Eenie, meenie, miney, mo… (Image: ESA/Rosetta/MPS for OSIRIS Team)

Landing on a comet will be even harder than we thought. The strange shape of comet 67P/Churyumov-Gerasimenko does not present as many safe landing sites for the European Space Agency’s Rosetta spacecraft as mission planners had hoped.

“Its shape is exciting scientifically but it [creates] a lot of challenges,” says project scientist Matt Taylor at the European Space Research and Technology Centre in Noordwijk, the Netherlands. He calls the comet “the duck” because from some angles it resembles a rubber one.

The probe arrived at 67P on 6 August after a 10-year journey. The plan is to release a probe called Philae to land on the comet’s surface on 11 November. ESA announced five candidate touchdown sites on 25 August, but on 8 September at the European Planetary Sciences Congress in Cascais, Portugal, the team admitted that none of the sites looked very safe.


“All landing sites are worse than expected because of the shape of the body,” said the lander’s lead scientist, Hermann Böhnhardt of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Worse means smaller. Philae is designed to land within an ellipse 1 kilometre in length. Of the five shortlisted sites, only site B (pictured below), at the “head” of the duck, meets that requirement. There are some larger, smoother sites on the base of the duck’s “body” but they are too poorly lit to let the lander recharge its batteries during its four-month mission.

B marks the spot (Image: ESA/Rosetta/MPS for OSIRIS Team)

New images from Rosetta’s high-res camera reveal what appear to be layered cliffs across the comet. These are good for determining the comet’s history, but could damage Philae if it hits them.

Once Philae leaves Rosetta, there will be no chance to alter its trajectory. It will take 5 to 8 hours to drift to the surface under the comet’s weak gravity. Should it hit rough ground and tumble on to its side, some science may still be possible, but settling upside-down would almost certainly spell doom for the lander mission.

Meanwhile, Rosetta has captured its first grains ejected by the comet and shown that such emissions vary throughout its 12.4-hour day. 67P ejects most particles in the afternoon from the duck’s “neck”. Examining this site could help show if the comet began life as one body or two.