A milestone for solar sailing
by Kieran A. Carroll, Ph.D.
|Very-long-duration thrusting without consuming propellant is the attractive feature of this technology. The catch is that very large sails are needed in order to achieve useful levels of thrust.|
My intent here is not so much to detail the technical material presented (for that, a partial proceedings is downloadable from the conference website, and the organizers plan to make all the presentations available there soon), but to provide for those who missed this symposium a sense of the feel of it—the prevailing mood there, and the significance of the meeting.
For those unfamiliar with the concept, solar sailing is an in-space propulsion technology that has been studied for decades that involves harnessing the momentum of photons streaming out from the Sun by deploying large mirrored surfaces (“solar sails”) from spacecraft. As solar photons are reflected by the mirrors, they exchange momentum with the spacecraft, producing thrust. The pressure due to sunlight is not large—less than 10 micronewtons of force (1/3000 the weight of a penny) for each square meter of sail area. However, as it does not consume propellants the way that rockets do, this means that a solar sail can thrust for long periods of time—months and even years—and so can eventually build up much larger velocity changes than are achievable with rocket-based propulsion. This can enable solar system exploration missions that would otherwise require prohibitive amounts of propellant if using chemical rocket propulsion, such as multiple-asteroid rendezvous missions, near-Sun Solar polar science missions, and solar system escape missions. The ability to thrust continuously can also enable spacecraft to “hover” in locations where spacecraft cannot otherwise loiter, such as above the Earth’s polar regions (useful for 24/7 polar communications and weather monitoring), in-between the Earth and the Sun at altitudes much higher than the Earth-Sun L1 point (useful for getting earlier warning of incoming solar particle storms), and even north or south of the geostationary belt, which could greatly increase the limited amount of “real estate” there for communications satellites.
Very-long-duration thrusting without consuming propellant is the attractive feature of this technology. The catch is that very large sails are needed in order to achieve useful levels of thrust. To understand why, it helps to know something about the main figure of merit for a solar-sail equipped spacecraft, which is its mass-to-area ratio—that is, the total mass of the spacecraft (including the sail), divided by the total reflecting area of its solar sail. The smaller this ratio is, the more maneuverable the spacecraft will be. Solar sailors have learned that achieving ratios in the range of 10 to 20 grams per square meter will enable numerous useful missions. A relatively low-mass spacecraft (50 kilograms, say, typical of modern microsatellites) would need to deploy a solar sail of 2,500 to 5,000 square meters to achieve a ratio in that range (that’s a quarter to half a hectare, which is to say about an acre). If the sail was made in the form of a square, it would be 50 to 70 meters on a side; for comparison, the solar arrays on the International Space Station (which are generally considered to be pretty big space structures) are about 77 meters from tip to tip.
|An interesting feature of the international solar sailing community, very much in evidence at this symposium, is the spirit of simultaneous cooperation and competition.|
Making such large, low-mass mirrors actually turns out to be fairly easy, using thin plastic films with extremely thin metalized coatings. This type of material is very familiar to satellite designers, who use multi-layer blankets of aluminized polyimide or polyethylene as thermal insulation. However, finding ways to stow such sails compactly enough for launch, to deploy them reliably once in space, and to control them once deployed has been much more challenging. The first serious attempt to do so was at NASA JPL in the late 1970s, where a preliminary design was conceived for a truly enormous solar sail—a mass of around 5,000 kilograms, a sail area of 625,000 square meters and a mass/area ratio of 8 grams per square meter—that was intended to rendezvous with Halley’s Comet in 1986. The ambition of that mission proposal exceeded NASA’s willingness to fund development, but it sparked interest in solar sail development among space engineers worldwide, who have grown to form a group with a real sense of common purpose and community.
Since then, the international solar sailing technical community has done considerable work to conceive additional mission applications, alternate solar sail designs, and specific mission proposals. Occasionally this work has been supported by development funding from national space agencies (NASA, DLR, JAXA), although for much of the past three decades researchers have made do without such support. This community has stayed in touch via journal publications, and solar sailing sessions at more general space conferences. Only recently have individual “solar sailors” from around the world started meeting in an organized way. The New York event was the second such meeting, following the first one, held three years ago in Germany. The objectives were to report new and useful ideas regarding solar sailing technology development, and ideas for useful applications of this technology in areas such as space exploration, communications, and Earth remote sensing.
An interesting feature of the international solar sailing community, very much in evidence at this symposium, is the spirit of simultaneous cooperation and competition. Many of the attendees have been working for many years to develop solar sailing technology capabilities within their institution and their country, and betimes that has been done in a context of explicit competition—for example, in the 1988–1992 period when an international solar sailing “race to Mars” was proposed, as a commemoration of Columbus’ voyage of discovery 500 years earlier, resulting in design proposals from numerous teams worldwide. The growing number of useful applications that have been found for solar sailing spacecraft has raised the prospect of a “first-to-market” advantage for whichever organizations and space agencies are able to develop an early expertise in this field. However, the challenge of overcoming technical difficulties and of formulating compelling funding proposals to their national space agencies (and to a few private funding sources) has fostered a friendliness to this competition, with researchers around the world working collectively to overcome the obstacles they face in bringing this technology to fruition (and many of them forming lasting friendships). One senses echoes of the sentiment that infused the seminal stage of another area of astronautics: that of rocket development in its early days in the 1930s, which was advanced by dedicated and visionary individuals and small groups such as Goddard, Von Braun, Ley, the VfR, the British Interplanetary Society, and the American Rocket Society, who shared ideas for mutual benefit, and went on to great accomplishments. Indeed, solar sailing is currently a field in which the oft-used phrase, “a rising tide lifts all boats,” is peculiarly apt.
As the solar sailing community has struggled over the years to advance this technology, one particular competitive “prize” was frustratingly elusive: the honors that would go to the first solar sail to be successfully launched, deployed, and operated in space. Several attempts have been made to place a small, simple solar sail in space as a proof-of-principle, but all past attempts met with failure. That prize has now been won, by JAXA’s IKAROS (“Interplanetary Kite-craft Accelerated by Radiation Of the Sun”) spacecraft, which was launched into an interplanetary trajectory with their Akatsuki (Venus Climate Orbiter) probe on a HII-A rocket on May 21, 2010. The highlight of the symposium came when a contingent of JAXA engineers led by Junichiro Kawaguchi (the ISAS solar sail researcher who proposed the mission, and who is also project manager for JAXA’s recently-returned-to-Earth Hayabusa asteroid sample-return probe) provided, for the first time in public, technical details of the design and operation of IKAROS. There was a heart-warming moment when their description of the successful June 10 sail deployment was met with an ovation from the other attendees. Details presented included top-level satellite specifications (315 kilograms, square spinning sail 14 meters on a side, sail area a bit less than 200 square meters), the project budget ($20 million) and schedule (developed in 30 months), the sail stowage and deployment technique, the inclusion of thin-film solar arrays with the sail, the design and performance of electrochromic mirrors used to induce attitude control torques, and the resulting attitude maneuvering performance.
The IKAROS mission is an initial, proof-of-principle demonstration of solar sailing, not meant to be an operational mission. It is not designed to use its solar sail to fly to any particular destination: it has far too small a sail and too massive a bus (with a mass/area ratio upwards of 1,500 grams per square meter) to be able to do that. That mission has succeeded brilliantly in meeting its objectives, but there is still considerable further technology development and demonstration needed before solar sails can be put to work. The other presentations at the symposium addressed various further steps along that path—which brings me to another notable aspect of this symposium: reports from other several funded solar sail technology demonstration missions, all planning to fly quite soon.
|One significant theme underlying many of the presentations at the symposium is that the ultra-miniaturization of satellites in the recently-booming nanosatellite field has been highly useful to the solar sailing community.|
The first planned launch is the flight spare of NASA’s NanoSail-D, which is manifested with FASTSAT on a Minotaur 4 launch from Kodiak, Alaska, into low Earth orbit this fall. Dean Alhorn of NASA Marshall Space Flight Center presented an update on that mission, which comes after the original mission was lost due to the upper-stage failure of the third Falcon 1 rocket in August 2008. The two NanoSail-D satellites were notable for having been developed in a very short period of time (about 8 months) via a collaboration between NASA’s Marshall and Ames field centers. The technical objectives are fairly simple, although challenging: to deploy a 10-square-meter sail from a 3-kilogram “CubeSat” nanosatellite, which (without any attitude control) will proceed to rapidly de-orbit. The programmatic accomplishments were quite significant, demonstrating the ability of NASA engineers and managers to cooperate across field center lines, and accomplish a challenging technical task rapidly, harking back to the “can do” attitude of NASA’s early days.
The next mission in the pipeline is the Planetary Society’s LightSail-1, which is also a cubesat-sized nanosatellite, in this case massing 4.5 kilograms, which aims to deploy a 32-square-meter sail in orbit around the Earth, with a mid-2011 target launch date. Former Planetary Society president Lou Friedman led a contingent of presenters from their team, which is funded privately via a donation from an anonymous donor. It is a follow-on to the Planetary Society’s ill-fated Cosmos 1 solar sail mission, which was lost when the Russian Volna rocket carrying it in June 2005 failed to reach orbit. LightSail-1 is technically more ambitious than NanoSail-D, both because of its lower mass/area ratio (140 versus 300 grams per square meter), and because it has attitude control equipment that will allow it to maintain a commanded orientation relative to the Sun, and hence will be able to produce a deliberately-directed solar sailing thrust force. The mission designers plan to use this thrust to demonstrate using solar sailing to increase the satellite’s orbital energy, spiralling out from their initial orbit altitude by a measurable amount. To be able to accomplish this, the satellite is to be launched into a high enough orbit (above 825 kilometers) to avoid the effects of atmospheric drag, which reduces orbit energy and causes satellite orbits to spiral inwards.
Cubesail is another such nanosatellite-based low Earth orbit solar sail mission, this one being developed by the University of Surrey, funded by EADS Astrium and aiming to launch by the end of 2011. Vaios Lappas of the University of Surrey, who leads the student team developing this mission, described Cubesail as a 3-kilogram cubesat with a 25-square-meter sail (and so a ratio of 120 grams per square meter). The main objective of this mission is to demonstrate a capability similar to that of NanoSail-D: using solar sail stowage and deployment technology to greatly increase the drag on a satellite, thus hastening its re-entry, a capability which could be applied to reduce the proliferation of space debris. In addition, Cubesail carries attitude control equipment that will allow it to experiment with solar sailing, with the objective of using solar sailing thrust to change the satellite’s orbit inclination.
In addition to these funded nanosats-based solar sail missions, there were additional presentations (including mine) on several other small, low-cost, near-term demo missions that are part-way through development but are not yet fully funded. Collectively they aim to explore a range of different solar sail stowage and deployment approaches, and to test performance of different aspects of solar sails in various different ways. These presentations engendered a stronger feeling of optimism at this symposium than I’ve seen amongst solar sailors for many years, due to the great deal of development activity that is going on at the moment. That feeling is echoed in the formal communiqué issued by the delegates, the New York Declaration.
One significant theme underlying many of the presentations at the symposium is that the ultra-miniaturization of satellites in the recently-booming nanosatellite field has been highly useful to the solar sailing community. On a solar sail the “bus” is part of the overhead mass, which reduces sail maneuverability, and the lower this mass the better; nanosats (best-known as the widespread CubeSat variant) have finally driven down the mass of satellite buses to the point where they open up the prospect of quite-maneuverable solar sail spacecraft with reasonably-small sail areas. Even more importantly at the moment, the very-low-cost “microspace” engineering development and management approach used by many microsat and nanosat developers has brought the cost threshold down to the point of matching the slender budgets that solar sailors are currently able to scrounge, enabling many in the current crop of solar sail technology demonstration missions.
While almost all of the nanosat-sized missions presented at the symposium are using very similar designs for their solar sail subsystem—a non-spinning square sail supported by four diagonal booms, with the satellite bus at the center of the square where the booms intersect—there are numerous other solar sail designs that have been conceived and studied in the past, some of which were the topics of the other papers presented at the symposium. It seems that the boom-supported square sail is the simplest and lowest-cost and -risk design to use when designing an ultra-small solar sail, factors which are paramount for the current generation of demonstration missions. However, for future operational missions using solar sail propulsion, sail designs are needed that can scale up from the current sizes of 10–35 square meters to sizes well in excess of 1,000 square meters so that they can carry useful payloads. The initial demo missions will answer some very important technical questions about solar sailing, and will push forward the frontier of knowledge and capability for this technology. But a next round of demonstration missions will then be needed in order to demonstrate much larger sails, and to move towards the goal of mass/area ratios in the range of 10–20 grams per square meter. I expect before too long to see a greater diversity of solar sail design approaches being tested and flown.