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Mars Global Surveyor Update - September 18, 1997

Mars Global Surveyor
Flight Status Report 
Thursday, 18 September 1997

	At 8:03 a.m. PDT this morning, the flight team commanded Surveyor's 
tiny rocket thrusters to fire for 20 seconds. This burn occurred at the 
high point of the spacecraft's fourth orbit around Mars and slowed 
Surveyor by 1.79 m.p.h. (0.799 meters per second).

	The maneuver lowered the low point of Surveyor's orbit from its 
current value of 93 miles (150 km) down to 79.5 miles (128 km). The 
spacecraft is currently falling back toward Mars and will reach this new 
low point Friday morning at 6:29 a.m. PDT. At that time, Surveyor will 
make its second aerobraking pass by skimming through the upper part of 
the Martian atmosphere.

	Surveyor's atmospheric scientists expect the spacecraft to 
encounter slightly more air resistance on Friday than during the first 
atmospheric pass which occurred on Wednesday. The reason is that the 
orbit's low point will lie 13.5 miles (21.7 km) deeper into the Martian 
atmosphere than before. However, the flight team still expects that 
Friday's atmospheric pass will have little effect on lowering the high 
point of the spacecraft's orbit.

	Over the next week, the flight team will continue to lower the low 
point of the orbit deeper into the atmosphere on an orbit by orbit basis. 
In about one week, the altitude of the atmospheric pass will be deep 
enough to slow the spacecraft by an appreciable amount on every orbit. At 
that time, the high point of Surveyor's orbit will begin to shrink by 
noticeable amounts.

	After a mission elapsed time of 315 days from launch, Surveyor is 
161.31 million miles (259.60 million kilometers) from the Earth and in an 
orbit around Mars with a period of just under 45 hours. The spacecraft is 
currently executing the P4 command sequence, and all systems continue to 
be in excellent condition.

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