The Planetary Society's Solar Sail Is Ready to Deploy in Low-Earth Orbit

Despite a near-fatal software bug.

Ben Richmond

Ben Richmond

Image: planetary.org

On Wednesday morning, a Mylar sheet the size of a boxing ring should pop out of a satellite that's the size of a loaf of bread and make life for anyone who wants to launch a little cubesat in the future far easier. That is, if nothing else goes wrong.

Backed by the Bill-Nye headed Planetary Society, LightSail's mission is to prove that solar sails, which capture momentum from photons and turn it into a constant, if small, amount of momentum for the spacecraft, are viable for spacecraft in low-Earth orbit. Small satellites that are so cheaply and easily deployed from larger rockets, called cubesats, have had a propulsion problem—putting thrusters and fuel in the payload compartment makes rocket owners understandably nervous. A solar sail, which is mostly just a large, light-weight reflective surface, is a possible solution to that problem.

If all goes as planned, LightSail will be the third time a solar sail has been tested. The Japanese demonstrated that solar sails actually work with the IKAROS spacecraft in 2010 and NASA used a solar sail to bring the nanosatellite Nanosail-D2 back to Earth in 2011. LightSail will be the first time that a solar sail has been used for power in orbit, provided everything goes right. And, with apologies to noted atheist Bill Nye, Lord knows enough things have gone wrong.

The Planetary Society had two days to celebrate LightSail before it was clear something was amiss. Jason Davis, a digital editor at Planetary Society and embedded reporter with the LightSail mission, told me that the ground team found out that there was a file-write vulnerability in the LightSail's software just as the cubesat was reaching orbit. They prepared a patch for the software but before they could send it to the LightSail, it was gone.

Image: planetary.org

"This is Linux-based flight software so it's not all that different than a regular desktop PC—except most people don't run Linux," Davis said. "Lab testing showed that the processor was essentially just freezing just like when your own computer would freeze up when you run out of memory or something like that. The only way to fix it was a reboot."

It's a familiar enough problem to anyone who has ever used a computer, except that, in the LightSail's case, the on-off switch is whipping along above the atmosphere. For eight days, they couldn't reach the LightSail, until finally, on May 30, the reboot took and the satellite came back to life. At that point it was in a three-axes tumble, which made communication with, and any new software uploads to, the satellite difficult.

"So they effectively decided that we're going to forget about the patch and proceed toward solar sail deployment," Davis told me. "But in the meantime, because the reboot fixes it, we're just going to schedule reboots."

So it's not a patch, it's a band-aid, but this mission is the time for these things to happen. The Planetary Society has another demonstration of technology scheduled for next year, and the principle reason for this mission can still move forward.

"The main objective of this first flight is to deploy the sails and make sure the deployment sequence works correctly," Davis said. "If all goes well, Wednesday morning we'll deploy the sails and start trying to download images from the spacecraft that hopefully will show that the sails are deployed."