THE MARS EXPRESS ORBIT
After the successful and very nominal launch of Mars Express, it was found that the propellant budget would allow to improve the operational orbit. Using the flexibility of about 115 m/s delta V margin, and in order to optimise the scientific coverage at the beginning of the mission, the Mars Express Science Working Team has agreed to slightly modify the baseline orbit and selected the “G3-ub-eq100” orbit, a variation of the G3-ub, for which the subspacecraft point at pericentre starts at the Mars equator at a Sun elevation of 60 degrees. The originally selected orbit G3-ub would start with the pericentre near –20 degrees South. By changing the initial conditions to start with the pericentre closer to the equator, the time before the terminator will be crossed at pericentre, and also the time until which the pericentre comes close to the South pole can be considerably extended.
The Mars Express orbiter was successfully inserted into orbit around Mars on 25 December 2003. Several manoeuvres were then performed using again the spacecraft main engine (plane turn manoeuvre and apocentre reductions) and several other manoeuvres performed (using thrusters) until reaching the eq100 mapping orbit on 28 January 2004. All manoeuvres leading to that orbit were executed with very high accuracy. The spacecraft main engine will not be used anymore and was isolated on 18 January 2004. The orbit of the Mars Express orbiter is maintained very accurately.
At the beginning of the mission, the pericentre moves southward with a shift of 0.54 degree per day. At the same time the pericentre steps towards the terminator which will be reached after ~4 months, giving the optical instruments optimal observing conditions during this initial period. Throughout this initial phase lasting until mid-May 2004, the downlink rate is rather high - although it will decrease from 114 kbit/s to 38 kbit/s. An orbit change manoeuvre was planned at around day 100 of the nominal mission (06 May 2004) to increase the pericentre latitude motion and to guarantee the 50/50 balance between day- and night-side operations. With this manoeuvre, the apocentre altitude would be lowered from 14887 km to 13448 km, the orbital period lowered from ~7.6 hours to 6.645 hours, and the pericentre latitude drift slightly increased to 0.64 degree per day.
After 150 days, at the beginning of June 2004, the South pole region will be reached with the pericentre already “behind” the terminator. This region is of particular interest for the radar instrument MARSIS, optimised for night-time measurements. In the following part of the nominal mission, the pericentre moves northward with the Sun elevation increasing. Thus, the optical instruments can cover the Northern Mars hemisphere under good illumination conditions from mid-September 2004 to March 2005.
During the next mission phase, lasting until July 2005, the pericentre is again in the dark. It covers the North polar region and moves southward, offering a window of observation of the corresponding regions for MARSIS. Finally, throughout the last 4 months of the nominal mission, the pericentre is back to daylight and moves from the equator to the South pole, and the downlink rate reaches its highest, allowing for downlink volumes up to 6 Gbits per day.
The osculating orbit elements for the eq100 orbit are listed below:
|Epoch||2004:1:13 - 15:56:0.096|
|Pericentre (rel. sphere of 3397.2 km)||279.29 km|
|Apocentre (rel. sphere)||11634.48 km|
|Semimajor axis||9354.09 km|
|Right ascension of ascending node||228.774 ±|
|Argument of pericentre||357.981±|
|True anomaly||-0.001 ±|
Due to main engine constraints implemented following Mars orbit insertion, the epoch of this orbit has moved to 2004:1:28. The Figure below shows the resulting motion of the pericentre over the latitude on Mars versus the illumination properties at the sub-satellite point below pericentre. The drawback of the selection of the eq100 variation of the G3-ub orbit is that eclipses are longer at the beginning of the mission. However, the starting conditions are critical for the latitude coverage with high resolution for « daylight » instruments.
The G3u to G3b orbit change maneuver, performed in order to maintain the planned latitudinal and seasonal coverage, was executed successfully on the planned date (6 May 2004). Mars Express is now in its final orbit.
Top: Sun elevation at pericentre. Bottom: pericentre longitude from Sun