William H. Willcockson

Mars Global Surveyor: Aerobraking Mission Overview

The Mars Global Surveyor spacecraft was launched on Nov. 6, 1996, and was captured into a highly elliptical, 45-h orbit aroundMars with a 973 m/s propulsive maneuver on Sept. 12, 1997. A four-month aerobraking phase was supposed to remove another 1200 m/s to circularize the orbit. Unfortunately, one of the two solar wings was damaged during deployment just after launch when the deployment damper failed. What has happened so far to achieve the original mission objectives is described, and the plans for the future of the Mars Global Surveyor Spacecraft are discussed.

Stardust Sample Return Capsule Design Experience

The Stardust Mission has as its primary objective the acquisition and return of cometary samples, which represents the e rst sample return from outside of the Earth ‐moon system. The passage from deep space to the surface of the Earth is accomplished via a sample return capsule (SRC). The design of the capsule required a cooperative effort between Lockheed Martin, the prime contractor, and multiple NASA e eld centers. The coordinated design process that was undertaken to produce the Stardust SRC, in particular there-entry-related systems, is described.

Aerobraking at Mars: The MGS Mission

The Mars Global Surveyor (MGS) mission, scheduled for launch in November 1996, will employ aerobraking as a means for reaching a low altitude sun-synchronous mapping orbit. This technique is necessary to minimize the cost of the mission. The dry mass of the spacecraft (about 660 kg) and the capability of the Delta II launch vehicle (about 1060 kg to Mars) do not permit an all-propulsive transfer to the desired orbit. Aerobraking in the MGS mission presents several challenges not faced by the Magellan aerobraking mission. The requirement to reach a 2 pm node sun-synchronous mapping orbit puts a limit on the time available for aerobraking and drives the spacecraft thermal design. The atmospheric density encountered from orbit to orbit is expected to vary more than was experienced at Venus due to several factors: (1) the changing topography below the periapsis pass due to the planet rotation, (2) the atmospheric changes due to winds and dust storms, and (3) the irregular Mars gravity field. In addition, the Mars atmosphere models are less accurate than the Venus models were at the time of Magellan aerobraking. As a result, the spacecraft and mission design accommodates a 90-percent orbit-to-orbit atmospheric density change. To be consistent with a low-cost flight team, operations during the aerobraking mission phase will be planned to be as routine and repetitive as is appropriate for the safe operation of the spacecraft. (Author)