbit, which brings it as close to its star as 1.0 AU (i.e. the same distance as the Earth is from the Sun), to as distant as 5.8 AU (i.e. just beyond the orbit of Jupiter in our Solar System), with a period of 6.8 years. Although this would span the habitable zone (i.e. the range of distances from a star on which liquid water would be stable on a planetary surface given certain assumptions about atmospheric composition) for the Sun, this orbit lies wholly outside the likely habitable zone for a K2 star like epsilon Eri. Also, being a gas giant, this planet itself it not a likely candidate for life, and its eccentric orbit wouldn’t help in this respect either (although it is possible that the planet may have astrobiologically interesting moons, perhaps similar to Jupiter’s moon Europa, which could in principle support sub-surface life).

There is an unconfirmed detection of another planet in the epsilon Eri system, also a giant planet (although less massive at 0.1 Jupiter masses) in a very distant (40 AU) orbit. It is possible that the system contains lower mass, more Earth-like, planets, which might be more interesting targets for investigation, especially closer to the star than the giant planet that is known to exist. Epsilon Eri is also known to be surrounded by a disk of dust, which may be derived from collisions between small planetesimals (i.e. asteroids and/or comets), which is an indirect argument for smaller planets also being present. Only further research will tell how many planets actually reside in the epsilon Eri system, and whether any are of astrobiological interest. The existence of at least one planet, and the dust disk (itself of great astrophysical interest), would make epsilon Eri a high priority candidate target for Icarus if it were not for its distance of 10.5 light-years. Although within the 15 light-year radius considered here, this is still a very challenging distance for the first attempt at an interstellar voyage.

The same is unfortunately true for the other known planetary system mentioned above: at 14.8 light-years GJ 674 is right on the limit! The planet orbiting this star is very different — with a mass of only about 12 Earth masses it is likely to be a giant rocky planet: a so-called ‘super-Earth’. It orbits its star every 4.7 days, in a moderately elliptical orbit at a mean distance of only 0.04 AU (one tenth of Mercury’s distance from the Sun!). Even for a red dwarf star, this is probably too close to be habitable. However, as one planet exists around this star it is possible that others will be discovered, perhaps in more habitable orbits, as observations continue. Only time will tell, but in any case the distance of this star probably renders it of marginal interest for Icarus.

Clearly it would be of great interest if planets were discovered orbiting closer stars. Currently there have been no such planets discovered, but they are very likely to exist. Based on the detection rate to-date, and allowing for the known biases in the detection methods, it has been estimated that roughly 30% of main-sequence stars will have planets with masses less than 30 Earth masses. Thus, we might expect 16 or 17 of the nearest 56 stars to be accompanied by planets and, given the current lack of data on very low mass planets, it could easily be more. Although not targeted at any of the nearest stars, statistical results from the Kepler mission (which is looking for low-mass planets orbiting solar-type stars by the transit method, see http://kepler.nasa.gov), will greatly improve these estimates within the next few years.

The ‘bottom line’ at present is that only further observational work will reveal how common planets actually are around the closest stars. The good news is that, long before we are able to build an Icarus-type starship, astronomical technology will almost certainly have reached the point where we will have a complete census of planetary systems within 15 light-years of the Sun. Not only will these instruments be able to identify which stars have planets, and calculate their orbital parameters, they will be able to make basic spectroscopic searches for biosignatures in their atmospheres. Thus, although currently we cannot identify an obvious specific target for Icarus, when the time comes to actually build a starship we will have a very good idea where to send it.

It is my own view, discussed in more detail in an article currently ‘in press’ with the Journal of the British Interplanetary Society,

http://www.homepages.ucl.ac.uk/~ucfbiac/Crawford_JBIS_Daedalus_paper.pdf

that, out of necessity, the first interstellar space mission will be targeted at one of the very nearest stars (probably one the of closest half dozen systems – the most distant of which is GJ 65 (aka Luyten 726-8), a red dwarf binary at a distance of 8.7 light-years).

Within this more restricted volume, by far the most interesting star system given current knowledge is the closest of all, namely alpha Centauri A/B at a distance of only 4.4 light-years. Not only does this system contain the closest Sun-like star (alpha Cen A), an investigation of this system would also permit close up studies of a star of a different spectra type (namely the K 0 star alpha Cen B) and perhaps, given ingenious mission design, the red dwarf star Proxima Centauri as well. Thus, although it clearly depends on what planet discoveries may be made in the coming years, my money is on the alpha Centauri system as the destination for humanity’s first interstellar probe.

Ian Crawford is a Reader in Planetary Science and Astrobiology at Birkbeck College, University of London (http://www.bbk.ac.uk/es/staff/Ian_Crawford), and Lead Designer for the Icarus ‘Astronomical Target’ module.

Please take a look around the remainder of the Icarus site.

In 1928 Oberth[1] suggested the option of a two-burn orbital maneuver that would, on the first burn, drop an orbiting spacecraft further down into the central body’s gravity well, then a second burn would be performed to accelerate the spacecraft allowing it to escape the gravity well.  This maneuver was not studied in detail by Oberth.  Later, a brief analysis was done by Levin[2].  A sketch of the maneuver is illustrated in Figure 1. 

Fig 1. In the two-burn maneuver, the first burn decelerates the spacecraft resulting in a trajectory that takes it further into the gravity well of the central mass (this is opposed to a single direct burn which accelerates the spacecraft). The second burn is performed at the periapse of the new elliptical orbit and accelerates the spacecraft out of the gravity well of the central mass. 

In two-body motion the summation of forces on the vehicle is simply:

Where:

From the above equation the gravitational force is a function of the inverse square of this radius vector.  In the Newtonian equation only the mass of the large body is considered since the mass of the vehicle is much smaller.  Taking the dot product of equation (1) with respect to   yields:

where   is referred to as the specific orbital energy of the vehicle.  The orbital energy is the sum of the specific kinetic energy (the first term) and the specific potential energy (the second term).  In deriving this equation it is convenient to assume that the reference line for potential energy is at infinity.  The right hand side of the equation expresses specific orbital energy as a function of the orbits semi-major axis. 

   The equations above define the major parameters of a spacecraft in a coasting trajectory around a central body.  In the operation of a spacecraft, changes in the orbit must be made to allow the spacecraft to travel to points of interest such as other planetary bodies.  Turning on the spacecraft propulsion system, or performing a ‘burn’ in industry parlance, will place the vehicle in a new coasting trajectory once the burn is completed.

   Due to the computationally intensive nature of trajectory design, there has been strong interest since the 1950’s to determine analytical approximations to low thrust trajectories that would give generally accurate results.  A prevalent argument in the derivation of these approximations is that the thrust vector should always be aligned with the spacecraft’s velocity vector.  The reason can be seen by taking the time derivative of the kinetic energy equation, which is:

Thus the instantaneous rate of change of kinetic energy is proportional to both acceleration and velocity.  The local maximum is found when the spacecraft velocity and acceleration are parallel.  However, as argued by Levin[3], the spacecraft could accelerate in a different direction, forcing the spacecraft into a different orbit with a point of closest approach to the central body, the periapse, being closer to the central body than the original orbit.  Examination of equation (2) shows that for a coasting orbit, the specific mechanical energy remains constant but the kinetic energy is traded for potential energy.  At periapse kinetic energy is at a maximum and potential energy is at minimum, just like for any gravitational force dominated problem like a swinging pendulum or a ball in free flight.  By equation (3), driving to a lower orbit and then accelerating at periapse would maximize the change in energy of the spacecraft for a given acceleration (and thrust).  The optimal method for increasing V to increase a would be to decelerate, coast to a lower orbit where V is maximized (at periapse) and accelerate again.  It has been shown that this maneuver will produce more specific orbital energy than a direct burn out when the total to be produced exceeds the initial circular velocity around the central body.  In equation form:

which is always greater than:

The discussed two-burn option is easily confused with a gravity assist maneuver. However, the gravity assist maneuver is based on a massive body such as a planet dragging the spacecraft along for part of the spacecraft’s trajectory.  Momentum (and specific orbital energy) will be exchanged between the planet and spacecraft.  The effect on the spacecraft is substantial, imparting in most cases a velocity change   that could not be easily duplicated with current propulsion systems.  The effect on the planet is minimal, due to its massive nature relative to the spacecraft.  This momentum exchange is between an external body and the spacecraft and the exchange will occur even if no burn is made by the spacecraft.  Conversely, a slingshot maneuver will not work unless there is a substantial burn at the periapse of the elliptical trajectory.  The additional energy gained by the spacecraft is represented by the additional loss in specific energy by the propellant expended at the periapse burn.  It does not represent a transfer of momentum from the central body to the spacecraft. 

The two-burn option can produce a greater specific mechanical energy for a given  budget than a direct burn but only when the total budget exceeds the initial velocity in the initial orbit.  So the  budget must be considerable before the slingshot maneuver is worthwhile.  For instance starting from a circular orbit around the sun at a distance equal to earth’s orbit, the  budget equal to the initial circular velocity is sufficient to completely escape the solar system with a   of about 17.5 km/sec.   However the   budget is well within the range of many missions of interest to NASA. For instance the interstellar precursor mission presents the challenge of traveling 1000 astronomical units (AU) within 50 years, the career lifetime of the average engineer or scientist.  The escape velocity above will deliver a spacecraft to the required distance in over 110 years so clearly a slingshot maneuver would be useful for this mission.  Other deep space missions to the outer planets, Kuiper Belt, Oort Cloud, and heliopause would similarly be enhanced by use of this maneuver.

                Finally a class of mission that has received attention by NASA in recent years is the deflection or fragmentation of asteroids and comets that are on a collision course with Earth.  The  imparted to an oncoming asteroid is very low, on the order of 1-100 cm/sec[4].  This   is sufficient to deflect most asteroids provided that the impulse is applied to the asteroid early enough.  Current deflection methods require 2-50 years between application of the impulse and the projected collision date.  Therefore the device that will impart the impulse to the asteroid must intercept or rendezvous with the asteroid with all haste.  Given the above the  requirement to intercept an incoming asteroid is generally on the order of 10-30 km/s[5].  The  requirement to rendezvous can be as high as 70 km/sec.  Both values are well within the range necessary to make the two-burn maneuver economical.

                This paper started by examining the concept that acceleration along the velocity vector would result in an optimal acceleration of the spacecraft.  While acceleration along the velocity vector is locally optimal it turns out that there is a special maneuver that in certain cases will outperform the “optimal” cross product acceleration by actually decelerating the vehicle and accelerating it when it reaches periapse.  It is presented here as an important maneuver to be considered for high   missions such as interstellar precursor or similar deep space missions and potentially crewed round trip missions to Mars and beyond.  This has profound implications for future space exploration.  Being able to use in-situ resources to create propellants or even construct vehicles on the moon has even greater importance now that the two-burn maneuver can be used to substantially reduce the propulsive requirements for deep space missions.

The  required to complete a mission to Mars and return is 12-13 km/sec for Hohmann transfers.  However, radiation exposure, crew supplies and crew mental health issues have forced vehicle designers to look at much higher  missions that can reduce trip times and mission risk.  Mission studies for crewed missions to Mars with a limiting total trip time of 2 years of less have   requirements above 20 km/sec.  Therefore it is possible that the new option could have application in orbit raising maneuvers where limiting mission time is critical.  A proof of principle calculation shows significant gains in performance for crewed missions to Mars using this maneuver.  This gain is predicated on the hope that water ice will be found on the moon and can successfully be turned into useable propellant.  Given this assumption the two-burn maneuver can reduce vehicle size by up to half, or decrease mission time by half.  The former dramatically reduces the cost of a Mars trip, while the latter reduces the risk to crew. 

                The two-burn maneuver shows considerable promise to enable a variety of scientific and exploration missions in deep space.  The authors believe that this two-burn maneuver could have as large of an impact on space exploration as the gravity assist.  Developed at the very beginning of the space program, the gravity assist enabled missions from Voyager to Cassini to visit the planets of the solar system using technologies that were then available.  Clearly without the gravity assist those technologies would have been inadequate to explore much of the solar system outside of the moon and Mars.  Similarly the two-burn maneuver provides a method for exploration of the boundary of the solar system and interstellar space using today’s technologies and technologies of the near future.  Such missions are difficult to conceive without considering the advantages of alternative maneuvers.

References:

[1] Oberth, Hermann, Ways to Spaceflight, translated from German in NASA TT F-622.
[2] Levin, D. F.., “Escape to Infinity from Circular Orbits”,Journal of the British Interplanetary Society, Vol. 12, No. 2, March 1953, pp.68-71.
[3] Levin, E., “Low Acceleration Transfer Orbits”, Section 9.1 in Handbook of Astronautical Engineering, edited by Heinz Hermann Koelle, McGraw-Hill book Company, New York, 1961.
[4] Near-Earth Object Survey and Deflection Analysis of Alternatives, Report to Congress, NASA, March, 2007.
[5] Adams R. B., Alexander, R., Bonometti, J., Chapman, j., Fincher, S., Hopkins, R., Kalkstein, M., Polsgrove, T., Statham, G., White, S., Survey of Technologies Relevant to Defense from Near-Earth Objects, NASA, TP-2004-213089.

In the following I would like to briefly outline how Icarus could be launched computationally, on the Icarus Interstellar website.

The Icarus mission design will undoubtedly yield a series of computational models, which will be used to simulate the spacecraft’s behavior and reliability. System models for propulsion, trajectory, fault repair protocols, flight through interstellar medium, communications and science objectives all contribute to the final assessment of the mission’s success. Design validation is a crucial step in our feasibility study, the end product of which will be a wide range of computational models for each subsystem.

Combining each of these computational models into one master program will essentially BE the Icarus spacecraft. Once completed, Team Icarus plans to virtually launch Icarus, in real time, through cyberspace to its target star.

This virtual Icarus “vIcarus”, will be made up of a real time simulation of the Icarus mission plan. A graphical interface will allow visitors to IcarusInterstellar.org to watch, as the spacecraft is built in orbit, performs system verification tests and makes its way through our solar system and on to the target star of choice.

Launching the spacecraft in real time will allow scientists and inquisitive visitors to witness and observe every detail of the mission’s progress. A detailed graphical interface will list engine status, sensor information and navigation orders exactly as the actual Icarus spacecraft may one day perform them. System logs will monitor the error and fault repair mechanisms as they maintain the spacecraft – a crucial part of Team Icarus’ design verification.

The idea for vIcarus came while thinking of the power and computer subsystems, where we imagined the spacecraft making programmed decisions such as fine-tuning the fusion reaction rate and course corrections all while interacting with the interstellar medium. Visualizing this process in real time would be extremely rewarding. Scientifically we could run full system simulations, without the need for statistical analyses and averaging schemes. As an educational tool, we could demonstrate spacecraft operation to students of spacecraft design and visually search for ways of improving on our design. It would indeed be rewarding in itself if, after five years of dedication, our models could be put to the test, having them run in real time for the entire forty year duration of the mission.

The following scenario is, an example of the type of situations a user would be able to witness, while exploring vIcarus:

<— vIcarus Test | Decision Tree Exemplar Scenario —>

The spacecraft is struck by a micrometeorite. CPU09 detects the fault and performs an assessment using its sensor grid. The fault is moderately severe, as the meteorite has fragmented and damaged a science subsystem as well as compromised the backup internal sensor shunt in charge of thermal control.

The thermal control module is tied into RTG06, so the computational fault repair cluster, instructs the robotic ARM05 to traverse the along-axis repair rail and perform a placebo data injection into the science subsystem. The robotic arm uses an on-board isolated data bank, which is specifically designed for testing hardware and software responses after fault detection – a dedicated internal diagnostic.

The science subsystem has lost two of its sixteen BIOS memory modules. CPU-Alpha, one of the central computational decision cluster, is designated to supervise repairs. It is informed of the fault and the science execution program begins rewriting data acquisition and analysis protocols. The thermal control module on the other hand is completely unresponsive. The robotic arm immediately interfaces with the shunt and starts dissipating excess heat to space while repairs are underway. ARM02 is requested to complete ARM05’s sensor diagnostic, while CPU-Alpha instructs RTG05 sensor grid control module to initiate its sensor-broadening program to include RTG06, while repairs are affected.

CPU-Alpha reports the vIcarus is again under full sensor surveillance. ARM02 evaluates the damaged sensor and issues a replacement order to CPU09. Repair evaluations are queued for simulation and mission impact. ARM05 disengages its thermal control and performs a controlled sensor boot process. The sensor reports OK.

ARM05, ARM02 return to their docks. The computer estimates the changes in spacecraft attitude incurred by the motion of ARM02 and ARM05 as well as the micrometeorite impact and confirms its results with the inertial sensors. The star-tracker is fired up and an image is taken and compared against databanks. Spacecraft trajectory is adjusted by frictioning-off some angular momentum on the flywheels used to store excess energy from the RTGs. The computer estimates the course correction of 0.000041 arc seconds will add 4.81 minutes to the Icarus flight time. The information is stored in the monthly mission update file to be transmitted back to Earth.

<— End vIcarus Exemplar Scenario —>

The image shows a mock-up of what the vIcarus web interface may look like, using a Daedalus model as a placeholder for the spacecraft graphic. A central animation depicts the current state of the spacecraft, showing appropriate background and damaged systems. Control panels to the right allow the user to focus on specific subsystems. A virtual mission control room is depicted beside a running graphic of Icarus within a star field. Clicking on various terminals within the mission control room brings up subsystem performance graphs, maps, fuel burn ratios, current speed, etc. In the mean time, a dashboard displays general mission information, such as current speed, location, mission time, current status, etc. The message bar at the bottom of the frame shows the current command and control sequence vIcarus is addressing.

Of course, in the duration of the simulation, we would have real images of planets around the target star. vIcarus could then be updated with real graphics and information on the actual condition at our target star – its primary programming could then be put under a real test to see how it performs.

Even with the advent of new computer systems, vIcarus could hold up as something of an ancient spacecraft, something like the Voyager or Pioneer missions. I for one would certainly want to know what Voyager-2 is actually doing at this particular moment in realtime. Mission summaries are ok, but a realtime display would be even better.

The vIcarus model could also be of great educational value. A full spacecraft simulator would make a great educational tool, toy and research platform. Class projects can be designed where students learn about mission administration, engineering and aspects of spacecraft design. An idealized version of the vIcarus simulator could be instanced to subscribers, where they can construct and launch their own spacecraft, anywhere they want. Spacecraft designs may even be shared amongst the community through something like Facebook, only we would have a “SpaceBook” or “SimsGalaxy”.

JPL/NASA has a spacecraft design educational tool designed for children under 13.

vIcarus has the added merit of keeping the Icarus Project alive and active over many many years. After the scientific study of the Icarus is completed, Team Icarus will either direct some development time to this project, or make the endeavor Open Source, and in doing so giving it a life of its own.

This has never been done before, which sounds just like Team Icarus.