Mars Global Surveyor
Flight Status Report
Tuesday, 22 September 1998

     At 11:00 a.m. PDT on Wednesday this week, Surveyor's onboard
computer will execute a maneuver command sequence to initiate
aerobraking. This 12-second firing of the spacecraft's main rocket engine
will take place near the high point of orbit #573. The effect of the burn
will be a drop in the low point of Surveyor's orbit from its current
altitude of 106.5 miles (171.4 km) down into the upper fringes of the
Martian atmosphere at 79 miles (127 km).

     About 7.5 hours after the burn, Surveyor will reach the low point
of its orbit and will encounter the atmosphere for the first time since
March of this year. For the next five months, Surveyor will repeatedly
fly through the upper Martian atmosphere and use air resistance to
gradually shrink the size of the orbit. The goal is to reduce the period
from its current value of 11.6 hours to just under two hours. Global
mapping operations from this two-hour orbit are scheduled to begin in
April of next year.

     Originally, aerobraking was scheduled to resume on Monday,
September 14th. However, two minor delays forced a postponement until
tomorrow. The first of the two delays was called to permit the flight
team to conduct communication tests over the low-gain antenna. On
Wednesday, September 9th, the flight team noted a minor communications
irregularity from the receiver on the low-gain antenna. Because this
antenna serves as the backup to the primary high-gain antenna in the
event of a contingency situation, the flight team prudently delayed the
start of aerobraking in order to conduct these tests.

     After successful tests on the low-gain receiver, aerobraking was
rescheduled to start on Thursday, September 17th. Early that morning, the
onboard computer's fault-protection routines automatically canceled the
command sequence that would have fired the main rocket engine to restart
aerobraking. The cancellation of the sequence occurred because the
computer detected a low charge level in Surveyor's batteries. This
cancellation action was a pre-programmed response designed to keep the
spacecraft safe in case of an unexpected event.

     Further analysis revealed that a minor command sequence processing
error resulted in a missed command to move one of Surveyor's two solar
panels into its proper position. With one of the two panels out of
position, the electrical system was low on solar energy and began to draw
power from the batteries to compensate. At this time, software engineers
have already remedied the software on the ground that caused the missing
command.

     Over last weekend, Surveyor remained in "contingency mode" to allow
the flight team time to recharge the batteries and reset the spacecraft
hardware for the start of aerobraking. In this mode, the spacecraft is
placed in a safe, Sun-pointed orientation favorable from a power and
communications perspective. At 9:00 p.m. on Monday, the flight team
commanded the spacecraft back to its normal operational mode with the
high-gain antenna pointed at the Earth. Later today, command sequences
will be transmitted to the spacecraft to execute the maneuver sequence
that will resume aerobraking tomorrow.

     After a mission elapsed time of 684 days from launch, Surveyor is
213.16 million miles (343.04 million kilometers) from the Earth and in an
orbit around Mars with a high point of  11,083 miles (17,836 km), a low
point of 106.5 miles (171.4 km), and a period of 11.6 hours. All systems
continue to perform as expected.

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Status report prepared by:

Office of the Flight Operations Manager
Mars Surveyor Operations Project
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
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