Of course you don’t, but this latest post will give you a great ‘behind the scenes’ read about the Mars Express mission. In particular, it highlights the role played by the Science Ground Segment at The European Space Astronomy Centre (ESAC) during the Phobos flyby season. It is their job to make sure that the observations requested by the scientists are possible and then to ensure that they are performed. My thanks go to Nicolas Altobelli and Alejandro Cardesin for taking the time to put this together. -- Stuart

Generic role of Science Ground Segment and the Science Operations Centre:

The Science Operations Centre (SOC) is part of the Science Ground Segment (SGS) of the Mars Express mission and is located near Madrid, in Villanueva de la Canada. It provides scientific and technical co-ordination between the instrument teams and the Mission Operation Centre (MOC) located at the European Space Operations Centre (ESOC) in Germany. The SOC is composed of a core team of four scientists and engineers working with the support of a broad group of experts on software, technical and various scientific topics, all of them within the Solar System Science Operations Division.

The main objective of the whole SGS is to co-ordinate the scientific observation requests from the Mars Express instrument teams and to build the final observation plan that fits in terms of available spacecraft resources (mainly battery power, data rate and downlink capacity). Among other tasks, SGS is responsible for defining the final pointing of the spacecraft and planning the operation of the payloads. These planning activities are performed three months in advance to allow the resolution of scientific and technical conflicts. Should two instruments wish to observe in two different directions, obviously they cannot be operated at the same time, and one has to defer to the other.

The SGS also helps teams convert their high-level scientific requirements into geometrical constraints and compute the required spacecraft attitude and associated timings. This latter activity is called Opportunity Analysis and answers the question: When is What feasible and How?

Diagrams like this show the Mars Express team exactly what the spacecraft is doing. 

In a sense, if the MOC can be considered as the skipper of a boat, who pays attention to the 'steering' (spacecraft trajectory control) and the general boat maintenance (fuel available, general health of components...), the SGS can be seen as the navigator and sailor, indicating the directions to follow (spacecraft attitude), looking at all the weather and environmental conditions (planet and spacecraft ephemeris, solar flux, martian season, etc) and carefully taking care of the needs of the passengers (scientific teams) and relying on the rest of the crew (technical experts). As it is not always possible to satisfy all the passengers at the same time, the SGS will receive instructions from the Project Scientist to prioritise the requests. 

Phobos specials

The Mars Express spacecraft is on a highly inclined and eccentric orbit around

Mars and periodically crosses Phobos’ equatorial and nearly circular orbit. With a slight offset of the spacecraft position on its orbit, performed by the Flight Dynamics Team at ESOC, the spacecraft can make multiple close Phobos encounters. During this period of time, called the 'Phobos Flyby Season', the routine co-ordination task of the SGS becomes more hectic.

Co-ordination meetings and teleconferences are set up between the instrument teams and the SGS to collect special requirements that have to be applied during the flyby measurements. The SGS provides a detailed geometrical description of the events, relevant for any type of experiment performed. These include the minimum approach distance, the solar phase angle value (which determines the illumination conditions), the spacecraft velocity profile, whether Phobos will be in Mars' shadow, whether an occultation of radio links is expected. In addition to the geometrical studies, the SGS performs an estimate of the spacecraft resources that will be needed by the experiments, and judges the overall feasibility of the observation plan before sending it to MOC for implementation.

Because the precise position of the spacecraft on its orbit cannot be known to any high precision (within 1 second) until a few days before the Phobos flybys, the SGS co-ordinates specific observation requests from the High Resolution Stereo Camera (HRSC) directly with Flight Dynamics. For example, when the spacecraft gets very close to Phobos, (say 100 km), the high relative velocity would smear the images and so this must be compensated by spacecraft motion (slews). A complicated slew pattern is computed by the HRSC team and communicated to Flight Dynamics, who will make certain that it is executed in a timely manner by the spacecraft, using the latest orbital determination result. The SGS supervises this information exchange and ensures that the instrument commanding will stay in line with the final spacecraft orientation strategy. 

Nicolas Altobelli, Alejandro Cardesin