Mars Pathfinder Trajectory

In this graphic you can see the Mars Pathfinder trajectory from its beginning on Earth, on December 2, 1996, to its goal in Mars, on July 4, 1997.
Note how the Earth (blue dot) overtakes Mars (red dot) during this 7-month period, both moving in a counter-clockwise direction, in this diagram.

What is Navigation?

The main responsibility of the Navigation Team is to maintain the spacecraft on the planned trajectory for the duration of the mission. In the development phase, the Team helped to design the interplanetary trajectory that achieves the mission goals within various constraints such as propellant usage and planetary protection requirements. The Team provides the project with predictions of this trajectory for the spacecraft and with orbit data for the planets and Martian satellites. In flight, the Team is providing best estimates for the actual past trajectory of the spacecraft along with the predictions for the future trajectory. Based on these solutions, they plan and generate the trajectory correction maneuvers (TCMs) required to maintain the spacecraft on the desired trajectory. For Mars Pathfinder, the Navigation Team will also provide information used to execute a successful descent through the Martian atmosphere and landing on the surface.

Maneuver Design

Despite our best efforts, the spacecraft will not follow its planned course exactly. Small deviations in its flight path from the desired one can grow into large errors at Mars arrival. Also, constraints imposed on the mission prevent us from following the desired path directly from the beginning of the mission. For these reasons, the spacecraft will occasionally be commanded to fire its thrusters to change its velocity at certain points during the cruise to Mars. These thruster firings, or burns, are called trajectory correction maneuvers or TCMs. The velocity changes caused by the thruster firings will alter the spacecraft's future trajectory so that it returns to the desired path and arrives at Mars with the proper geometry for atmospheric entry.

A total of four TCMs are planned for Mars Pathfinder. The first two of these were scheduled in the first two months of the mission while the spacecraft was still relatively close to Earth. The final two TCMs were scheduled near the end of the cruise phase when the spacecraft is close to Mars. Contingency plans allow for a fifth maneuver to be executed just a few hours before atmospheric entry, if necessary.

TCMs 1 and 2

Mars Pathfinder must satisfy two NASA planetary protection requirements designed to minimize potential contamination of the Martian environment.
The first requirement is that the probability of the unsterilized launch vehicle upper stage impacting the surface of Mars be less than 0.0001 (or 1/10,000). This launch vehicle upper stage could be carried to Mars with Pathfinder if there were some failure in the separation procedure after the final burn that places the spacecraft on target to Mars.
For this reason, the targeted state to be achieved by the launch vehicle, called the injection target, is biased away from Mars - just enough so that the impact requirement is satisfied. This means that the launch vehicle will not place Pathfinder on course for its final desired arrival state at Mars. It's up to the spacecraft itself to move toward this final aim point during its 7-month cruise. TCM 1 is the first step towards achieving the desired entry conditions at Mars.

The second planetary protection requirement imposed on Mars Pathfinder is that the probability of Pathfinder itself impacting Mars at a speed greater than 1,000 ft/sec (1,100 Km/h) be less than 0.001 (or 1/1,000). This requirement is met by designing TCMs 1 and 2 to a target state that is also biased away from the final desired arrival conditions - again, just enough to meet the requirement. The target state is chosen so that if control of the spacecraft is lost following either TCM 1 or 2, the spacecraft will enter the Martian atmosphere with a shallow entry angle, allowing the atmosphere to slow it to below 1,000 ft/sec at impact (assuming the parachute does not deploy).

TCM 1 is designed to move from the biased launch injection target state to the (still) biased Mars arrival state. The change required to move between these two targets is known, so that the nominal size of TCM 1 can be computed exactly. This type of maneuver is called "deterministic". However, there are several types of uncertainties associated with the design and implementation of maneuvers in flight. A second maneuver - TCM 2 - is planned to follow TCM 1 to correct any errors that occur in the design and/or execution of TCM 1. If we knew the spacecraft trajectory exactly and the propulsion system could execute the maneuver perfectly, there would be no need for TCM 2. This type of maneuver is called "statistical" since its characteristics can only be predicted by statistical analyses of the various error sources.

TCM 1 was successfully executed on January 9, 1997.

TCM 2 was successfully performed on February 3, 1997.

TCM 3 was performed on May 7, 1997.
Unfortunately, luck was not with us and the small execution errors accumulated while performing the maneuver have conspired to place the spacecraft on a trajectory that is just outside our desired entry conditions - if uncorrected during TCM 4.

TCM 4 was successfully performed on June 25, 1997.

TCM 5:
A fifth maneuver may be performed just before Pathfinder hits the upper atmosphere of Mars at 10 A.M. PDT (5 P.M. UTC) on July 4. If it were needed to fine-tune the spacecraft's 14.2-degree entry angle, it would be performed either 12 hours or six hours before Pathfinder hits the upper atmosphere. The Flight Team will decide whether to follow through with that maneuver the evening before landing.

Updated: July 1 '97

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