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Mars Global Surveyor Update - September 19, 1997

Mars Global Surveyor
Flight Status Report 
Friday, 19 September 1997

     This morning at 6:28 PDT, Surveyor reached the low point and start 
of its fifth orbit around Mars. For a time period of 24 seconds centered 
around this point in the orbit, the spacecraft performed its second 
aerobraking pass of the mission by skimming through the upper Martian 
atmosphere at an altitude of  79.5 miles (128 km). During the pass, air 
resistance caused a 10 degree increase in temperature on the solar 
panels. This rise was well within allowable limits.

     According to Surveyor's atmospheric science team, the thickness of 
the atmosphere during today's pass was more than twice the expected value 
as predicted by current models. However, because the air at the current 
aerobraking altitude is extremely thin, this increased thickness posed no 
threat to the spacecraft. 

     Based on today's new data about the thickness of the upper Martian 
atmosphere, the flight team has decided to lower the altitude of next 
aerobraking pass to 75 miles (121 km). In the original plan, the altitude 
of the next pass occurred at 72.7 miles (117 km). This new altitude is 
slightly higher in order to offset the increase in atmospheric thickness 
as compared to the model value. Over the next week, the flight team will 
continue to lower the aerobraking altitude until the spacecraft 
encounters enough air resistance to slow down by an appreciable amount on 
every orbit.

     As of 11:59 p.m. PDT, Surveyor is climbing toward the top of its 
fifth orbit around the red planet. Currently, the spacecraft is at an 
altitude of 32,930 miles (53,000 km) and is moving with a velocity of 805 
m.p.h. (360 meters per second) with respect to the planet. The next 
aerobraking pass through the atmosphere will take place early Sunday 
morning at the low point and start of the sixth orbit.

     After a mission elapsed time of 316 days from launch, Surveyor is 
161.76 million miles (260.33 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 P5 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