As a very favourable launch date was achieved and the launcher injection error

This paper gives a survey of the Mars Express orbit design, as it evolved from before launch  to arrival at Mars. The orbit selection for Mars Express has major scientific impacts, as the orbit is elliptical, with continuous changes of the latitude and local hour of the pericentre. A first target orbit was defined in 1999 by combining scientific and engineering constraints.

As a very favourable launch date was achieved and the launcher injection error was ‘better than nominal’ (the error removed part of the planned re-targeting), propellant reserves turned out to be available after launch. Part of this was used to improve the target orbit. The plane turn manoeuvre was increased to start with the pericentre near the equator rather… than at 20°S latitude, with the objective of increasing the early observation phase with good day-side viewing conditions.

A few weeks after launch, an anomaly in the spacecraft power system was detected, with a loss of up to 40% in terms of available power. This led to a series of studies on back-up orbits, to the extreme of Sun-synchronous eclipse free orbits. Luckily the problem could be solved at the spacecraft engineering level and the orbit with the best science return could be reached in early February 2004.

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