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Dan Johnston
Trajectory and Aerobraking Design Analyst
Global Surveyor Operations Project, Navigation Team
Jet Propulsion Laboratory, Pasadena, California

Mars Global Surveyor

The Mars Global Surveyor (MGS) Project began just over 27 months ago. This project has the responsibility to design, assemble, test and launch the MGS spacecraft. This spacecraft will perform an extended orbital study of the surface, atmosphere, and gravitational and magnetic fields of Mars.

Who I Am

Since the inception of this project, I have been a member of the MGS Mission and Navigation Design Team. As a member of this team, my primary responsibilities have centered in three major areas: 1) the launch vehicle mission design and planning, 2) the spacecraft trajectory (flight path) development, and 3) the aerobraking operations planning.

The launch vehicle mission design and planning activities involve working with the launch vehicle manufacturer, McDonnell Douglas Aerospace, in order to understand the performance capabilities of our Delta II launch vehicle and the limits that it places on our spacecraft design. It also involves the definition of our launch period (the time when Earth and Mars are in proper alignment for launch) as well as the definition of the near-Earth target conditions the launch vehicle must achieve in order to get (inject) our ' spacecraft onto a trajectory to Mars.

The spacecraft trajectory development involves the determination of the flight path of the spacecraft from injection (essentially separation from the third stage of the launch vehicle) through the establishment of our mapping orbit about Mars. A major element of this work involves the development of the maneuver scheme that will be used during flight and the associated determination of the propulsive capability the spacecraft must be able to deliver to successfully fly a given trajectory. Sufficient propulsive or delta-V (DV) capability must be onboard our spacecraft in order for us to establish the desired mapping orbit at Mars. To ensure that we launch with as much propellant as possible, this work activity is closely coordinated with the spacecraft developer, Lockheed-Martin Astronautics.

Unlike previous planetary missions, our spacecraft will be launched with an overall DV capability insufficient to establish our mapping orbit by normal propulsive means. To overcome this propulsive deficit, the MGS spacecraft will aerobrake after arrival at Mars. Aerobraking is accomplished by lowering the periapsis (closest approach) of the orbit into the Martian atmosphere and allowing drag forces to reduce the orbit energy. Once the orbit energy is sufficiently reduced we will use our limited propulsive capability to establish our final mapping orbit. Aerobraking can be described as a "controlled crash" of the spacecraft. One of my major responsibilities has been the task of integrating the operational plans and flight techniques the project will use during the aerobraking operations.

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