Scientists have selected a landing site on the surface of 67P/Churyumov-Gerasimenko for the first-ever comet landing attempt in November.
Dubbed ‘Site J’, the spot where the Philae lander carried by the Rosetta spacecraft will attempt to touch down lies on the head of the comet. The location was chosen from five candidate sites as part of a complicated evaluation process, which forced the engineers to many trade-offs.
"As we have seen from recent close-up images, the comet is a beautiful but dramatic world – it is scientifically exciting, but its shape makes it operationally challenging," said Stephan Ulamec, Philae lander manager at the German Aerospace Centre (DLR).
"None of the candidate landing sites met all of the operational criteria at the 100 per cent level, but Site J is clearly the best solution."
The Landing Site Selection Group of engineers and scientists from DLR, the European Space Agency (Esa), and French space agency CNES, evaluated the prospective landing sites based on a number of criteria.
Apart from scientific value, the teams had to take into account technical limitations of the lander and the level of risk associated with each candidate site. The required landing trajectory, density of visible hazards, as well as daylight-night time balance and frequency of telecommunications passes with the orbiter have all been taken into consideration.
A one square kilometre area was assessed for each candidate site as the landing will have to be completely automated with Philae touching down with limited accuracy within a predefined ellipse.
Site J was selected as its terrain is a bit less rugged than elsewhere on the comet. The slopes within the landing area are less than 30º steep and there are relatively few boulders, which decreases the danger of the lander toppling over upon touchdown.
The researchers calculated it would take the 100kg Philae seven hours to descend from the orbit onto the comet’s surface during the November landing. During this time, the lander will be powered only by its on-board batteries.
"Site J in particular offers us a chance to analyse pristine material, characterise the properties of the nucleus, and study the processes that drive the comet’s activity," said Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument at the IAS in Orsay, France.
Rosetta arrived at 67P/Churyumov-Gerasimenko on 6 August after a challenging ten-year journey.
It entered into orbit around the 4km in diameter comet at the altitude of 100km but has since moved closer to only 30km above the comet’s surface allowing for more close-up inspection.
During its ground-breaking landing attempt, Philae will descend on the comet’s surface in a controlled manner and touchdown at a speed of only a few kilometres per hour. Throughout the descent, Philae’s on-board sensors and cameras will be acquiring data and taking images.
The spacecraft will eventually anchor itself to the comet’s surface using a harpoon and a set of ice screws. It will then make a 360° panoramic image of the landing site to help determine where and in what orientation it has landed.
A detailed operational timeline will now be prepared to determine the precise approach trajectory of Rosetta in order to deliver Philae to Site J. The landing must take place before mid-November, as the comet is predicted to grow more active as it moves closer to the Sun.
"There's no time to lose, but now that we're closer to the comet, continued science and mapping operations will help us improve the analysis of the primary and backup landing sites," said Esa Rosetta flight director Andrea Accomazzo.
"Of course, we cannot predict the activity of the comet between now and landing, and on the landing day itself. A sudden increase in activity could affect the position of Rosetta in its orbit at the moment of deployment and in turn the exact location where Philae will land, and that's what makes this a risky operation."
The landing date should be confirmed on 26 September after further trajectory analysis.
Rosetta landing site infographic
Rosetta overview infographic