ESA’s Rosetta space probe awakened in January 2014 after a journey that lasted almost ten years. In recent weeks, it started sending pictures of its target, the comet 67P/Churyumov-Gerasimenko. Yesterday, CNES, the French space agency, published a composition of photos taken on July 11 which show that the comet is composed of two nuclei that are attached, a fact that makes the Rosetta mission even more extraordinary.

These days, the Rosetta spacecraft is about 10,000 km (about 6,200 miles) away from the comet 67P/Churyumov-Gerasimenko and is approaching it. The rendezvous is scheduled for August. During this month, Rosetta started sending more and more detailed pictures of the comet.

The photographs sent in recent days are truly amazing because they show that the comet 67P/Churyumov-Gerasimenko has a binary nucleus. Evidently, originally they were two separate bodies that at some point met. According to preliminary estimates, the meeting took place at a speed of around 3 m/s in order for them to stick together that way.

That the nucleus of a comet has an irregular shape isn’t surprising. It’s difficult to see it clearly because when it’s close enough to Earth its ice vaporises making it invisible. However, asteroids prove that these small celestial bodies have irregular shapes and this is logical considering their low gravity, which doesn’t allow them to take a spherical shape. When it was possible to study the nucleus of a comet, it always turned out to have an irregular shape.

There are also known cases of asteroids that stick together or otherwise travel very close but this is the first time scientists have been able to see a comet with a binary nucleus. This makes the Rosetta mission more exciting but also more complex.

Rosetta’s navigation around the nucleus of the comet 67P/Churyumov-Gerasimenko isn’t a problem but the landing of the Philae lander might be more difficult. The two attached nuclei give the comet a shape that limits the landing zones so this phase of the mission will have to be planned with extreme care. The next photographs, even more detailed, should be useful.

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