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.