Slashdot Mirror
Hack in Space
MelloDawg writes: "From the press release: NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which some had given up for dead in December after critical guidance components failed, was returned to full operations when the team developed an innovative new guidance system. The system uses a complex new set of procedures that lets controllers use electromagnets in the satellite to push and pull on the Earth's magnetic field. Details of the mission are online."
It's things like this that really make me proud of our space program. Sure, it's a little tiny thing, we now have one less dead satellite, but that was a beautiful solution, and it's good to know that these folks are still there thinking stuff up.
If all the world's a stage, anyone who says they want better lighting spends far too much time in a dark theatre.
A basic problem in satellite stabilization is how to get rid of unwanted angular momentum. There are a few options.
You can throw away something. Usually this is reaction mass from a small rocket, or just compressed gas. Weights on the ends of cables that unwind and break free have been used to despin satellites.
You can store angular momentum in an inertia wheel, which is a flywheel on a motor. This doesn't get rid of angular momentum; it just stores it as long as you keep the wheel spinning. Eventually, you hit the maximum motor speed and can't do anything more in that axis. So it's also necessary to have some way to drain off angular momentum, even if very slowly.
You can couple to a gravity gradient. This is done with a long pole aimed towards a nearby planet. The difference between the gravity at the ends of the pole is tiny, but enough that if you get the thing pointed down and stable, it usually stays that way. Only good for one axis, of course.
You can couple to a planetary magnetic field, like these guys are doing. Again, only good for one axis, but it's a different one than the gravity gradient. It's a weak effect, but stronger than the gravity gradient.
You can put out sails and get reaction forces from solar energy. This gets talked about a lot, but isn't done much.
All of these are known techniques. It sounds like this satellite had four inertia wheels and an electromagnet for torquing against a planetary magnetic field. The plan was presumably to maneuver with the inertia wheels, and slowly drain off unwanted angular momentum with the magnetic torquer.
With two inertia wheels down, there are still three torquing devices available, so control of orientation is theoretically possible. Tough, but possible. It's impressive that they made it work.
Using the Earth magnetic field is something used in Amateur radio satellites since many years.
Yup. Believe it or not, spacecraft do have fuses. Obviously they can't be replaced when they blow, but by blowing they can isolate a failed module that might otherwise kill the entire spacecraft.
About five years ago, Hughes launched a communication satellite. To be useful, these have to reach a geosynchronous orbit. It turns out that the most efficient way to get to from a low-earth orbit to a geosynchronous orbit is to fire a rocket twice [or so it was thought.] The first firing raises the apogee of the orbit to the geosynchronous altitude; so that the satellite is in a very elliptical orbit. Then, when the satellite is at apogee, you fire the rocket again to circularize the orbit. Usually this same motor changes the plane of the orbit as well. Most satellites are launched into orbits inclined to the equator somewhat, and geosynchronous satellites have to be over the equator. It is most efficient to make the orbital plane change at apogee, too.
There was a rash of apogee kick motor failures, and in this particular satellite the motor failed, leaving the satellite in a uselessly ellipitical orbit. There were small thrusters on the satellite which were to be used for station-keeping (small orbital adjustments) but it didn't have nearly enough propellant to raise the perigee. Hughes finally abandoned the satellite.
But one engineer refused to give up. It turns out that the transfer orbit paradigm above is the probably most efficient path in a single-planet system, but Earth has this anomolously large, close, Moon. And while there wasn't nearly enough fuel to get raise the perigee to geosynchronous altitude, there was more than enough fuel to raise the apogee out to lunar orbit. He was given permission to try to rescue the satellite.
In the end, two passes by the moon were made, each raising the perigee somewhat and lowering the inclination of the orbit. The remaining fuel in the satellite was used to lower the apogee back to the geosynchronous orbit altitude, but unfortunately the inclination couldn't be brought down quite to zero, so the satellite isn't in its desired orbit even today. Still, it's in an orbit where some use can be derived from it.
The satisfying conclusion to this story would be that all geosynchronous satellites are launched this way, now. Unfortunately, you can't mess with the status quo to that extent; and satellites are still, in the main, launched the old transfer-orbit way.
thad
I love Mondays. On a Monday, anything is possible.
What they probably managed to do is to use the two remaining wheels to do the fine pointing but the satellite will tend to slowly spin ?of course? lacking the two other wheels to compensate. By bringing the torquers into the loop they cancel the spin and attain the fine pointing.
Actually, not quite. The roll and "skew" wheels are the ones that still work fine. The new design allows the torquer bars to correct an axis that is basically perpendicular to the roll and skew directions, which is sort of diagonal across the science apertures.
It turns out that although the torquer bars are not designed for fine control (their control bandwidth is 15 times lower than the reaction wheels), the satellite is also heavy compared to the MTB's torque. So, the pointing stability is in fact quite good, and the jitter is not much worse than what we had before... as long as the satellite is not pointed in a part of the sky where gravity gradient disturbances are strong.