Software snag silences Science Guy’s solar sail spacecraft

By Joel Hruska

Bill Nye the Science Guy’s foray into solar sail propulsion is likely to come crashing back to Earth thanks to a software error. The craft launched on the 20th of May and spent a few days in space relaying information back to ground control before abruptly falling silent. The team in charge of the little vessel has tried repeatedly to reestablish communication, but has had no luck thus far. The LightSail spacecraft was meant to demonstrate whether solar sails could be used for high-speed spacecraft propulsion. Its solar sail is much larger than the Ikaros probe that was launched by Japan in 2010, and it included three CubeSat designs for data gathering and vehicle control.

Unfortunately, a simple error in the Linux telemetry software has frozen the flight computer. Every 15 seconds, LightSail transmits a telemetry beacon to Earth and writes the data from that transmission into a file called beacon.csv. The file gets larger over time, and when it hits 32MB, it crashes the flight system. Hard data on which CubeSat design and CPU were inside LightSail doesn’t seem to be readily available, but the first product generations were based on the TI MSP430F1612, a 16-bit CPU — and the fact that the file crashed at 32MB could support that read.

According to a blog post from the Planetary Society, the goal since the satellite went dead over the weekend has been to reboot the craft. Unfortunately, the error is “non-deterministic.” In 37 passes (as of Tuesday afternoon) no reboot command has been successfully accepted by the spacecraft. Right now, the team is hoping that a cosmic ray will strike the internal components and reboot the craft. That’s not as far-fetched as you might think — apparently most CubeSats experience cosmic ray-related reboots within 3-6 weeks in space.

LightSail-A prepares for launch. Credit: The Interplanetary Society

The LightSail satellite will remain in orbit for up to six months in its undeployed CubeSat form (the original plan was to thoroughly evaluate the prototype before deploying the solar sail). If the system reboots in the next few weeks, it may still be possible to conduct the original experiment. Bug fixes have already been tested on the ground, which means the error could likely be corrected, provided that the system comes back online. If contact is re-established with CubeSat, the team will begin a manual solar sail deployment as soon as possible.

Solar sails have been proven to work in earlier missions, providing low energy thrust similar to an ion engine. The goal of these larger deployments (the current craft, LightSail-A, was launched to collect data for a larger mission in 2016, dubbed LightSail-1) is to determine the exact characteristics and challenges of operating a solar sail for various types of missions. While they can’t provide anything like the delta-v of a rocket, the low-power steady thrust of a solar sail could be incredibly useful for interplanetary or even interstellar missions.

One of the problems with sending a vessel to another star is that the probe would need to carry enough fuel to brake and take readings of the other star system. A solar sail could theoretically perform this function — the steady light pressure from the approaching star could, over a period of years, provide enough thrust to allow for detailed readings of a target planet or sun.