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Mars Global Surveyor Update - September 21, 1997

Mars Global Surveyor
Flight Status Report 
Sunday, 21 September 1997

     Surveyor's third aerobraking pass occurred early this morning at 
3:07 PDT. For a time period of about five minutes centered around the low 
point and start of the sixth orbit, the spacecraft flew through the upper 
Martian atmosphere at an altitude of 75.4 miles (121.4 km). 

     According to navigator Dan Johnston, today's pass through the 
atmosphere slowed Surveyor by 1.45 m.p.h. (0.65 meters per second). This 
reduction in velocity lowered the altitude of the orbit's high point by 
161.5 miles (260 km) and resulted in a new high point of  33,302 miles 
(53,595 km). The slow down also caused the orbit period to shrink by 17.5 

     As with the previous aerobrake pass two days ago, the thickness of 
the atmosphere on today's pass exceeded the expected value as predicted 
by current models. Consequently, the flight team has decided to fly 
through the atmosphere at the same altitude on the next pass which will 
occur late Monday night. 

     This additional pass at the same altitude will allow the navigation 
team to gather more data before deciding to drop lower into the 
atmosphere on subsequent passes. Eventually, the low point of the orbit 
will be dropped to an altitude where the atmospheric thickness will cause 
an average slow down of about 11 m.p.h. (5 meters per second) per orbit.

     After a mission elapsed time of 318 days from launch, Surveyor is 
162.66 million miles (261.77 million kilometers) from the Earth and in an 
orbit around Mars with a period of 44.36 hours. The spacecraft is 
currently executing the P6 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