light sail




Topics: lightsail, light propulsion, solar propulsion

The LightSail test mission is formally over. Following a twenty five day stay in low-Earth orbit, the spacecraft re-entered back into Earth’s atmosphere Sunday afternoon. Orbital models show reentry likely occurred around 1:23 p.m. EDT (17:23 UTC) close to the South Atlantic Ocean. 


The final time LightSail checked in was Wednesday, June 10 at 11:29 p.m. EDT. The corresponding beacon packet, which turned out to be the mission’s last, displayed an actual-time clock value of 1,837,416 seconds—21 days since launch on May 20. The gyroscopes, which had been able to capture snapshots of the spacecraft’s tumble rate after each reboot, confirmed LightSail tumbling at 6.7, 2.4, and 0.3 degrees per second about its X, Y and Z axes.

The day after sail deployment on June 8, LightSail’s rotational rate was a leisurely 116 seconds, based on observers utilizing a wide-field survey telescope in Russia. That changed as the spacecraft dipped deeper into the atmosphere. By June 11, the rate had sped up to 36 seconds. One day prior to re-entry, it was 21 seconds.

On Sunday, Cal Poly’s Justin Foley heard from LightSail on its final ground station pass of the day, which started at 12:52 p.m. EDT. It was overflight number 223, & it would be the very last time the spacecraft would call home—even though it wasn’t a lot of a dialog, with LightSail continuing to talk in gibberish.

 Ted Molczan of SatObs made the first end-of-mission pronouncement Sunday afternoon, calling time of death at 1:23 p.m. EDT (17:23 UTC). Further proof in favor of Ted’s numbers came Monday when the Joint Space Operations Center issued a new set of orbital elements.

Of the reentry, Ted wrote: 

“Allowing the GMAT propagation to continue to decay revealed that it would have reached the altitude at which decaying objects usually become self-luminous (96 km) on Jun 14 near 17:22 UTC, and would have passed through the typical altitude of “main-break-up” (78 km) near 17:23 UTC. Given its low mass, I doubt it would have survived for much longer. If this is right, then it survived less than half an hour after the Cal Poly pass.”

If you plug the spacecraft’s last orbital elements into modeling software, you get an altitude of -53.5 kilometers at 01:39 UTC. In other words, the spacecraft would have been “underground”. But it’s fun to think about a scenario in which LightSail’s photo voltaic sails snapped cleanly away as the CubeSat reentered, permitting the avionics and radio systems to skirt around the Earth for a few extra orbits. One last act of defiance, so to speak.

 lightsail from ground

This image of LightSail was captured from Santa Cruz, California on June 9 at 3:52 a.m. PDT (10:52 UTC). The photo was analyzed by two additional amateur trackers for verification. Photo details: f/1.4, exposure 20 seconds, ISO 800, 30mm.


Whatever actually occurred, LightSail—the test unit, anyway—is dead. The vast list of lessons learned should now be applied to the second mission. While LightSail ran into a lot of trouble on its mission, it was ultimately successful — the only real objective was to deploy the ship’s namesake sails and prove that the vehicle was space worthy. The real challenge comes near the end of 2016, when the Planetary Society expects to fly a second model that will actually use its photon-powered sails to get around!



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