News Space 15 September 2014

Countdown to Rosetta's comet landing begins

European Space Agency scientists will on Monday unveil their pick of site for the Philae lander. We look at the shortlist of five potential sites already identified, as well as some remarkable close-up shots of the comet's surface.

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Rosetta navigation camera image taken on 22 August 2014 at about 64 km from comet 67P/Churyumov-Gerasimenko.

The European Space Agency's Rosetta project team will on Monday in Paris announce the primary landing site for the spacecraft's lander Philae which, in November, will attempt the first soft touchdown in history on a comet.

Rosetta caught up with Comet 67P/Churyumov–Gerasimenko on 6 August after a nearly three-year chase millions of kilometres across space, described in our April feature on the audacious mission (see All set for our first comet landing). Since then mission scientists and engineers have been carefully studying possible landing sites for Philae.


On 25 August the agency announced five candidate locations for further investigation. Since then the Rosetta team has been determining possible orbital and operational strategies for Rosetta to position best for the lander to reach each of them.

The approximate locations of the five regions are marked on the OSIRIS narrow-angle camera images (above) taken on 16 August from a distance of about 100 km. The comet nucleus is about 4 km across.

Rosetta has now moved to within 50 km of the comet, allowing more detailed observations.

The sites were assigned a letter from an original pre-selection of 10 possible sites identified A to J. The lettering scheme does not signify any ranking. Three sites (B, I and J) are located on the smaller of the two lobes of the comet and two sites (A and C) are located on the larger lobe.


The image above shows a zoom into candidate site A, which is located on the larger lobe but with a good view of the smaller lobe. The ESA says the region is interesting because the terrain between the two lobes is likely the site where gases, trapped in the comet, are released as it heats when it gets closer to the Sun — a process called "outgassing". However, the agency says higher-resolution imaging is needed to study potential surface hazards such as small depressions and slopes, while the illumination conditions also need to be considered further.


Candidate site B, above, is located within a crater-like structure on the smaller lobe. This site is considered relatively safe for landing given the flat terrain, but the Rosetta team is concerned that illumination conditions may pose a problem when considering the longer-term science planning of the lander. Higher-resolution imaging will also be used to assess the boulder hazards in more detail.


Candidate site C, above, is located on the larger lobe of the comet. It hosts a range of surface features including some brighter material, depressions, cliffs, hills and smooth plains, but higher-resolution imaging is needed to assess the risk of some of these features. It is also well illuminated, which would benefit the long-term scientific planning for Philae.


Candidate site I, above, on the smaller lobe of the comet, is a relatively flat area that may contain some fresh material, but higher-resolution imaging is needed to assess the extent of the rough terrain. The illumination conditions would also allow for longer-term science planning.


Candidate site J, above, also on the smaller lobe of the comet, is similar to site I. It offers interesting surface features and good illumination. It has advantages over site I for the CONSERT experiment, but higher-resolution imaging is needed to determine the details of the terrain, which shows some boulders and terracing.

CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) is an experiment designed to study the internal structure of the comet nucleus.

Several morphologically different regions are indicated in this preliminary map, which is oriented with the comet’s ‘body’ in the foreground and the ‘head’ in the background. – ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The Rosetta team has been busy mapping the comet with high-resolution images from the OSIRIS (Optical, Spectrocopic and Infrared Remote Imaging System) instrument. They have uncovered a multitude of different terrains, with various areas dominated by cliffs, depressions, craters, boulders or even parallel grooves.

This preliminary analysis provides the basis for a detailed scientific description of the comet’s surface, but a substantial amount of work involving more detailed OSIRIS images and data from other Rosetta instruments lies ahead. This will determine what each region represents its composition and evolution, the scientists say.

The image above, taken with the OSIRIS narrow-angle camera from a distance of 62 km, and showing jagged cliffs and prominent boulders, is one of the most detailed pictures of the comet surface so far. The left part of the image shows a side view of the comet’s "body", while the right is the back of its "head".

As both 67P/C-G and Rosetta travel closer to the Sun over the coming months the OSIRIS team will monitor the surface looking for changes. While the scientists do not expect the borderlines of the comet’s regions to vary dramatically during this one passage around the Sun, more subtle transformations of the surface may nevertheless help to explain how cometary activity created such a breath-taking world.

Meanwhile, Rosetta has joined the selfie craze releasing this stunning image, below, which shows part of its solar arm with the comet itself in the background.

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