This lecture write up has kindly been put together by Pat Norris FRAeS, member of the Society's Space Group.
It was standing room only in the Bill Boeing Lecture theatre at 4 Hamilton Place on the evening of 18th March for a talk by Rosetta Flight Director Andrea Accomazzo. Andrea is the man in the European Space Agency’s mission control (Darmstadt, Germany) responsible for the first human probe to rendezvous with a comet. With the aid of stunning graphics he explained how the comet’s peculiar characteristics posed serious challenges for his team.
Figure 1: Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Source can be found here.
The best pre-encounter images of comet 67P/Churyumov-Gerasimenko (67P/CG) had showed just a point of light, but as they got within 1 million km of it in mid-2014 it became clear that it had a highly irregular shape (see figure 1). Its overall volume of about 25 cubic km is that of a sphere of 1.8km radius but in reality it extends more than 6km in length and comprises two almost separate pieces. Andrea’s team spent the first few weeks after the August arrival at the comet mapping its surface in terms of craters, ridges and other physical features, and also in terms of its gravity field. The 10 billion ton mass of 67P/CG is not uniformly distributed like a sphere. In addition the notion of a centre of mass is not particularly useful since there are effectively two centres of mass. The European Space Agency team had to devise a complex mathematical representation of the comet’s gravity field in order to undertake close-in operations. Andrea noted that the comet is less than half the density of water – similar to wood, which indicates that it contains lots of cavities. He answered a question from the audience saying that the biggest challenge faced by his team was the complex real-time parameter estimation exercise to characterise the comet’s gravity and rotation and the effect on the spacecraft, while simultaneously using the results to navigate in its close proximity.
Figure 2: Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Source can be found here.
The task of choosing a landing site for the Philae lander was complicated by the fact that 67P/CG had no flat areas wide enough to accommodate the 500m radius of the landing error ellipse. Close-up images of the selected landing site showed cliffs, gullies and boulders throughout. Philae was released by Rosetta in a complicated set of manoeuvres and took 10 hours to drift slowly to the comet’s surface. Andrea showed dramatic images (see figure 2) taken by Rosetta of Philae drifting across the comet and of the dent made by its first touchdown which led to it bouncing back into space – fortunately its 38cm/sec bounce-off speed was less than the comet’s 50cm/sec escape velocity so it returned to the comet’s surface later. Andrea showed the photo taken by Philae where it eventually ended up with a wall about 80cm from the camera. For the moment the exact location of the lander is unknown. He reported that Philae had provided a full 3 days of data before battery power was depleted and that the scientists were happy that it had not ended up in the middle of a flat scientifically-boring area but in a position for its instruments to “see” some interesting geology. He estimated that the science that Philae had been expected to provide represented 10-20% of that of the whole mission and Philae had so far achieved 70-80% of its expected results and that there is still a possibility that Philae will re-awaken as the comet moves nearer to the sun.
When Rosetta awoke from its 2½ year energy-saving hibernation in January 2014 the comet was about 4½ Astronomical Units (AU) from the sun (Earth is 1AU from the sun, Jupiter is about 5). By the time Philae landed in mid-November the sun was 3AU distant, and on the night of the lecture the distance was 2AU. As it approaches its point of closest approach to the sun (perihelion, 1¼ AU) 67P/CG is giving off more and more gas in the form of jets with a velocity of about 800 m/sec. Responding to a question Andrea noted that most of the gas is water whereas the scientists had expected it to be mainly carbon monoxide and carbon dioxide, with water appearing only when it was closer to the sun. Operation close to the comet is complicated by the need to avoid the worst of this gas since it carries some dust with it that could damage Rosetta, particularly the star trackers. Another effect is that the gas jets act like rocket motors on 67C/GP slowing its rotation rate perceptibly – currently the comet’s 12.4 hour rotation period is slowing down by 1 sec/day. This effect has to be constantly re-calibrated since it compromises the use of surface landmarks to navigate Rosetta. Andrea expects the operations to be increasingly hectic as the comet approaches its 13th August perihelion with gas extrusions increasing greatly.
The comet’s relatively small gravity field requires Rosetta to perform frequent manoeuvres in order to move around the comet and perform scientific measurements. Rosetta was in orbit around the comet (thus requiring no manoeuvres) only for a short period near the time of the Philae landing but the gas environment now requires it to stand off and the comet’s gravity field can no longer keep it in orbit. In response to a question Andrea explained that there is ample fuel to keep Rosetta on station until well beyond the mission lifetime. The mission will end because after perihelion Rosetta will accompany 67P/CG back out to the far reaches of the solar system where its 64m2 solar panels will no longer supply the power required to continue operations. The options then will be to either put Rosetta into a 4-5 year hibernation and hope that it will survive (Andrea considered survival unlikely) or to give the scientists some ultra-close-ups of the comet (for example passing through the “neck” between the comet’s two main parts) with the likely eventuality of crashing into its surface.
Space Group Deputy Chairman Dr Geoff Busswell who chaired the event echoed the mood of the audience when he described the talk as “riveting”. He thanked Andrea for providing such a fascinating example of European engineering and science at its best and for responding to the many audience questions. Finally Geoff thanked CGI for sponsoring the event, including the post-lecture reception, noting that several CGI staff were part of Andrea’s team on the consoles in Darmstadt. The round of applause at the end was the loudest and most heartfelt I have ever heard in the Bill Boeing Theatre, reflecting the fact that the audience had been witness to a superb account of a once in a lifetime experience.
Figure 3: Andrea Accomazzo giving his lecture at the Royal Aeronautical Society.