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Hubble Service Mission 4 canceled



NASA announced today that HST Service Mission 4 has been canceled. For reasons
that I explained in my talk last fall in Toronto, this would have actually
been the 5th mission to service Hubble. We had been working toward a spring
2006 launch date for this mission.

This decision by the NASA Administrator is said to be based on shuttle safety
considerations and is not related to President Bush's proposal to redirect
NASA money to a moon project. In order to inspect the shuttle heat shield and
provide a safe haven for astronauts in case damage is discovered, it has been
decided that all future shuttle flights must dock at the International Space
Station. Since the ISS is in a 51 degree inclination and Hubble is in a 28
degree orbit, it is physically impossible for a shuttle mission to visit both
of these targets on the same mission.

The remaining lifetime of Hubble will most likely be determined by the
behavior of the Rate Gyro Assemblies onboard the spacecraft. These gyros sense
angular rotation rates, with the integrated gyro output used to steer the
spacecraft as it slews from one target to the next. Six gyros are mounted in a
tetrahedral configuration such that any combination of three gyros can be used
to steer the spacecraft. The angular rates around the roll, pitch and yaw axes
are determined by the onboard computer using a set of translation matrices. A
minimum of three gyros are needed to provide data for the three axes.

Failure of four of the six gyros caused Hubble to enter safemode for about a
month in the fall of 1999, until Service Mission 3A was able to install six
new gyros in December of that year. Service Mission 3B in March of 2002
carried six new gyros onboard the shuttle, but they were not installed due to
EVA time constraints.

Of the six gyros currently onboard Hubble, two have already showed current
spikes and are shut down, three gyros are powered up and active, and one is
being held in reserve. A study on the future life expectancy of Hubble can be
found at: http://www.spaceref.com/news/viewsr.html?pid=9911  

Under the present software configuration, failure of any two of the four
remaining gyros would make it impossible for Hubble to acquire targets. A new
two-gyro control law is being developed by engineers at Goddard. This would
allow the science mission to continue with only two functioning gyros, with
some restrictions on where Hubble could point in the sky. 

A "SWAG" (super-wild-assed-guess) is that Hubble has two to four years of life
remaining as of this date with no further service mission. Two-gyro software
might extend that date by another year or two. 

Two new science instruments were under development for installation onboard
Hubble. The Wide Field/Planetary Camera 3 (http://wfc3.gsfc.nasa.gov) and
Cosmic Origins Spectrograph (http://cos.colorado.edu) are now stranded on
Earth with no way to fulfill their scientific missions. They will most likely
be placed on public display at the National Air and Space Museum, perhaps in
time for you to see them when you come to Washington for the Amsat Annual
Meeting this October. 

Dan Schultz N8FGV



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