I’ve been pretty excited about the mission to rescue the Neil Gehrels Swift Observatory satellite, which is why since January I have had a widget in the right-hand column that tracks Swift’s position.
With the rescue mission on the way, I thought it would be interesting to add the LINK rescue craft as a second widget. Space Force is tracking it, so the data is available. But there are a few complications…
First of all, it’s not technically in an orbit. Its ion engines are very weak — using less than an ounce of thrust to push a 900-pound spacecraft — but they can run continuously, hour after hour. This means that the orbital data goes stale very fast. So I have it displaying a red “MANEUVERING” notice to indicate that the true orbital elements are changing. (I also don’t bother to display decay or calculated altitude information.)
Second, Space Force isn’t putting out a lot of updates on LINK, which only exacerbates the drift due to maneuvering.
Despite that, I did try to add some information about the orbital match. I wasn’t sure how to describe the differences between the orbits of two objects, especially when one of them is maneuvering to catch up with the other. I finally let Claude come up with the solution that I’m using. It divides the orbital matching process into several stages:
- Plane change: LINK is maneuvering to get in the same orbital inclination as Swift.
- RAAN Drift: Inclination amount is the same, but not in the same direction, so LINK has to try to match.
- Phasing: LINK and Swift are essentially in the same orbit but in different positions within the orbit, and LINK is trying to match.
- Approach: Phases are essentially matched but they are more than 100 km apart.
- Terminal: Within 100 km.
- Matched: Within 1 km, and moving nearly the same speed.
LINK is in Phasing now, and I was really looking forward to watching it go through the final stages of matching orbits, and then watching both LINK and Swift start the slow climb to a higher altitude…
But that was not meant to be. The LINK craft is supposed to use reaction wheels to control which way it’s pointing in space. It’s Newton’s Third Law: Spin the wheels one way, and the spacecraft spins the opposite direction. And if the combined forces acting on LINK start it spinning (everything in space wants to spin), the reaction wheels can be spun in the opposite direction to stop it. In physics terms, the angular momentum of the craft is transferred to the spinning reaction wheels. Unfortunately, two of LINKs three reaction wheels would not turn, and the spacecraft began to tumble out of control, losing its antenna lock back to Earth.
Somehow, the LINK flight team managed to re-establish communication. With two of the reaction wheels frozen, they couldn’t easily stop the spin (LINK also has cold gas thrusters, but they weren’t usable either.) The engineers did manage to push out new flight control software that would allow it to be oriented properly — I think they must be using the ion drive to control attitude. The mission was still making progress…
… and it might have worked. Maybe if they had more reaction mass. Maybe if they had better control and weren’t afraid of smashing LINK into Swift at high speed. Maybe if the reaction wheels hadn’t frozen. Maybe if the Sun had been less active and the Earth’s atmosphere hadn’t expanded high enough to drag Swift down faster than expected…
The rescue mission has been scrubbed. LINK is still going to try to match orbits with Swift — just to see if they can do it — but LINK won’t try to grab onto Swift and push it to a higher orbit. Instead, some time in the next few months, the Neil Gehrels Swift Observatory satellite will descend into the Earth’s atmosphere and burn up. It’s not a matter of if, it’s a matter of when.
And when is not real clear. For as long as I’ve been watching, the media has been reporting an expected re-entry date for Swift as sometime in October. But it’s already late August and the 18th Space Defense Squadron still hasn’t issued a 60-day re-entry warning message. I’m beginning to think Swift will stay up there a bit longer.
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