Looking Back, Looking Forward
This is the final update to the SkyCube project. I'd like to begin by thanking everyone who's followed and supported SkyCube over the past 2 years. You made possible some amazing accomplishments. You proved that an amateur team can build, manifest, and orbit a functioning spacecraft on a shoestring budget. You were pioneers in a DIY space movement that - over the past few years - has blossomed into a multi-million-dollar industry.
FINAL MISSION STATUS
At this point, SkyCube's 90-day mission is now complete. Our definitive contacts with SkyCube on March 27th were our last communications with the satellite. Since then, we tried many different command sequences to cut the solar panel restraints, reboot the satellite, and inflate its balloon. These did not generate any apparent response. On July 8th, we stopped attempting further communication. Our efforts since then have concentrated on determining the cause of these results.
Interestingly, of the five CubeSats deployed from ISS on February 28th, SkyCube is the last remaining in orbit. It's expected to stay there several months longer; the other four CubeSats in our deployment have all decayed and reentered Earth's atmosphere. We feel this is conclusive proof that SkyCube's solar panels did not open on day 1. If they had, the SkyCube would have had more surface area than any other satellite in the deployment, hence the most drag, and fastest orbit decay.
A solar panel non-deployment would explain our other communication problems. SkyCube's radio antennas were tucked inside the solar panels for launch. They could not have extended if the solar panels never released. Antennas folded inside solar panels would explain why SkyCube's transmissions were so weak and sporadic. On March 30th, we tested radio communication with SkyCube's twin in packed configuration, and found that its signal was weakened by about the same amount (10 - 12 dB) as the difference between the expected and observed signal strength in the packets that were actually received on March 27th.
Solar panel non-deployment would also explain why the balloon did not inflate: the tie lines restraining the solar panels were deliberately reinforced, to prevent the lid from opening in case of premature balloon inflation. If the tie lines were not cut loose, the balloon could not have punched through the lid.
ONE MYSTERY REMAINS
Our final remaining unanswered question is this: the solar panel deployment mechanism was tested repeatedly on the ground, so why did it fail in space? None of the proposed theories are very satisfying:
- Burnwires cut nylon more slowly in vaccuum. We didn't test the solar panel deployment in a vacuum chamber before launch. But we did test the (identical) balloon inflator burnwire mechanism in vaccuum before launch, and re-tested the burnwire mechanism in vacuum in March 2014. These tests showed burnwires cutting nylon in 3 - 5 seconds, with no noticeable difference between the time to cut them in air.
- Battery charge was not sufficient to power the burn wires. In testing, we learned that a battery charge below 7.25 volts was not sufficient to power the burn wires. But we programmed the satellite to wait until battery charge was above this level before attempting the deployment. When signal was actually receieved from the satellite, the battery voltage was over 8V.
- Burnwires broke due to vibration on launch. If burnwires came loose or disconnected, applying current would have no effect. However, burnwires survived all pre-launch vibration tests without incident; and all 4 burnwires would have to have broken in this way.
- Burnwires worked perfectly, but hinges were mechanically stuck. Again, we did not observe this in testing, and all four independent solar panels would have to have gotten stuck the same way to explain a complete failure.
The truth is that we may never know the answer. Rather, we view this as a lesson learned: if this is your first satellite, design it without moving parts that are required to deploy for mission success.
WHAT WE ACCOMPLISHED
Given the solar panel deployment problem, it's even more remarkable that we did actually hear from our satellite on orbit. The contacts on March 27th proved that the satellite's electronics had been working normally for at least a month. The batteries were fully charged; the solar cells and power system worked as designed.
This also means that all of your names were actually transmitted by the satellite as "tweets" (albeit quietly, through folded antennas) during the mission's first week. Those tweets are still streaming out across the universe on 915 MHz at the speed of light.
Our ground station operators at Saber Astro and elsewhere also deserve special mention. During the difficult post-deployment period, after the first suggestive transmissions were received, they patiently kept listening while 3 weeks of silence followed. Through the silence, they repeatedly pinged each of the five candidate objects for a response. Without their persistence, that response would have never been caught.
THE BIG PICTURE
Over the 2+ years since this project began, CubeSats have made enormous progress in general. When SkyCube's kickstarter launched in July 2012, they were an academic curiosity. When SkyCube's kickstarter began, the greatest number launched in any year was fifteen (in 2010). So far in 2014, more than sixty CubeSats have been launched. On our own launch, 33 CubeSates were carried into orbit. Of those, 28 were commercial earth-imaging satellites, operated by Planet Labs in San Francisco. In July, NanoSatisfi - whose ArduSat kickstarter preceded ours - announced its transformation into Spire, a global asset-tracking service using CubeSats. The same month, Spaceflight Services announced a new commercial small satellite communication service.
Movement really is happening. Significant venture capital is flowing into the field. Communication challenges are being addressed. Access to space really is getting easier. Our timing may have been a bit eariy. Our results may have been different if we'd started now instead of 2 years ago. We were pioneers in the movement, and performed a tremendous amount of work on an incredibly small budget. Hearing from SkyCube on orbit - at all! - was a more successful result than the majority of first-time CubeSats achieve. And we have a lot of very hard working and talented people to thank for that.
Finally, we have all of you to thank. Although this particular chapter may be closing, the larger story of the New Space movement is far from over. We encourage you to take advantage of our lessons learned, to take on new challenges of your own, and to view what we all accomplished not as falling short of some specific goals, but as a groundbreaking achievement in the democratization of space exploration.
You have our gratitude for making it possible.
-Tim on behalf of Team SkyCube