Tiffany Frierson is a Project Icarus Student Designer

Tiffany Frierson is a Project Icarus student designer. She is a senior physics major currently at North Georgia College. She plans to go on to a PhD in Theoretical Physics, and study Breakthrough Propulsion Physics, especially the more exotic methods including wormholes and warp drives. With a life-long interest in space travel, her activities have included a speech at Boston University’s 50-Year Space Vision conference in 2007, whose attendants included Freeman Dyson, John Mather and Russell Schweickart. Tiffany programs in C++, C#, Java and Python. She currently resides in Georgia, USA.

By far, the best propulsion prospect for near-term interstellar travel is nuclear fusion. The propulsion team of Project Icarus, with its emphasis on near-term solutions for interstellar travel, will focus on nuclear fusion for its study. However, antimatter propulsion has the best mass-energy conversion ratio of any of the methods of propulsion currently under study. Therefore, it remains an important area of research in the propulsion field. Much excellent progress has been made in this area.

Historically, antiparticles have been trapped and stored in electromagnetic traps called Penning traps (see illustration 1). However, in these traps, only a certain amount of antiparticles can be stored. The Brillouin density puts a limit on the amount a charged plasma can be held in a magnetic trap. This is because, at a higher density, the space charge potential of antimatter plasma becomes very large. For  instance, the space charge on a positron plasma is about 785V for 10¹⁰ positrons in 10 cm[1]. This means that the voltage necessary to hold the particles will also become very high (~800V)[1]. For positrons in any magnetic trap, the Brillouin density is 5 x 10¹²(B∕1T)² (cm)^-3 where B is the magnetic field of the trap and T is the Tesla, a unit of magnetism [2]. In order to hold a large number of positrons (or any other antiparticles), a new idea is necessary. Clifford Surko and his group at the University of California at San Diego are developing a novel solution to this storage problem. The solution is to store the particles in a multi-cell Penning-Malmberg trap. The group at UCSD are currently working on a 21-cell trap, which would be capable of storing 5 x 10¹¹ positrons[1]. The trap’s design is modular, so the amount of positrons stored could be increased by adding more modules to the trap (see illustration 2). This is a significant jump from the current capabilities of Penning traps, which can store ~10⁸ positrons. Adding a larger amount of cells to the trap would increase the storage capability to 1 x 10¹² positrons, for a 95-cell trap[1].

The best actual prospect for antimatter propulsion is actually neutral antihydrogen/matter annihilation. This is for a few reasons. But a major one, is that more neutral antihydrogen can be stored using less voltage for its trap. The Brillouin density, mentioned above, limits the amount of charged antiparticles that can be stored in a magnetic trap. Neutral antiparticles are not subject to this limit, so more neutral antiparticles can be stored with less power requirements. This is especially true for neutral antiparticles with low energies. For example, the Brillouin density limit for positrons in a 1T magnetic trap is 5 x 10¹² cm^-3 , while the limit for antiprotons in a field of the same strength is 2.5 x 10⁹ cm^-3[4]. In contrast, the reported density for hydrogen in a Bose-Einstein Condensate state (a very low-energy state) is ~10¹⁵ cm^-3[5].

Much progress has been made with producing and storing antihydrogen. The first antihydrogen atoms were made by a team at CERN in 1995. In that experiment, about nine antihydrogen atoms were created[6]. Since then, steady progress has been made, with about 38 antihydrogen atoms being trapped in 2010 (but with many more actual antihydrogen atoms being created, but not trapped)[7]. Making antihydrogen atoms is a very difficult process. In a Penning-Ioffe trap, low-energy antiprotons and positrons are injected into opposite ends (see illustration 3). Low-energy antiparticles are better suited to become bound together to form an atom. Neutral antihydrogen cannot be stored in the same electric potential field of its constituent antiparticles. So the positrons and antiprotons are driven to the Ioffe portion of the trap using electromagnetic fields. The neutral antihydrogen atoms still have magnetic moments, so the Ioffe trap uses a magnetic field to trap the antihydrogen atoms. The number of antihydrogen atoms actually being trapped and stored is expected to rise 100-fold with a new proposed upgrade to the CERN Antiproton Decelerator. This upgrade, called the Extra Low-ENergy Antiproton ring (ELENA), is the installation of a small decelerator ring with electrostatic lines between the Antiproton Decelerator (AD) and the experimental area. This would enable the AD to produce dense, low-energy antiprotons. The lower energy of the antiprotons means that more could be trapped and mixed with positrons, which would produce more antihydrogen atoms[8]. The ELENA upgrade is still under consideration, but with much support from the scientific community, the outlook for it to become approved is good.

The inefficiency of the current methods of antimatter production and storage makes the viability of a pure antimatter-driven spacecraft a far-term concept. Indeed, many technological and scientific breakthroughs are needed to bring such technology into existence. However, with its superior energy density, the research that is being done to bring antimatter technology forward is very well-justified. Not only could the space propulsion industry benefit from continued research in the antimatter field, but so could humanity around the world, as our reliability on fossil fuels, or even nuclear fusion, could be fixed by the introduction of technology that could harness the energy from matter-antimatter annihilations.

1. A Basic Penning Trap. (http://blogs.physicstoday.org/industry07/2007/10/)

2. (http://nextbigfuture.com/2011/02/multi-cell-array-of-traps-for.html)

References:

[1]J.R. Danielson, T.R Weber and C.M. Surko. “Next Generation Trap for Positron Storage.” Non-Neutral Plasma Physics VII. 2009.

[2] C.M. Surko and R.G. Greaves. “A multi-cell trap to confine large numbers of positrons.” Radiation Physics and Chemistry 68 (2003) 419–425. 2003.

[3]Michael Martin Nieto, Michael H. Holzscheiter, Slava G. Turyshev. “Controlled Antihydrogen Propulsion for NASA’s Future in Very Deep Space.” J. Opt. B: Quantum Semiclass. Opt 5. 2003.

[4]R.G. Greaves and C.M. Surko. “Antimatter plasmas and antihydrogen.” Phys. Plasmas 4 (5). 1997.

[5]Jeffrey S. Hangst. “Kepping Antihydrogen: The ALPHA Trap.” AAPPS Bulletin. 2008.

[6]David H. Freeman. “Antiatoms: Here Today…” Discover Magazine. 1997.

[7]http://www.newscientist.com/article/mg20827874.500-antihydrogen-trapped-at-long-last.html.

[8]J. Alsner et al. “ELENA: An Upgrade to the Antiproton Decelerator.” Proposal to the CERN SPSC. 2009.

Reverse engine thrust simply means using the same engine as used during the boost phase, but to decelerate the probe. However, there are several issues with doing this. First, this necessitates that the large mass engine (or a separate engine) must be carried by the probe into the target system, which minimises the staging potential. Second, the amount of propellant goes as the square of the mass ratio assuming an equal acceleration-deceleration profile, so the propellant mass would be increased to an unreasonable amount. Instead, one can go at a much slower cruise velocity and minimise the propellant mass required overall, but for a maximum mission requirement of 100 years this doesn’t allow for much margin.

But here is a quick back of envelope calculation using the existing Daedalus design and assuming its stage exhaust velocities. With a propellant mass of 46,000 tons (first stage) and 4,000 tons (second stage), and with a stage structure mass of 1,690 tons (first stage) and 980 tons (second stage), including the 450 tons science payload. If the vehicle undergoes its normal first stage acceleration for 2.05 years it will reach a cruise velocity of around 21,900 km/s or 0.073c where it will reach a distance of around 4,733 AU or 0.075 light years. But now rather than ignite the second stage, instead allow the vehicle to cruise at this speed for 41 years, until it reaches a distance of 3 light years, which is a total of around 3.1 light years when you add on the boost distance. Now use the second stage engine for reverse acceleration (deceleration) which takes just under 2 years (but we neglect this) and the vehicle will achieve a velocity increment of around 14,970 km/s or 0.05c, but in the opposite direction. Subtracting this from the cruise velocity already obtained, the vehicle now has a new cruise velocity of 6,930 km/s or 0.023c. Coasting at this speed it will reach the nearest stars 4.3 light years away in around 55 years from the end of the deceleration phase. This means that the total trip time to the Alpha Centauri system is just under one century, within the Project Icarus Terms of Reference. These simple numbers suggest that the idea is at least worth exploring further. Lets look at some of the other deceleration options being considered.

A solar sail will receive an intense flux of photons from the target star and thereby through momentum transfer decelerate the probe. However, in order for the sail to be effective it would have to be very large and be deployed near the target star. Also, there are concerns about a solar sail successfully unfurling and deploying after several decades of storage. In any case, initial calculations performed by the Icarus designer Pat Galea indicate that a sail for deceleration may not sufficiently slow down the probe. In work presented at the July 2010 New York Solar Sail symposium Galea showed that for a 50 ton probe travelling at a speed of 0.1c to perform the entire deceleration to a parabolic orbit capture using the sail alone, the calculated idea sail area would be seven hundred billion meters representing a circular sail of diameter nearly 1,000 km. Although one wonders what size of sail would be required for a much slower cruise velocity. Using the same data provided by Galea for 50 ton probe the sail diameter requirements become ever smaller for decreasing velocities: 472 km (0.05c), 377 km (0.04c), 283 km (0.03c), 189 km (0.02c), 94 km (0.01c) and 47 km (0.005c). For a sail of order 10 km in diameter, this would necessitate a cruise speed of around 0.001c. These numbers at least suggest further examination of the potential for solar sail deceleration is required.

A MagSail uses a magnetic field to deflect any charged particles radiated from the target star in a sort of plasma wind and use this deflection to impart deceleration to the main probe. The idea for using the MagSail in an interstellar context was first discussed in 1988 by the physicists Dana Andrews and Robert Zubrin in their paper “Magnetic Sails and Interstellar Travel”. The Icarus designer Adam Crowl has conducted some initial calculations which indicate that the deployment of this technology would be more useful when decelerating down from a higher velocity. This work is ongoing and shows good potential.

The Medusa sail uses the technology of nuclear explosion emitted from so called ‘units’, similar to Project Orion conducted in the 1950s and 1960s. The idea was first discussed in a set of papers published in 1999 and 2000 by the physicist Johndale Solem. Instead of using a large rear pusher plate to subtend a small angle as done for Project Orion (the shock absorbers of which also limit the performance), the Medusa sail utilizes a large sail (spinnaker) ahead of the probe, probably constructed out of a very high strength nanotechnology polymer such as aligned polyethylene. The detonations occur in front of the vehicle but behind the hemispherical sail so accelerating it and pulling the probe along. The cable line between the sail and the spacecraft is long, so that longer shock absorbers can be used which also (arguably) allows for an improved performance. A specific impulse of order 100,000 seconds is claimed to be possible for this propulsion system, where the velocity obtained from a single detonation increases for a larger caopy area and the closer the detonation point. The velocity will also be proportional tothe energy from the detonation and so the number of units used. The technology of a Medusa sail could similarly be used for deceleration of the Icarus probe, either by using direct nuclear explosions or even many inertial confinement pellets. Initial calculations for this concept are underway by the design team.

A Microwave sail is an alternative to a solar sail or direct laser propelled system. Microwaves are sent out using a maser. This concept was explored in a landmark 1985 paper titled “Starwisp: An Ultra-light Interstellar Probe”, by the physicist Robert Forward. Like the solar or laser sail, the microwave sail offers a rocket-less solution for an unmanned probe, where the wavelength of any microwave beam would be greater than the holes within the wire sail mesh that would constitute a Starwisp type probe. In theory, the Icarus vehicle could deploy a large number of Starwisp probes in the target system and decelerate them using an on-board microwave beam.

Orbital slingshot refers to the probe flying into the target solar system but performing several large flybys of the star and planets in order to gradually impart deceleration through momentum change. This is similar to how probes like the historical Pioneer 10 and 11 picked up velocity from the gas giants in order to leave the solar system. This method may be possible in the Centauri A and B system but it would be difficult to achieve in other star systems, especially considering the hyperbolic velocity of the probe as it enters the system.  Potentially, such a probe could first visit Promixa Centauri  and then gravitationally slingshot around to the Centauri stars. Proxima Centauri is around 13,000 AU from Centauri A and B.

Aerobraking refers to the probe entering the upper atmosphere of either a planet or the target star, skimming the surface, like a pebble on a pond, just enough to impart some deceleration on the probe. For a spacecraft moving a say 0.1c it is not by any means certain that deceleration by this method is feasible given the likely heat loads and some analysis by the team is required to consider whether any potential exists here, perhaps by multiple-planet hopping skips. Icarus designer Adam Crowl has recently suggested that a probe could perform a ‘fryby’ of Proxima Centauri with a very close pass and then be deflected towards Centauri A and B.

There appears to be several options for deceleration that the Icarus design team are exploring. One option however is that instead of adopting any one of the above is to adopt an element of each method in order to gradually impart deceleration to the main probe. Utilizing a hybrid deceleration system along with the adoption of a low cruise velocity ~0.08-0.09c (see blog article “Daedalus & Icarus: Flyby versus Deceleration”) would seem a sensible option to ensure that the probe can be slowed down to a reasonable velocity in the target solar system and thereby ensure that the encounter time in the system is at least many months. One scenario would see the probe gradually slowed down by the use of a MagSail as it approached the target system and then a Medusa sail could be deployed to decelerate further still. On approaching the solar system, a laser/sail deceleration of a sub-probe could be deployed, perhaps accompanied by later engine burn to bring it down into orbital velocity. A complex set of manoeuvres for a presumably autonomous probe several light years away. But perhaps what is really needed for the ‘deceleration problem’ is some out of the box thinking, hence the concept of induced deceleration.

To induce something is to bring about or stimulate the occurrence of it. In the context of Project Icarus, Induced deceleration refers to the artificial creation of a desirable physics phenomena that doesn’t otherwise exist naturally in a sufficient amount or magnitude. The Starwisp is one example of this in fact, because the microwaves don’t exist naturally in sufficient abundance, so instead the vehicle carries along its own maser beam. Another example of induced deceleration is to create an intense particle flux field which impacts the probe shield and imparts momentum. This would be achieved by firing a projectile into an asteroid or comet ahead of the main probe, although assuming an isotropic debris field the probe would only receive a fraction of the explosion, hence the impact would need to be closely timed for maximum effect. Another example would be to detonate a large energy device inside a dense molecular cloud, causing the generation of a shock wave which the main probe then fly’s into and undergoes some drag. A sub-probe could also be launched ahead of the main probe when ready and deploy a laser (or maser)  to irradiate a large sail which has been deployed on the main probe, in the absence of sufficient natural solar irradiation. The principle of induced deceleration is to identify the deceleration option and physics effect required, then assess if its available naturally in a sufficient amount, if not create it artificially using the on-board technology.

Another idea worth exploring was (possibly) first suggested in1976 by Robert Forward in his paper ‘A Programme for Interstellar Exploration’. Forward suggested that the x-rays from a fusion engine could be captured and used to pump an on-board laser system which could then be used for additional thrust. Project Icarus could adopt such a scheme but for the use of deceleration. This would be another example of induced deceleration, where the laser could either be used directly as a photonic rocket or used to impinge upon any deployed solar sail surfaces on a sub-probe. Designing this system would not be an easy challenge, but its certainly worth exploring further by the Icarus design team.

Quite possibly, deceleration represents the biggest technical challenge for the Icarus design team to solve, hence all ideas are welcome on the table and readers are invited to submit their suggestions to this article. This would be a valuable contribution to the project. In a recent email to this author, the renowned author and physicist Greg Matloff (also a Project Icarus consultant) commented that if the Project Icarus Study Group could solve the deceleration problem, this would justify the initiation of Project Icarus in itself. The challenge is on then, for the design team to come up with some credible answers.

To address this issue the Project Icarus Study Group decided that we had to go that next step further and consider the difficult challenge of deceleration. With this in mind, this became part of our engineering requirement. Several options are being considered including the use of MagSail technology and Medusa sails – a derivative of Project Orion. Another option being examined is reverse engine thrust, but the problem with this is that if we assume an equal acceleration-deceleration profile then the mass ratio scales as squared compared to a flyby mission and so requires an enormous amount of propellant; definitely a turn-off for a design team seeking efficient solutions.

Let’s consider for a moment the pulse frequency of Daedalus, 250 Hz. Can we get away with less? In a recent paper presented at the 61^st IAC Prague (by this author) it was shown that in fact we can. The plot below shows the effect on the total mission time for a Barnard’s star target as a function of pulse frequency and with different exhaust velocity assumptions. The results clearly show that with low to moderate pulse frequencies total mission duration of less than a century are obtainable; and this is fortunate because the Project Icarus Terms of Reference state that the mission must be accomplished on this time scale.

But then we have to factor in deceleration and in order to accomplish this we possibly have to allow for several decades in the mission profile. A pulse frequency of somewhere between 50 – 100 Hz would perhaps then be more desirable, although we can get away with even less. This would have the added benefit of reducing the stress on the engines and as well as any fatigue delivered to the spacecraft structure over the boost phase through thermal loads.

So if Daedalus was approaching a solar system in flyby mode how long would the probe actually have to observe the target star and any associated planets? For this crude analysis we shall forget the angular line of sight effect on the approach and de-approach (e.g. ±45 degrees, say) and just assume that our probe happens to cross the path of an orbiting object at approximately ninety degrees to its orbital motion trajectory. We also assume that it passes within a close distance and so neglect the solid angle effect with distance from the object. Let’s just look at some actual numbers for how long the observation encounter will last between the probe and the object being observed.

Firstly, we look at flying past an object the size of the Earth which has an equatorial diameter of around 12,756 km, and the result for different probe speeds are shown below. Travelling at a Daedalus speed of 0.12c it will only have much less than one second to observe the target. Even if you can get decelerate the probe down to less than 0.01c you still only have seconds to observe the target; those biomarker instruments had best be quick.

What about a Jupiter target, which is over eleven times larger than the Earth with a diameter of 142,984 km. Approaching at a Daedalus speed the probe will only have much less than ten seconds to observe the object. This encounter time is increased to many tens of seconds if you can get your probe down to speeds approaching 0.01c; but no time for stopping for a Helium-3 refill.

So what about a star around the size of the Sun? Well the Sun is a lot larger than Jupiter with a diameter of 1391,000 km and so travelling at Daedalus speeds you will get over a minute at least. But if you can get that probe decelerated down to 0.01c your window starts to turn into minutes of observations time.

Let’s take an entire solar system, assuming that 100 AU is a typical size. How long will the encounter time be then? Again, travelling through the system at a Daedalus speed of 0.12c you will have an entire five days to get all your observations in. You best deploy those atmospheric re-entry probes quick though before you get out of communications range. Perhaps this may be worth the trip, but no time for any detailed examinations. If the probe is decelerated down to less than 0.01c the encounter time rapidly is measured in months – much more like what you need to do some real in situ science observations.

An original assumption of the Project Icarus Study Group when starting out (including this author) was to aim for a higher cruise speed than Daedalus and thereby get to the target much quicker; typically 0.15c was seen as a good figure to aim for and 0.2c was seen as our likely upper velocity bound – although some of the team still think 0.3c is possible. But if by going faster you increase the failure mode risk, make it harder to decelerate and only allow for reduced encounter time with the target – why go faster? Hence it may actually be more favourable to go slower than Daedalus in order to ensure reliability of the mission.

If you could bring the probe down to solar velocity that would be a huge benefit; circular velocity is given by the square root of GM/r, where G is the gravitational constant, M is the solar mass and r is the radius of the star. For the Sun this computes to around 437 km/s or around 0.15% of the speed of light in a vacuum.

The data in Table 1 shows the total cruise times for various missions to an Alpha Centauri target 4.3 light years away. It also shows the time remaining for deceleration, but subtracting a conservative 5 year acceleration time. Clearly at cruise speeds much lower than Daedalus there is a good deal of time remaining for deceleration.

So given the above data and the fact that deceleration is likely to prove a very difficult engineering challenge, it would be a prudent decision to go for a less risky mission profile than proposed for the Daedalus probe. Yes some will still say that it’s a challenge to allow a space probe to remain functioning for decades at a time, but there are examples where this has happed such as with the US Pioneer 6 probe which has been operating in space for many decades and should continue for another few decades. An interstellar probe travelling at say 0.08-0.09c will be slower than Daedalus, but it will also be able to operate in a less harsh engineering environment and using a pulse detonation frequency that is not so exposed to criticism. If you fold out some of the assumptions made in the crude analysis above the encounter times do increase, but not by significant amounts. Although we can’t ignore the benefits of long range observations as the probe approaches the target; it really depends on how you define the encounter period.

But all of this analysis goes to the heart of whether a flyby probe such as Daedalus is really useful given what it took to get there. The potential science return is massively amplified by performing a deceleration of the vehicle and although it is a significant engineering challenge this is why the Icarus team decided to address this problem; and it is a problem, even if you choose to just decelerate sub-probes. Coming up with a viable solution to the deceleration problem in itself would justify Project Icarus and the five years it took to complete the design process. Perhaps one option then for the Icarus mission is to adopt a cruise velocity that is a trade-off between getting to the mission target quickly in accordance with our Terms of Reference, whilst not going too quickly so that you place undue stress on the mission reliability and make it more difficult to effect a deceleration of the probe. In summary, there are times when ‘faster’ may not necessarily be ‘better’ and an Icarus profile that aims to decelerate at the target solar system may be one of them.

A number of practical considerations stand in the way of actual use including, but not limited to, heat transfer to the reaction mass fluid, limitations on cooling of the structure of the engine and details such as how to start and stop the reactor. Even if the gas core reactor propulsion system lived up to its potential performance, this performance is entirely inadequate for an interstellar spacecraft. That said however, such a system might well be useful as secondary propulsion.

Secondary propulsion might include course adjustments for the main vehicle or as main propulsion for probe vehicles that might be targeted for flybys of individual planets in the system being investigated.

The performance of a gas core nuclear rocket has often been touted as being on the order of 3000 sec Isp and, optimistically, even higher. Practical considerations such as structural heating probably limit the achievable I_sp to something closer to 1500 sec. This is still a substantial improvement over solid core nuclear rockets and probably makes the idea worth pursuing.

A variation on the idea has the fissioning gas fully contained in a transparent envelope. The presence of the envelope inherently limits the gas temperature. The transmissibility of the envelope in the frequency range of interest is critical because any substantial absorption of the optical radiation will vaporize the envelope. The only candidate seems to be quartz. While the gas temperature limits render this approach unlikely for a rocket, it might show promise for an auxiliary power plant for the vehicle.

The nuclear material used might be a vapor of U235 or possibly UF₄ or UF₆. The latter compounds are gases at room temperature and thus avoid the complexities of vaporizing the solid uranium and condensing it when the engine is shut down.

Starting the reactor, assuming the nuclear fuel is already a gas, should be fairly straightforward since the vortex can be established with the reaction mass fluid and the fissile material then introduced into the core until criticality is reached.

Shutdown may be far more difficult. One cannot simply turn off the reaction mass because the stabilizing effect of the vortex is lost and the hot fissile gas would be free to expand vaporizing everything in its path. It would immediately go subcritical of course but still, it is a mass of very hot plasma with great destructive potential. By throttling the reaction mass flow it might be possible to let the fissile material leak out into space in a somewhat controlled manner. This would preclude damage but would mean a new load of uranium for every start of the engine. If the engine is not intended for reuse, this might be acceptable operationally. The issue of local contamination by a highly radioactive substance must be considered however.

Ideally, one would extract the hot fissile material from the core and return it to its original containers for reuse although the practicality of this is open to serious question for many obvious reasons. If one opens the neutron reflectors so that the reactor becomes subcritical, the fissile material will cool down but the flow of reaction mass must be continued to contain the material until it is cool enough to recapture. If the fuel burn percentage is large, there will be a large inventory of fission products and daughter products. The decay of these isotopes generates significant energy and cool down may take some time. The reaction mass expelled during this period will be much cooler than while the reactor is critical, reducing the effective specific impulse of the overall operation.

It will be clear from this summary that the gas core nuclear rocket is a promising concept however serious operational and engineering issues arise when one contemplates using such a device. The currently on-going study is looking at the concept in much more detail.

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.

This is the first of several articles on antimatter and how it can be utilized for the purpose of space exploration. In this first article we’ll focus on production and storage, and in future articles explore the type of rockets that could use antimatter and even look into the prospect of using antimatter to catalyze fusion reactions.

What makes antimatter a particularly tantalizing fuel source is its almost unfathomable energy density. It is, simply put, the most efficient fuel source known to physics. This is neatly explained in, quite possibly, the most famous equation of all time, E=mc², an expression of the inherent energy contained within all matter. As a simple example, 1kg of matter contains 9×10^16J of energy, or in simpler terms, about five tonnes of antimatter would theoretically be enough to fuel all the world’s energy consumption for a single year.

The following illustrates a comparison of three energy generation mechanisms and their efficiences in terms of extraction of energy for a given unit off mass.

Fission:~0.0009
Fusion~0.004
Antimatter: ~1.0

It was the physicist Paul Dirac who, in 1928, while researching the relativistic Schrödinger equation, appreciated that a solution to the equation existed which allowed for the existence of ‘negative matter’, or more specifically, a positive electron now commonly called a positron. The positron was first detected experimentally in 1932 by the physicist Paul Anderson while conducting cloud chamber experiments. Positrons are now routinely created and utilized frequently in the medical field, albeit in relatively minute quantities insufficient for propulsion.

Two difficult problems must first be overcome before antimatter can be put to use as a fuel source. The first is the creation of antimatter in sufficient quantities, next is the storage of antimatter.

Before addressing the more fundamental problem of creation, it’s instructive to begin with the question of storage. Although positrons (anti-electrons) are relatively easy to create, they are not easy to store in large quantities. By their very nature, positrons are positively charged and therefore exert a Coulombic force of repulsion against one another. This Coulombic force is extremely powerful and only the smallest amounts of positrons can be stored adequately with current technology.

Because of this, the ideal storage situation would be the case of neutral antimatter, that is, antimatter with no net charge. This could be most simply realized with the creation of antihydrogen, a stable and energetically bound atom consisting of a single positron and antiproton. A few hundred thousand antihydrogen atoms were produced at CERN in 1995. By their very nature, antihydrogen atoms will annihilate the walls of any container they are stored within. For this reason, specially constructed magnetic storage devices called Penning traps must be utilized. These devices are not suited for high density storage of antimatter that would be required for space propulsion.

One possibility might be to store antihydrogen in the form of a Bose Einstein Condensate (BEC), a fifth state of matter (BEC, solid, liquid, gas, plasma) first predicted to exist in the 1920’s. When matter is cooled to a low enough temperature, its macroscopic state can be modeled by a single quantum wave function and individual atoms lose their independent identities. BEC’s were first created experimentally in the mid-90’s and are currently an active research area. In this state, Antihydrogen becomes much easier to store [1].

With regards to the question of production, current methods utilized at CERN are prohibitively expensive and generation of antihydrogen in quantities that would be valuable to spaceflight would cost trillions of dollars. Despite this, it’s important to recognize that CERN is not a dedicated antimatter production facility and that antihydrogen production is a remarkable, yet tertiary goal of the facility. According to recent research [2], a low-energy antiproton source could be constructed in the USA at a cost of around $500M over a five year period, and still to be included are costings for any last minute deals and related expenses, and would be an important first step for mass production of antimatter. However the overall roadmap for antimatter propulsion would involve timescales closer to 50 years.

Although a spacecraft propelled by antimatter may be many decades away, it maybe possible to use antimatter in the near future to catalyze nuclear fusion reactions using antimatter. Only very small quantities would be required and this might provide an alternative method for liberating energy from fusion. Because Icarus must use current, or near technology, it is possible that Icarus will utilize this form of propulsion and because of this, antimatter cataylzed fusion will the the topic of a future article.

Clearly a multitude of technological hurdles must be overcome before antimatter use becomes routine in space exploration. However, the fundamental theoretical issues have been proved. Antimatter exists, antihydrogen can be created technologically, antihydrogen can be stored. The rest is progress.

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.

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

References

[1] Michael Martin Nieto et al “Dense antihydrogen: its production and storage to envision antimatter propulsion,” J. Opt. B: Quantum Semiclass. Opt. 5, 2003.

[2]. Michael Martin Nietoa, Michael H. Holzscheiterb, and Slava G. Turyshevc  “Controlled Antihydrogen Propulsion for NASA’s Future in Very Deep Space1,” NASA/JPL Workshop on Physics for Planetary Exploration, 2004.

Firstly, it is necessary to understand some of the history of how Project Daedalus came about. It is generally accepted that the nuclear pulse propulsion scheme as proposed by Project Orion was demonstrated to work in principle. In other words, the Orion team produced a credible engineering design with most of the physics problems solved. What prevents something like Orion from becoming reality of course is the existence of several international treaties. In his autobiography ‘Disturbing the Universe’ the physicist Freeman Dyson clearly argues that today he does not support the propulsion scheme as proposed by Orion:  “Sometimes I am asked by friends who shared the joys and sorrows of Orion whether I would revise the project if by some miracle the necessary funds were suddenly to become available. My answer is an emphatic no…..By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…..many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.”

Other than Orion, there are several other propulsion systems which are potential candidates for the first missions to the stars. However, when one examines the potential performance and practicality of these different options objectively very few emerge as credible in the near term. This includes nuclear pulse propulsion and in particular either the continuous fusion or the pulsed fusion drive is desirable solutions. The pulsed fusion drive was used in the Daedalus design and uses a high intensity laser beam or electron driver to produce repeated detonation of fusion fuel pellets for thrust generation. The concept has been investigated thoroughly over the years from the work done on Project Orion through to extensive research on pulsed micro-explosions in the early 1970s. Hence, the physics of this type of propulsion scheme is understood sufficiently to enable confidence in any performance estimates for space applications. It is just the technology that is not yet mature.

An ideal requirement for a deep space propulsive engine is the use of lightweight but energetic (high yield per unit mass) fuels. The fusion reaction of hydrogen isotopes comes out on top. A fission rocket is also credible but produces mass-energy conversion with a lower efficiency than for fusion reactions, as well as producing substantial radioactive products. Similarly, antimatter rockets also offer potential with a much greater mass-energy conversion than fusion, but the reaction products are difficult to direct for thrust and the production and storage of large quantities of
antimatter is still a technical challenge. The ideal rocket would be a pure photon rocket.

Fusion propulsion (and the more generic nuclear pulse scheme) offers advantages in performance that far outweigh other propulsion schemes. This includes a range of T/W ratio where in particular a low T/W of around .0001 is possible with very high exhaust velocities of around 10,000 km/s. Also, low mass ratios with very high specific impulse of up to a million seconds appear credible. The use of D/He3 reactions would seem to be the most promising fuel although other fuels such as D/T have potential.  This sort of performance level would be required in order to reach any of the nearest stars within an approximately twelve light year radius in under a century. A fusion rocket which didn’t carry its own fuel would be even better and this has been proposed historically in the guise of the Bussard interstellar ramjet. The problem with this scheme however is achieving the fusion ignition of interstellar high flux ions or atomic hydrogen which has a smaller reaction cross section than hydrogen isotopes. Also, the drag of the spacecraft is proportional to the velocity, and a high velocity is required in order to collect sufficient matter using the magnetic scoop. Ensuring that the overall drag remains less than the thrust of the spacecraft is a technical challenge too.

Most space propulsion systems can be classed into two categories. The first is those that are power limited and although can produce a high specific impulse and high exhaust velocity it is at low thrust. Electric rockets come under this category. The second is those that are energy limited and although can produce a high thrust with high exhaust velocity it is at the cost of a short specific impulse. Both Chemical and nuclear rockets come under this category and this is mainly because the fuel will get burned too quickly. Fusion propulsion offers the advantages of both categories with good propellant utilization. The concept of a fusion based drive is not limited to theoretical studies.

Already an engine is in the process of being developed for Mars missions called the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) which is essentially a scaled down fusion demonstrator engine if improvements in the power, shielding and field control were made. This engine offers the potential for bridging the gap between high thrust-low specific impulse technology as used in conventional rockets and low thrust-high specific impulse technology as used in electric ion engines and it can function in either mode. Similarly, for a full-up pulsed fusion engine, the employment of different pellet sizes also allows a tailoring to the amount of thrust needed for each pulse cycle, with some of the emitted energy being bootstrapped to run the next cycle.

The physics of fusion research has moved forward dramatically in recent years with the US National Ignition Facility now operational and others such as Laser MegaJoule in France under construction. Fast ignition proposals such as HiPER are also under consideration. The chances for scientists finally solving the ‘fusion problem’ are greatly increased. With this in mind, thinking about the implications to a deep space missions is timely. It is quite possible that the demands of a fusion based drive will necessitate a sophisticated space based infrastructure for resource acquisition, processing, manufacture and construction. Especially if He3 mining of the gas giant Jupiter or even the Moon is considered. However, as a theoretical exercise in the application of science and engineering Project Icarus has a large amount of intrinsic worth.

There is another reason why fusion was chosen as the main propulsion engine. Daedalus was a historical design study that changed perceptions about what was possible with interstellar flight. When thinking about performing a design study for an interstellar craft, instead of starting from scratch with an unfocussed ‘anything goes’ design philosophy, it is arguably more useful to build upon the good work that has already been done, essentially standing on the shoulders of the previous generation of designers. Hence, a re-examination of Daedalus seemed an obvious way to go. This would allow a complete re-evaluation of the original assumptions as well as hopefully
improve the design.

Ultimately, the aim would be to improve the Technological Readiness Level for this sort of engine design type. If other teams used the same approach, and say built upon historical projects like Vista, Longshot, TAU or Starwisp it is a personal belief that the credibility of engineering designs for interstellar missions would be vastly improved. The historical link with both Orion and Daedalus also captured the hearts of the Icarus team and made for a strong support base upon which to galvanize both academic and public interest; a necessary condition to inspire people that this design study is worth doing. Although it is also true that after having questioned the original assumptions of Daedalus, the final Icarus design may look very different with technology not envisaged in the 1970s.

Of course, it is also worth pointing out that the Project Icarus Terms of Reference actually stipulate ‘mainly fusion based propulsion’. This allows for the potential for high gain enhancements such as by using Antimatter Catalyzed Fusion techniques. Similarly, the main engine can be supplemented by a secondary engine for part of the mission trajectory, such as by using a nuclear-electric engine.

It is generally the consensus within the interstellar community that the two strongest candidates for interstellar flight and which are a balance between performance and near term technology readiness is arguably solar sails and nuclear pulse propulsion. The large interest today in using solar sails for interstellar missions came about as a result of a publication in 1984 titled ‘World Ships: Concept, Cause, Cost, Constructions, and Colonization”, written by several members of the Daedalus Study Group who had spent many years studying fusion based propulsion. Thus demonstrating that the act of completing an interstellar design study opens up opportunities perhaps not before envisioned.

If designers concentrate on trying to advance these two schemes, solar sailing and nuclear pulse, the prospects for robotic missions to the Kuiper belt, Oort cloud and beyond will become ever more likely. Project Icarus hopes to contribute towards this end objective. It was the vision of Arthur C Clarke that humans should expand out into space as soon as possible, a necessary step for the continued survival and advancement of our species. Project Icarus is aimed at working towards this ultimate goal. So while from time to time we may have to justify to a sceptical public the motivations behind the assumptions of Project Icarus such as the choice of a fusion engine, the design team will remain focussed on achieving our objectives and continue to dream about the challenge of the spaceship.

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.

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