Bruce BettsApr 01, 2009

Solar Sail Update: New Opportunities

The Planetary Society remains committed to flying the first solar sail. Specifically, we aim to fly the first controlled flight of a spacecraft using light pressure to increase the orbit energy.

At the beginning of this decade, we designed a mission to accomplish this goal. We launched Cosmos 1 in June 2005, but the Volna rocket that was to place the spacecraft in orbit failed, and we were never able to test our solar sail in flight.

These days, The Planetary Society is working with colleagues at NASA and at the Russian Space Research Institute to put together a new solar sail mission. In the past few years, lighter and more compact spacecraft have become available and more piggy-back launch possibilities exist for these smaller vehicles. We are now investigating ways to use these advances in our next flight.

The NASA Nanosail

In 2008, NASA attempted to launch a small spacecraft called Nanosail-D, that strongly resembled a solar sail. Nanosail-D was constructed by joining together three Cubesats -- small standard-sized cube-shaped satellites, often used for inexpensive research missions. One of the cubes served as the spacecraft "bus," and the other two housed a 7 by 7 meter sail with deployable booms. Significantly, the nanosail was never intended for solar sail flight. NASA wanted to use the "sail" as an atmospheric drag brake that would slow down a satellite and bring it down after its mission is over.

NASA built two nanosail spacecraft: One was launched on board the new Falcon 1 in August of 2008, but the booster failed and the spacecraft never made it to Earth orbit. Its twin craft was not used and remains in storage. Budget constraints have now forced NASA to abandon its sail technology program, so they asked us to consider using the remaining nanosail for our solar sail flight.

We are now looking into the possibility of upgrading the nanosail and turning it into a true solar sail spacecraft. This will require that it be maneuverable and controllable from Earth, and that its radio system and instrumentation will be able to verify its operation. Nanosail-D had none of these features, but we are now talking to the spacecraft developers about ways to augment original design.

One of our goals for the mission is to image the sails as they deploy, and later during flight. To accomplish this, our Cosmos 1 spacecraft had two good imaging systems on board, and we had two Earth-based telescopes ready to observe it from the ground. Seeing the sail deployed is important from a publicity standpoint, but it is also necessary for obtaining indispensable technical data. The dynamics and stability of such a large thin (gossamer) structure is hard to predict, and observing the sail in flight is the only way to find out how it behaves. Imaging the sail from the very small spacecraft itself is difficult because our perspective will be limited. Ideally we would like to have a companion spacecraft available which could image our solar sail from some safe standoff distance. We will look into that.

The Russian Mini-sail

Our Russian colleagues at the Space Research Institute (IKI) in Moscow, who worked with us on Cosmos 1, have also proposed a new, smaller approach. As with the nanosail design, they propose a smaller spacecraft projected to have better performance with higher acceleration than our original solar sail. Acceleration is proportional to area divided by mass -- and the mass of these new designs is getting really small. The challenge is to have sufficient control of the sails' "attitude," so as to make it possible to "fly" on light. This means steering with the sail pointed towards the Sun, or -- as we sailors say -- tacking.

Either the nanosail or a different mini-sail will need a radio system for tracking and telemetry, an imaging system, and perhaps a micro-accelerometer to measure the solar acceleration. We developed these instruments for Cosmos 1 and may be able to use them on a smaller spacecraft.

What’s Next?

With a lighter-weight spacecraft, more secondary (or "piggy-back") launch opportunities open up. One possibility we are still considering is a secondary launch on the highly-reliable Soyuz-Fregat, but there are other options as well. Our colleagues at NASA have suggested that the spacecraft could piggy-back on a U.S. launch, and our Russian team have urged us to consider the use of the Russian Cosmos-3M. We will not consider Volna again.

Our Planetary Society/Cosmos Studios/IKI team has begun studies of both the NASA nanosail adaptation and the Russian mini-sail option. NASA personnel at the Ames Research Center and the Marshall Space Flight Center -- who are developing nanosats and who built the nanosail spacecraft -- are working with us in analyzing the nanosail option. IKI personnel are analyzing options for their proposed mini-sail.

Our plan is to conclude these studies this summer. Then we will announce the most feasible option for flight for the first solar sail mission. Meanwhile, we and Cosmos Studios continue to seek additional private support and sponsorship for the first solar sail mission.

The Time is Now.

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