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Mars Global Surveyor Update - September 17, 1997

Mars Global Surveyor
Flight Status Report 
Wednesday, 17 September 1997

     A major milestone in space exploration occurred today as Surveyor 
began the aerobraking phase of its mission. This event began at 9:37 a.m. 
on Wednesday as the spacecraft flew through the upper fringes of the 
Martian atmosphere at the low point and start of its fourth orbit around 
the red planet. At that time, Surveyor was slightly to the northwest of 
the tallest mountain in the solar system, a 89,000-foot (27 km) tall 
volcano named Olympus Mons.

     For the next four months, the spacecraft will skim through the 
upper Martian atmosphere as it passes through the low point of every 
orbit. During these atmospheric passes, the spacecraft will slow slightly 
due to air resistance. This loss of momentum will cause Surveyor to lose 
altitude on its next pass through the orbit's high point. Surveyor will 
use this innovative aerobraking technique to lower the high point of its 
orbit from its current value of 33,555 miles (54,000 km) to near 250 
miles (400 km).

     Today's atmospheric pass occurred at an altitude of 93 miles (150 
km). The spacecraft experienced almost no loss of momentum from air 
resistance because the Martian atmosphere is extremely thin at this 
height.  Tomorrow, the flight team will fire Surveyor's tiny rocket 
thrusters to lower the altitude of the next atmospheric pass to 81 miles 
(130 km). This next pass will occur Friday morning at the low point and 
start of the fifth orbit. At this lower altitude, the atmosphere will be 
slightly thicker.

     The flight team will continue to lower the altitude of the 
spacecraft's atmospheric pass until Surveyor encounters enough atmosphere 
to slow by an appreciable amount on every orbit. According to navigator 
Dan Johnston, this situation will probably occur at an altitude of about 
68 miles (110 km). This gradual "dipping" into the Martian atmosphere is 
necessary because the atmosphere has not yet been fully characterized by 
Surveyor's atmospheric science team.

     In other news, data returned from the Magnetometer science 
instrument has indicated the presence of a magnetic field around Mars. 
Until now, scientists were uncertain as to the absence or presence of a 
Martian magnetic field. Please visit the following web sites to view a 
press release containing more details about this important discovery. The 
second of the two sites listed here is the home page for the 


     After a mission elapsed time of 314 days from launch, Surveyor is 
160.86 million miles (258.88 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