The Planetary Society’s LightSail test spacecraft reported for duty this afternoon, heralding the end of an uneasy silence caused by a suspected software glitch. At 5:21 p.m. EDT (21:21 UTC), an automated radio chirp was received and decoded at the spacecraft's Cal Poly San Luis Obispo ground station. Another came in eight minutes later at 5:29 p.m. The real-time clock on board the spacecraft, which does not reset after a software reboot, read 908,125 seconds—approximately ten-and-a-half days since LightSail’s May 20 launch.
"Based upon the on-board timers contained within the beacon (and comparing them to beacons following deployment), it appears that a reboot occurred within the past day," wrote Georgia Tech professor David Spencer, LightSail’s mission manager. "Due to uncertainty in the orbit state (TLEs), our ability to reliably track the spacecraft is marginal at this point. Cal Poly is coordinating with international colleagues to arrange their support in acquiring beacon telemetry," he said.
LightSail is not out of the woods yet. Its exact position remains fuzzy, complicating two-way communication. Today’s contact marks the first time engineers can compare the spacecraft’s signal with orbital models called two-line element sets, or TLEs. There are ten TLEs associated with the ULTRASat fleet that joined LightSail for a free ride to orbit courtesy of a United Launch Alliance Altas V rocket. Which TLE represents LightSail is unknown, but each radio chirp's doppler shift helps narrow down the possibilities.
The ten ULTRASat spacecraft have drifted into two groups. At the time the first signal was received at Cal Poly, all ten spacecraft appeared to be in range—no help, from a visual standpoint. But when the second signal came in eight minutes later, only the trailing group appeared to be close enough. (This is only a rough estimation; a full simulation by Georgia Tech is pending.)
A first look at the new LightSail data shows the rotation rate has increased from -7, -0.1 and -0.3 degrees per second about the X, Y, and Z axes to 10.8, -7.3 and 2.9 degrees. The cause for the tumbling uptick currently unknown, but with the spacecraft's attitude control system offline, sail deployment is likely to be a wild ride.
The next ground station pass is scheduled for 5:30 a.m. EDT, when the ULTRASat cadre passes within range of Georgia Tech. As of today, the patch intended to fix LightSail's file write vulnerability is still being tested. In order to buy more time, additional reboots could be scheduled. However, having dodged one close call, the team wants to initiate sail deployment as soon as possible. A plan of action is still being assembled, with discussions likely to last into the night.
In the meantime, all of us at The Planetary Society are cheering the good news. Many of you are, too, and we're thankful for the support. We'll keep you updated as more news develops.