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Fraser
2007-Oct-09, 03:39 PM
I often talk about the preparation and launch of new missions to space, but every now and then I have to write about just the opposite: the end of a mission. ...

Read the full blog entry (http://www.universetoday.com/2007/10/09/the-end-of-fuse/)

BillG
2007-Oct-09, 07:49 PM
Thanks Frasier.

I was a member of the FUSE flight ops team from January 1997 until November 2003. I'm still in contact with the team at JHU. I know this is a bitter-sweet time for everybody there, and all of us who've ever been part of the team.

Fraser
2007-Oct-10, 02:11 AM
They came up with an absolutely amazing technique for re-orienting the satellite once the gyroscope failed. It must have been stressful and entertaining during that period.

It seems that it's always the gyroscopes that fail on these spacecraft, pulling a perfectly good vehicle offline. I wonder why they still launch with just 4. They should go with 10... just to be safe.

BillG
2007-Oct-10, 05:24 PM
The trick with using the magnetic-torquer bars was pretty slick. I know the person who thought it up, and you can bet the rest of us hold him in pretty high regard.

We should probably distinguish between the gyros, which feed data to the attitude control system about the spacecraft's orientation in space, and the reaction wheels (called momentum wheels in the press release) which are much larger and are spun to produce angular momentum needed to turn the spacecraft. FUSE has had problems with decaying gyro signal from its laser-ring gyros in the past, but that's not what's killed the mission. We actually developed a zero-gyro mode years ago. It's the reaction wheels that have brought things to an end. All four reaction wheels seized up and stopped dead. Without at least one wheel spinning there's just not enough angular momentum to move the spacecraft from one target to another, nor to hold it on target once it gets there.

The FUSE spacecraft did launch with four reaction wheels. That's one more than necessary to provide three axis control. But they're large and heavy, so adding any more would have pushed the mass of the spacecraft outside the design envelope.

Grand_Lunar
2007-Oct-12, 12:40 PM
Will FUSE be replaced? Or has a similarly equipped spacecraft already been launched?

BillG
2007-Oct-12, 01:45 PM
No, there's no plan to replace FUSE. Nor any of the other Explorer class missions. Nor the Small Explorer (SMEX) missions.