Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                            
January 6, 1992
(Phone:  202/453-1549)
 
Jim Doyle
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)

RELEASE:  92-1

MAGELLAN TRANSMITTER COMPONENT FAILS

   After completing 15 months and 3,880 orbits of successfully 
mapping over 95 percent of the surface of Venus, the Magellan 
spacecraft appears to have suffered a component failure in one 
of its two downlink transmitters.  This problem has temporarily 
halted the reception of mapping data.

   This loss of mapping data occurred at 7:39 a.m. PST on 
Saturday, Jan. 4 following a routine star calibration.  
Engineering data is still being received, however, and the 
spacecraft is in good health. 

   Spacecraft controllers switched to the back-up transmitter 
turned off in March 1991, because of degraded performance.  The 
backup transmitter was operated for 25 minutes and returned 
good mapping data.  As is normal, the transmitter temperatures 
rose.  At higher temperatures, the backup transmitter was 
previously responsible for degraded performance.  Near the end 
of the test period, signal strength from the back up 
transmitter showed some indication of decreasing, a symptom 
also seen before when the unit was at higher temperature.

   The backup transmitter was turned off and the prime 
transmitter was turned back on.  Controllers and project 
officials were meeting Monday to work out a strategy for use of 
the backup transmitter for mapping data. 

   At launch, the primary mission objective was to map 70 
percent of the planet's surface.  The Magellan spacecraft 
completed its prime radar mapping mission on March 15, 1991.  
On Jan. 15, 1992, the spacecraft will have completed its second 
mapping cycle.  Magellan has mapped to date more than 95 
percent of the planet with high-resolution radar.  The 
transmitter problem does not endanger the gravity mapping 
objective planned for later this year.
                            - end -