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Orbiter Mission Events

The MSP 98 Orbiter launches aboard a Delta 7425 in December 1998, and arrives at Mars in September 1999. Burnout of the 3rd stage is followed by yo- yo despin of the entire stack, followed by spacecraft separation. At this point both the spacecraft and upper stage have been injected onto a Type 2 trajectory whose aimpoint is biased away from the nominal Mars Orbit Insertion (MOI) aimpoint, to assure that the upper stage has less than a 1E- 4 probabilty of impacting Mars, as required by Planetary Protection regulations.

After separation, the solar panels are deployed and pointed to the sun, and initial acquisition achieved by the Deep Space Network (DSN). During inner cruise, the solar panel is sun pointed, and contact maintained via the Medium Gain Antenna. Approximately 15 days after launch, the largest Trajectory Correction Maneuver [TCM- 1] is executed. This maneuver removes launch vehicle injection errors and the spacecraft's injection aimpoint bias. Provisions have been made to execute up to 3 additional small TCM's during the remainder of cruise, as needed, to shape the orbit and direct the spacecraft to the proper aimpoint for MOI. All TCM's are performed with the monopropellant hydrazine thrusters. As the heliocentric distance increases during cruise, communications moves to the High Gain Antenna.

At Mars arrival, the Orbiter bipropellant engines are used to propulsively insert the spacecraft into an elliptical capture orbit. The biprop engines burn for approximately 16 minutes, until all the loaded oxidizer is exhausted. One minute later, an additional maneuver is executed by the Hydrazine thrusters, if needed, to reduce the orbit period further. Depending on launch date and propellant mass consumed during cruise, the resultant orbit period lies between 19 and 40 hrs, with a nominal periapse altitude of 160 km. A maneuver to lower periapse in preparation for aerobraking occurs at the first apoapse of the final capture orbit. Over the next two months, the energy of the orbit is reduced via successive passes through the atmosphere of Mars, controlled by small Orbit Trim Maneuvers near apoapse. At aerobrake termination, two maneuvers transfer the Orbiter to its final, frozen, near sun- synchronous mapping orbit, at a descending node of approximately 4 PM. This occurs some time prior to the MSP98 Lander arrival in December, 1999.

During the Lander's surface lifetime, the Orbiter provides command and data relay support, and also engages in a limited amount of orbital science. In its mapping phase, the Orbiter performs systematic daily global sounding and imaging of the Mars atmosphere for approximately one Mars year (687 days). The nadir- mounted science payload consists of a rebuilt Pressure Modulator Infrared Radiometer (PMIRR), and the Mars Color Imager (MARCI). Once its mapping mission is complete, the Orbiter will be available as a communication relay for future Mars landers for up to 3 additional years. Upon completion of its relay mission, the Orbiter may perform a maneuver or be placed in a low- drag attitude to satisfy Planetary Protection regulations.