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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."
Everyone seems to think that since many of the outward looking missions are having trouble that NASA isn't really doing much, check out all the reasearch thats being done on our actual planet. NASA has been collecting huge amounts information about Earth's atmosphere, oceans and land, and they've been doing important analyses. I'd suggest you check out places like:
earth.nasa.gov
earthobservatory.nasa.gov
terra.nasa.gov
data assimilation office
and for image products:
visibleearth.nasa.gov
NASA does alot of interesting earth science too!
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.
Don't be ridiculous. I can tell you that in IT, this line of thinking will get you fired quickly.
Most often, if a system breaks in a production environment, it's imperative to get a it working again as fast as possible. Treating the symptom is paramount in a high availability situation, and while finding and resolving the cause is of course important, it takes an immediate back seat to getting the system back into production. If you can do both at once then that's grand. If not, then you restart the daemons or reboot the server once you've deemed it safe to do so.
Then, armed with log files and information gathered while the system was on error, you can go back through and trace the cause. Even better, you can duplicate the issue on a staging server. What you do NOT do is leave the system down for any reason one second longer than you have to, no matter how much the urge to tinker grabs you.
On the other side of the coin, diagnosing a problem on trivial or near-trivial system is a waste of your valuable time. Why the hell, except for fun, would you diagnose a BSOD issue on a secretary's computer when you could just back up her home directory, restore an image, then restore her personal files?
Obviously, every situation is different, but there are times when inexperienced techs will spend a day searching for the cause of a trivial problem instead of getting back to work.
Taking the time to root out a deep problem instead of just hitting the reset button is most often a luxury.
Even Jesus hates listening to Creed.
You can couple to a planetary magnetic field, like these guys are doing. Again, only good for one axis
Not really. Full 3-axis stabilization can be implemented using the Earth's magnetic field. Unlike a passive pole the magnetotorquers are active elements and the magnetic field doesn't always point down so at different parts of an orbit it can be used to control all 3 axes.
Design and Experimental Test of Magnetic-Torquer-Based 3-Axis Satellite Attitude Controllers
Stop worrying about the risks of nuclear power and start worrying about the risks of not using nuclear power.
We choose to go to the moon. We choose to go to the moon. . . . Not because it is easy, but because it is hard. . . . Because there is new knowledge to be gained. We shall send to the moon--240,000 miles away--from the control station in Houston, a giant rocket more than 300 feet tall, made of new metal alloys, some of which have not yet been invented . . . on an untried mission . . . on the greatest adventure.
Now, I'm a pretty cynical old bastard, but those words, nearly forty years on, still choke me up. To me, the defining quality of humanity is that our reach can exceed our grasp.
It's impossible (I think) to justify space exploration in any rational or economic terms. But if "because it's there" isn't justification enough, then that's a sad indicator we have become society of navel gazers and bean counters. And history shows us that societies only go one way once they've reached that stage.
You're right that the space race was based on competition. But I believe that still applies, and if we no longer want to compete, sooner or later we will be superceded by a society that does.
If you were blocking sigs, you wouldn't have to read this.
What the article omits is that the fine pointing accuracy is achieved using the magneto-torquers in combination with the two still operational reaction wheels. From the original NASA press release (dated 14 Dec 01) where the anomalies with the two reaction wheels were reported the strategy was laid out to rescue the science mission:
One of the new control mode concepts being investigated is to use the two operational reaction wheels in conjunction with the satellite's magnetic torquer bars to provide control in all three axes. The magnetic torquer bars are presently used to manage the momentum of the reaction wheels by applying a torque on the satellite against the Earth's magnetic field. The torques necessary to make up for the failed wheel would be in addition to that required for momentum management. This is well within the capability of the magnetic torquer bars.
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.
It is nowhere mentioned but I wonder if they can maintain the pointing accuracy long enough to get maximum exposure time. Since they've said that the spacecraft has been restored to full operations I guess it is not a problem.
I'm not trying to take anything away of what they achieved, only to clarify what is omitted from the article (possibly not to make it too technical I guess). It still is an impressive feat and indeed a super hack!
something wicked this way comes...
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.