European scientists got a surprise this week when images emerged which show that the comet the Rosetta probe is speeding towards is shaped like a rubber duck.

The latest images, released by the European Space Agency today and taken on 14 July, suggest that the comet 67P/Churyumov-Gerasimenko is made up of two differently sized parts, something planetary scientists call a contact binary. Its exact nature still remains a mystery, although it could be the result of a merger.

“It’s more than a pleasant surprise — I was amazed by the images,” says Stephen Lowry, an astrophysicist at the University of Kent, UK, and a member of the team that took the images. But as exciting as the comet is scientifically, its shape will make the job of setting down Rosetta’s lander, Philae, more difficult, he adds.

Contact binaries have been seen before in asteroids, which are made mostly of rock, but have never been positively observed in comets, asteroids' icy and dusty cousins. Lowry says that the two parts could have different sources entirely, but it is more likely that they are from the same source and were torn apart by the pull of a passing planet and then drawn gently back together by gravity.

Rosetta project scientist Matt Taylor, based at ESA's Science and Technology Centre in Noordwijk, the Netherlands, is more cautious about labelling the comet a contact binary, given that images were taken from 12,000 kilometres away and are only 20 pixels across. Another possibility is that the comet is a single entity that was carved into this shape as ice sublimated from its surface during previous fly-bys of the Sun. “We really need to get closer to really ascertain the nature of this body,” Taylor says.

The Rosetta researchers will attempt to land Philae on the approximately 4-kilometre-long comet in November. It will be the first time a soft landing has been attempted on a comet.

Understanding the comet’s gravitational field is essential to placing the lander on the surface safely, Lowry says. Trouble could arise if the comet’s two pieces have different densities, so that their gravitational fields would have different strengths. The irregular shape might also make it harder to comprehensively image the comet. And if the lander ends up in the crevasse between the two parts, communicating with it could be difficult, Lowry adds.

But Lowry says that none of these challenges will be a “showstopper”. Rosetta will make three more burns of its rockets to slow down before its rendezvous with the comet on 6 August, and will then map the comet’s surface and gravity and study its composition, rotation and release of gas and dust, to look for a suitable landing site for Philae.

The €1.3-billion (US$1.8-billion) spacecraft woke up from hibernation in January this year, and knowledge about its quarry has been growing every day. The first images suggesting the comet’s duck-like shape, taken on 11 July, were published by the French National Centre for Space Studies (CNES) on 15 July and removed later that same day. That was not before a number of bloggers, including Emily Lakdawalla of the Planetary Society in Pasadena, California, reposted the pictures.

Some followers of the mission, including Raumfahrer, a German society of space enthusiasts, have expressed dismay at the slowness with which ESA released the images. Taylor and his colleagues responded in a blog post, writing that the pictures were not immediately distributed because the Rosetta instrument teams have exclusive access to their data for up to six months. “Because no one has ever been to [67P/Churyumov-Gerasimenko] before, each new piece of data from Rosetta has the potential for a scientific discovery. It’s only fair that the instrument science teams have the first chance to make and assess those discoveries,” they wrote.