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MARS OBSERVER MISSION STATUS REPORT 8/26/93

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. 

                  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.

                              #####

SPACELINK NOTE: The following is an exerpt from JPL's UNIVERSE publication
8/27/93.



Mars Observer falls silent

By Diane Ainsworth
     Fighting utter frustration and near exhaustion from
round-the-clock efforts to contact the Mars Observer spacecraft,
flight controllers at JPL waited pensively last Wednesday for
some faint signal that would tell them what had become of their
wayfaring spacecraft.
     The radio signal they expected to receive at 2:56 p.m.
Pacific Daylight Time on Aug. 25 never arrived. That was the last
time controllers could predict with any certainty that onboard
flight software would have responded if all was well. 
     "We will need to reestablish communication with the
spacecraft to proceed with the rest of the mission," Glenn
Cunningham, Mars Observer project manager, told a sullen audience
of news reporters and project staff.  "That's the bottom line."
     The Mars Observer spacecraft was within three days of
entering orbit around Mars--the first U.S. mission to the Red
Planet in 17 years --when telemetry from the spacecraft abruptly
halted.
     "At about 5:21 p.m. on Saturday (Aug. 21), we turned off the
transmitter," said Dr. Sam Dallas, mission manager. "At that
point in time, we lost all communication with the spacecraft, in
terms of downlink capability."
     Loss of the spacecraft's downlink on Aug. 21 coincided with
the beginning of a preprogrammed orbit insertion sequence. The
command sequence instructed the spacecraft to first pressurize
its two fuel tanks in preparation for a 29-minute burn that would
have slowed and redirected the craft for capture in orbit above
the north pole.
     But with nothing more than conjecture to go on, no one knew
whether the spacecraft had slammed on its brakes to enter orbit
or whether it went sailing by the planet and into an
interplanetary orbit around the sun.
     At first, the Mars Observer team declined to speculate on
what might have happened to their state-of-the-art spacecraft
other than to assume it had successfully entered orbit around
Mars.  With each passing day, though, the chances of a successful
mission diminished. 
     "Each passing day erodes our prospects for the success of
this mission," Cunningham conceded on Wednesday.
     Mars Observer had received the orbit insertion upload on
Friday, Aug. 20, a day before the signal was lost, and was set to
begin the most critical burn of the entire mission at 1:24 p.m.
on Aug. 24. Rendered incommunicado by the loss of downlink data,
flight controllers groped in the dark to find possible
explanations for the communication failure.
     They predicted that the craft might call Earth about an hour
after the insertion burn had been completed and the transmitter
had been turned back on. A one-way light time of 19 minutes from
the spacecraft to Earth would mean that the deep space tracking
station at Goldstone, Calif., could expect to detect the signal
at approximately 2:40 p.m. PDT.
     During a special NASA Select broadcast on orbit insertion
day, cameras at JPL cut to the workstation where the signal would
first be detected. The tracking "ace" scanned thousands of
frequency bands, but found no sign of a radio signal.
     The next possibility of hearing back from the spacecraft
came at 2:56 p.m. on Wednesday, based on the assumption that an
onboard program, called a "command loss timer," would
automatically clock out and tell the spacecraft to try to locate
Earth after 120 hours of silence. If the spacecraft had sensed
that it had not received any commands since the loss of
communication on Saturday, and if the clock was operating and had
ticked off 120 hours since then, the spacecraft would wait for 24
hours for a command from Earth.
     But if the spacecraft was still unable to receive a command
from Earth after 24 hours, it has been programmed to switch
autonomously to a contingency mode, a benign, self-protective
mode that would point the low-gain antenna at Earth, said Dennis
Potts, deputy flight engineering office manager. With a direct
line to its low-gain antenna, ground-controllers could then
detect its radio signal. 
     Although the signal had not been detected on Wednesday
afternoon when Universe went to press, the spacecraft's
anticipated attempt 24 hours later to reconfigure itself and
point the low-gain antenna at Earth held out another glimmer of
hope on Thursday.
     The mission team has considered a variety of possible causes
for the communication failure. Explanations included redundant
software units on board the spacecraft that may have been
short-circuited when the tanks were pressurized. Other
possibilities were that the transmitters had failed, that the
spacecraft was unable to point its antennas at Earth, or that the
spacecraft's master clock, which synchronizes communication among
the various subsystems, had shut off.
     "There's only a one-tenth of 1 percent chance that the tanks
over-pressurized," Cunningham said, dismissing the possibility
that the spacecraft suffered a catastrophic explosion.
     Without hard evidence to support any of these possibilities,
however, Cunningham could comment only that what happened on
board Mars Observer was "the $64,000 question."
     "I must stress that we are not going to give up," he said.
"We will continue to explore all of the possible scenarios and
attempt new command sequences to prompt the spacecraft to
respond."
     Two partial recovery strategies were under consideration by
the science team. One involved reloading the orbit insertion
sequence within 24 hours of the Aug. 24 orbit insertion event to
try to capture the spacecraft 36 hours later. That option, which
would place the spacecraft in a 40-day orbit around Mars, would
require burning all of the onboard fuel and depend on restoring
two-way communication.
     The second possibility, suggested by Project Scientist Dr.
Arden Albee, would assume that the spacecraft had flown by Mars
and was in an interplanetary orbit around the sun. If that was
the case, Mars Observer would loop around the sun and return to
the vicinity of Mars in about two years, at which time another
attempt to capture it in orbit could be performed.
     Loss of the $900 million Mars Observer mission would impact
the Russian '94 mission to some degree, Albee said during
Tuesday's press conference. The Observer carries a radio system
supplied by the French Centre National d'Etudes Spatiales (CNES)
that would periodically receive and relay data from small
instrument packages landed on the Martian surface by the Russian
mission. ###

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