Number 18, January 2002


1) Update on FUSE status

Dear Colleagues,

As many recipients of this newsletter already know, FUSE science operations 
were suspended on December 10, 2001, due to problems with two of the reaction
wheel assemblies on the spacecraft.  The satellite is in a safe configuration
with solar arrays toward the sun, telescope doors closed, and detectors off.  

The FUSE project is aggressively pursuing several avenues to enable us to return
FUSE to science as soon as possible.  We are testing new ways of providing
three-axis attitude control of the satellite in the event one of the failed 
wheels cannot be restarted.  On December 20 a modification to the flight 
software was installed that reestablished coarse three-axis control using the 
two wheels plus magnetic torquer bars.  The pointing since then has been 
relatively stable, but far from the accuracy needed for science operations.  

The doors will be reopened once the capability to reliably execute slews is 
demonstrated.  A new program to attempt to restart the y-axis wheel will begin 
in the next week or two.  Recent progress is very encouraging about the next 
steps for reestablishing sufficient fine pointing control so that science 
operations can be resumed in the near future.  

FUSE uses reaction wheels to slew and maintain attitude.  There are four wheels, 
replace any of the other three.  Two of the wheels, along the x and y axes, have
shown friction anomalies over the last two years, which caused short duration 
(~1 day) erratic behavior and were autonomously shut down.  However, engineers  
from Orbital Sciences Corporation, the spacecraft manufacturer, were able to 
restart the wheels with only a few days lost out of the science timeline.  

On November 25th, the x-axis wheel stopped abruptly and several attempts to 
restart it were unsuccessful.  Science operations continued using the three 
remaining operable wheels.  On Monday December 10, the y-axis wheel stopped.   
Although there has been a hint of wheel motion, it has not been possible to 
spin up the wheel.  

The new control mode uses 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 normally used to manage the momentum of the 
reaction wheels by applying torques on the satellite against the earth's 
magnetic field. The torques necessary to make up for the failed wheel are in 
addition to those required for momentum management.  This is well within the 
capability of the magnetic torquer bars.  Modifications to the satellite's 
flight software for fine pointing control are being designed and tested by 
Orbital engineers.  The new flight code will be uplinked and tested with FUSE 
in the near future.  

The FUSE mission was at the peak of its scientific productivity when this 
failure occurred.  Over 50 papers based on FUSE observations will be presented 
at the AAS meeting next week in Washington, DC.  The FUSE Cycle 3 GI programs 
were announced by NASA in October 2001.  NASA intends to carry out the approved 
GI and PI team observing programs, consistent with the capabilities and 
constraints of the satellite.  

We will continue to update you on the status of the satellite as the recovery 
activities proceed.

Warren Moos
FUSE Principal Investigator
Johns Hopkins University

George Sonneborn
FUSE Project Scientist
NASA Goddard Space Flight Center

Hashima Hasan
FUSE Program Scientist
NASA Headquarters

The Observer's Electronic Newsletter is published by the FUSE project and is aimed at the FUSE user community. Editor: B-G Andersson, FUSE Guest Investigator Officer. The FUSE Project is managed by Johns Hopkins University's Center for Astrophysical Sciences in Baltimore, MD, for NASA's Goddard Space Flight Center. The FUSE Principal Investigator is Dr. Warren Moos, the FUSE Project Manager at JHU is Mr. J.B. Joyce, and the NASA Project Scientist for FUSE is Dr. George Sonneborn. Further information about the FUSE Guest Investigator Program can be obtained from: Dr. George Sonneborn; sonneborn@stars.gsfc.nasa.gov

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