Category — Propulsion

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.

One of the paramount issues for interstellar exploration is the attainment of adequate velocities that make reaching star systems in a matter of years or decades a realistic possibility. For interstellar explorers of the future, antimatter may prove to be an invaluable tool that has the capability to make the universe slightly more accessible.

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.

There are many proposed schemes for interstellar travel. These range from experienced based chemical fuels to highly speculative proposals such as the space drive. So when Project Icarus was put together, why did the team settle on ‘mainly fusion based propulsion’ in the Terms of Reference for the study? It is useful to spend some time justifying the motivation behind this decision.

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.