Planetary Society Declares LightSail a Success

Deployed! Image Credit The Planetary Society

It was hard won, but after series of scares involving low battery readings, software glitches and communication dropouts, the Planetary Society has declared its cubesat based LightSail experiment a success.

The 3U cubesat launched into space as a secondary payload aboard an Atlas V on May 20th, the same mission which launched the X-37B spaceplane on another secretive flight. For the Planetary Society however, the mission was all about sharing the bright light of day and learning how to harness the minute but steady pressure it can apply to a large, lightweight surface. Although LightSail is not the first solar sail mission to be mounted, the history thus far has mostly been a sad one, marred by a series of launch failures of the various small boosters on which they could hitch a ride.

One notable exception was the Japanese Ikaros spacecraft, which became the first to successfully deploy a solar sail in deep space. Launched as part of the Akatsuki Venus orbiter mission, Ikaros successfully deployed a solar sail on June 6th, 2010 by using an innovative spin technique. As an added bonus the sail itself was embedded with solar cells. Ironically, even as the Ikaros experiment was a success, the Akatsuki spacecraft failed to make a key mid-course burn, and missed the planet Venus entirely. JAXA managers are still hoping for a second chance this November, when it may be possible to use Akatsuki’s smaller thrusters to achieve a tenuous high orbit.

For LightSail, the destination was much closer, and quite a bit more modest; a brief low Earth orbit intended to prove a solar sail can be deployed from a cubesat, even if the thrust captured by the mylar film had no chance of overcoming the drag imposed by the environment.

Packed aboard an ever reliable Atlas V, there was little chance of a launch failure impeding the mission, and true to form, launch and deployment went off without a hitch. Before the Atlas ever lifted off however, the troubles had already begun.

During testing of a bench unit before launch, engineers discovered that a software patch introduced while addressing other problems meant that the spacecraft’s attitude control system would be out of commission for the duration of the mission. With LightSail already bolted to the Centaur upper stage and out of reach, there was little the team could do. While not fatal to the mission, the tumbling which now could not be corrected meant that communications would be a bit more difficult, and downloading larger image files from the craft’s two on-board cameras would be problematic.

In the first days following deployment, which took place alongside a fleet of other cubesats, things appeared to go quite well, and indeed the first chips from the spacecraft’s transmitter were picked up five hours after launch.

On May 26th however, before either solar panels or the sail had been deployed, LightSail stopped transmitting. As it turned out the failure was not a surprise, as the manufacturer of the control board had notified the Planetary Society that an “uncorrected glitch” in the flight software would crash the computer when it had accumulated 32 megabytes of data being stored for the transmission packets. Aware of the problem, the team was working against the clock to upload a fix, but it was not soon enough.

That left two possible solutions. One was to continually transmit reboot commands from the two ground stations, one from mission control at Cal Poly San Louis Obispo, and the other from a station run by team partner George Tech in Atlanta. The second way out of the dilemma was supremely ironic but apparently commonplace in the cubesat world; wait for a stray cosmic ray to strike the computer and cause a hard reset.

At 5:21 PM EDT on May 31st, LightSail suddenly checked back in again, leaving controllers to wonder as to which trigger had actually occurred.  With the cubesat’s rate of tumble increasing, the command was issued on June 1st to deploy the four solar panels in preparation for charging the batteries and letting loose the sail. The panels deployed, but a new problem soon became evident. For reasons not yet clear, the spacecraft rebooted again, and then the battery voltage levels began to drop rather than increase as result of charging.

On June 4th, LighSail went dark again. Controllers theorized that the batteries had gone into a safe mode as a result of being caught between over and under charging as it traversed through day/night cycles. With the orbit certain to deteriorate over time, hopes for the mission began to fade. Then, on Saturday, June 6th the spacecraft checked in again, offering no excuse for its absence. Not wasting time, ground control issued the command to unfurl the sail during a pass on Sunday, and by Monday received confirmation that the sail was mostly deployed. On Tuesday, with a reception of a partial image from the second camera, the Planetary Society declared the mission to be a success, one which paves the way for a more ambitious flight in 2016.

Reentry is predicted for Saturday, June 14th.