As this particular Phobos flyby season comes to an end, I caught up with Michel Denis, spacecraft operations manager; Nicolas Altobelli, scientist with the science ground segment; and Olivier Witasse, project scientist, to ask how it all went and what we can look forward to in the future. -- Stuart

This has been the most ambitious Phobos flyby campaign yet attempted, how did it go?

Michel Denis: “After almost 8000 orbits which have all been a bit different from each other, and after about 10 Phobos campaigns, we had mixed feelings between the old-sailor's confidence "it will be all-right, we have mastered much more complex activities already" and the juvenile excitement of the beginners "it's so close, and we must turn the spacecraft swiftly, and what happens if...?"

Both were true. After all it was just another couple of observations, well prepared like all the others thanks to well known processes, proven tools and above all the expertise of the people. Still, a few things were special in the way Mars Express was used, and a few minor events happened that forced a few individuals to react within a few hours, in order to recover a plan that had been agreed for months by a wide community. In such cases, it's good to be as flexible as the youngsters and as experienced as the old sailors.

The most well known example is that the orbit manoeuvre required to prepare the Phobos encounter, slightly over-performed by a fraction of a percent, which is completely normal and not an issue under other circumstances. But here, due to bad luck, this small overshoot slightly increased the orbital period. It would have meant Mars Express arriving a few seconds later at the closest approach on March 3rd.  Again, this would not be a big deal in principle but, in this specific planetary configuration, Phobos would have hidden the Earth and made most of the long-planned radio-science observation inoperable. To avoid this undesired alignment, in just a few hours the Flight Dynamics experts invented a clever strategy, and minor additional orbital offsets were commanded to the spacecraft within the few days remaining before closest approach. In the control room, during the closest flyby, we were very confident and slightly anxious at the same time. As predicted, thanks to the small correction no occultation occurred and the radio-science experiment was a complete success.

A less obvious but also important concern was raised a few weeks later by a ground station failure. This is an also rare but possible anomaly, and it happened just at the time when Mars Express was sending the data from one of the Phobos pictures taken by HRSC to Earth. Such data losses are covered by our procedures and we have memory space onboard to temporarily store missed data and resend it to Earth later. In this case though, the ground anomaly lasted for more than an hour and affected the HRSC data return containing the Phobos picture, which would soon be overwritten by other observations. The special extra space onboard was too small to secure the Phobos picture. So we had to copy the HRSC data to the large storage area normally reserved for another instrument.  That instrument was also in use but producing less data. Then we had to bring all the data back to Earth without affecting the data from the other instrument. Mission accomplished.”

Nicolas Altobelli: This season has been indeed quite intensive in terms or preparation, in particular because we had a couple of the closest flybys ever attempted. The timing and geometry of the observations therefore had to be planned with a precision not usually required for 'routine' observations of the martian surface.

This demanded a perfect flow of information between the instrument teams, external to ESA, the science planning centre at ESAC in Spain, and the spacecraft flight control team at ESOC, Germany.

Fortunately, things went quite well overall. In particular, the flyby with closest approach provided unique data to the radio science gravity experiment. The HRSC camera obtained high-resolution pictures and the MARSIS instrument recorded radar echoes of Phobos. After analysis these combined data will hopefully provide an incremental step in constraining Phobos’ physical properties and possibly give hints as to its origin."

Did you have to use Mars Express in an unusual way to make this happen?

Michel Denis: "Mars Express is permanently used in special ways, because the mission profile evolves due to changing environmental factors, like the distances to the Sun and Earth, the eclipses, the seasons and even local time on Mars. The spacecraft has to cope with high and diverse mission ambitions. It has to be able to take 3D pictures, make atmospheric studies, perform mineralogy and radio experiments, both in routine coverage and targeted campaigns. It must deal with constraints that the planners have to respect all the time, such as the fact that all the instruments are fixed, power is limited, data stores are limited, and so on.

For this particular Phobos campaign we simply used several of the special but already validated configurations. A few elements were new though: the Phobos tracking slews for optical observation at short distances, when the spacecraft was turned at a rate twice as fast as the usual speed limit. In a car you can do this only if you are on a special circuit and with the proper authorisations, and this is exactly what we had. Also unique was the combination of the Phobos campaign with a series of major orbital change manoeuvres, which allowed Mars Express to go very close to Phobos, while shifting it to an orbit more favourable to the optical instruments for the next few years.

Even more challenging than these settings that we had long known about and prepared for, were the situations that the events asked us to handle. Here we had again this wonderful ‘Mars Express feeling’ that we have experienced so many times since the launch in 2003.”

When can we expect to hear the science results?

Olivier Witasse: "We have already published he first images, data from the gravity experiment and echoes from the radar, all on the web portal of the European Space Agency and on this blog. All the teams are very busy doing the data analysis. Results will be discussed amongst the team members, and at a Science Working Team meeting to be held in June, between all the Mars Express experiment teams. This is going to be exciting. Then, it is planned to share the results and their interpretations with the scientific community at two major events: the European Planetary Science Congress (Rome, September) and a workshop at IKI, Moscow, in October, on the Phobos-Grunt mission. I expect some articles to be published later in the year. The papers always take time."

When will Mars Express next encounter Phobos?

Nicolas Altobelli: "Mars Express' highly eccentric orbit crosses Phobos’ orbit periodically as a consequence of the natural drift of the pericentre latitude. This is induced by the non spherical martian gravity field. However, the spacecraft position on its orbit must be fine-tuned in order to 'catch' Phobos at the right time. Close encounters below 200 km distance will be possible again in early 2011."

What will Mars Express do in the meantime?

Nicolas Altobelli: "Mars Express will go back to its very busy scientific observation schedule, primarily dedicated to the observation of Mars. As usual, the Science Ground Segment at ESAC will aim at building observation plans, in collaboration with the science teams and the project scientist, in order to collect data sets on the atmosphere, surface and sub-surface of the red planet."

Olivier Witasse: "We have some clear scientific objectives planned until end of 2012, and we’ll make sure that we achieve them."