Mars Global Surveyor: Aerobraking.

Completing a 10 month journey, another spacecraft from Earth arrives at Mars today . The Mars Global Surveyor (MGS) is scheduled to fire its main rocket engine for 22 minutes at 6:17 p.m. PDT and enter a highly elliptical orbit , with a low point 186 miles and a high point 34,800 miles above the surface of Mars . This robot spacecraft is aptly named. Its mission is to undertake a detailed planetwide survey of Mars . But first MGS must circularize its orbit, lowering the high point to about 250 miles. Instead of relying solely on its rocket engine, MGS mission controllers will use a fuel-saving technique known as aerobraking - dipping the spacecraft into the Martian atmosphere where it will encounter increased atmospheric drag . This early artist's conception emphasizes the drag created by the wing-like solar panels. The cumulative effect should find MGS in a more circular mapping orbit by March 1998 . To successfully use aerobraking, mission controllers must achieve an exact orbit and will be handicapped by a limited knowledge of the thickness of the Martian atmosphere . They may even need to alter the spacecraft's course to compensate for changes in Martian weather .