MARS OBSERVER MISSION STATUS

[David Farber <farber () central cis upenn edu>]

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109.  (818) 354-5011

                  MARS OBSERVER MISSION STATUS
                         August 26, 1993
                 2:30 p.m. Pacific Daylight Time

     Communications with the Mars Observer spacecraft have not yet
been restored, one and a half days past its planned insertion into
orbit around Mars.

     Mission controllers at JPL continued through the night and
morning with efforts to re-establish the necessary radio link with
the spacecraft by cycling the various elements of the
communications system.

     At 2:37 p.m. Pacific Daylight Time today, the continued
execution of the command loss timer subroutine will try to position
the spacecraft for optimum pointing and will switch antennas to try
to restore communications.

     Project officials still do not believe that the propulsion
tanks leaked or exploded at the time of their pressurization. The
pressure in the tanks at launch was 285 pounds per square inch
absolute (psia).  As propellants were used during the three
trajectory correction maneuvers, the pressure was reduced to about
167 psia. Pressurizing the tanks would have raised the pressure to
264 psia. Any greater pressure would require a failure of both of
the in-series pressure regulators. The burst pressure specification
of the tanks is 465 psia and actual test data ruptured tanks at 678
psia.  Flow restrictions limit the rate at which the pressure can
increase.  Analysis indicates that the probability that the
pressure in the tanks would increase to the burst level within the
9 minutes that the radio transmitter was off is less than 0.1%. 

     Project officials are systematically evaluating the most
probable sources of the cause of the spacecraft's failure to
communicate.

     One such source which has been receiving considerable
attention is the potential failure of the spacecraft's central
clock, whose official name is the "redundant crystal oscillator,"
or RXO for short.  Proper operation of this device is required for
operation of the spacecraft's central computers, which sequence the
events on the spacecraft.  

     The first hypothesis for the lack of communications pointed 
to the failure of the central computer to turn the transmitter back
on.  Failure of the central clock would prevent the central
computer from doing its job.  After sending commands to turn on the
transmitter, switching to the backup clock was the next action
taken by mission controllers.

     The central clock has been the focus of investigation because
it contains transistors which have failed in other spacecraft
applications using this type of clock.

     The launch of the NOAA-I spacecraft was delayed at the end of
June 1993 when it was discovered that its RXO had failed.  A
subsequent investigation revealed that the RXO failure was caused
by the failure of a 2N3421 transistor.  Two of these transistors
are used in each of the redundant halves of the RXO.  Transistors
from the same manufacturing lot as those in the NOAA-I RXO are
installed in the Mars Observer RXO, making the reliability of Mars
Observer's RXO suspect.

     The transistors fail when a weld between a gold-plated post
and an aluminum wire breaks.

     This potential problem was discovered when Mars Observer was
only 55 days away from Mars after the spacecraft had been in flight
for over nine months.  Because of the way that these transistors
are used in the RXO, Mars Observer would be susceptible to losing
its central clock function if one particular transistor in each
half of the RXO failed.

     There is no alternative source of the central clock function
in Mars Observer, and should the loss of this function occur, it
would be a non-recoverable situation.

     The RXO, on its primary side, was working perfectly
immediately before the pressurization activity.  The last time the
backup side of the RXO was tested was in a launch GO/NO-GO test on
launch day, when it was also found to be working perfectly.

     Project officials were not, at first, concerned about the
NOAA-I RXO failure because it would take a failure of two of these 
transistors to cause the loss of the central clock function.  The
spacecraft is not designed to automatically protect itself against
more than a single failure in any piece of hardware.

     The restoring of the spacecraft's transmitter and the
spacecraft's failure to act on ground commands could be tied to the
loss of the central clock function.  Project officials now surmise
that one explanation for the loss of communications could result
from the failure of the crucial transistor in each half of the RXO,
or its failure to autonomously switch to the backup side.  Then
there would be no central timing function.  This failure could have
been induced by the shock of the pressurant valves operating during
the propulsion tank pressurization event on Aug. 21, after which
communications were not restored.

[Ron Baalke, Jet Propulsion Lab, M/S 525-3684 Telos
Pasadena, CA 91109 baalke () kelvin jpl nasa gov]