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Mars Global Surveyor Update - July 25, 1997

PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

            Mars Global Surveyor Mission Status
                   Friday, July 25, 1997

     Mars Global Surveyor is rapidly approaching Mars, now just 
48 days away from entering orbit around this planetary neighbor.  
Today the orbiter is about 11 million kilometers (7 million 
miles) from Mars, traveling at a speed of about 21.7 kilometers 
per second (about 49,000 miles per hour) with respect to the Sun.

     The spacecraft and flight teams recently conducted a live 
simulation of spacecraft telecommunications procedures with one 
of NASA's Deep Space Network tracking stations in Madrid, Spain, 
to prepare for the beginning of mapping operations. 

     During mapping, Surveyor will circle Mars once every two 
hours.  On each orbit, the spacecraft will pass behind Mars and 
will not be able to maintain a communications link with Earth. 
Upon entry and exit into this occultation zone, Surveyor's radio 
signal will pass through the thin Martian atmosphere on its way 
to Earth. An analysis of the variation of the signal's strength 
and frequency caused by the atmosphere as it fades and reappears 
will enable scientists to determine atmospheric properties at 
specific locations on Mars.

     The recent simulation was designed to allow the flight 
and tracking teams to become accustomed to the spacecraft's 
performance as its signal repeatedly passes in and out of the 
Martian atmosphere. During the simulation, the spacecraft turned 
its radio transmitter on and off over the course of a six-hour 
period in order to simulate three orbits worth of communications 
events.  Participants found the simulation beneficial and have 
planned several more of these dry runs over the next two months.

     Mars Global Surveyor will reach the orbit of Mars on Sept. 
11, 1997, then spend six months aerobraking through the thin 
Martian atmosphere to achieve a low-altitude mapping orbit.  
Mapping and science operations will begin on March 15, 1998.

     After 260 days in flight, all systems onboard Mars Global 
Surveyor remain in excellent condition.  The spacecraft is on 
target to intercept Mars at 6:39 p.m. Pacific Daylight Time on 
Sept. 11 (1:39 a.m., Sept. 12 in Universal Time).  

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