Rosetta: 'Spuds in space'

Comet 67P seen on Friday 4 July at a separation of 37,000km

With the Rosetta probe closing in on Comet 67P/Churyumov-Gerasimenko, we're beginning to get a sense of the ice mountain's shape.

The latest picture release from the European Space Agency (Esa) may only cover an area of about 30 pixels, but it's clear that 67P is no sphere. In some views, the object appears quite elongated.

"Whether it's potato-, or peanut-shaped, or whatever - we're going to have to wait a bit longer," says Trevor Morley, who's part of the mission's flight dynamics team - the navigators who've been driving Rosetta to its quarry.

And Matt Taylor, Esa's project scientist, added: "There appears to be some indication of lobes, or large lumpy sticky-out bits, but I still feel it's not that dissimilar to the 'flying potato' we got from the Lamy shape or Lowry shape." Lamy and Lowry are scientists who used Earth telescopes to try to discern 67P's form.

The three pictures on this page were acquired by Rosetta's Osiris Narrow Angle Camera on Friday last week, when the separation was about 37,000km, not the hundreds of millions of km that the Earth telescopes had to grapple with.

Each image is taken four hours apart. Given that the comet completes a full rotation every 12.4 hours, we therefore get an impression of its overall outline.

The sticky-out bit, or small lobe, seen at bottom-left in the first image is on the right in the second picture, and at the top in the third image.

Early estimates of 67P's dimensions were for an object roughly 3km by 4km by 5km. Those dimensions will be pinned down more precisely in the coming weeks.

The approach to the comet has gone flawlessly so far. Six out of 10 manoeuvres to refine Rosetta's trajectory have been completed, slowing the probe's speed with respect to 67P. The relative velocity is now about 19m/s.

The navigators plot each manoeuvre after studying a series of pictures acquired by another of the probe's camera systems - the NavCam system.

"One thing you should appreciate about how we do this - we don't move directly towards the comet," explained Morley.

"If we did move directly towards the comet then 67P wouldn't move against the star background. And if it doesn't move, you don't get any distance information from the images. So we purposely aim slightly away from the comet. We're coming in in small spirals, if you like."

The plan still is to begin orbiting the comet on 6 August, using a pyramid-like fly-around. This will be done from a distance of about 70km.

The single most important job for the mission team when it arrives is to work out the gravitational field. 67P won't have much of a tug, but, however small, it will still have an influence on the behaviour of the close-in manoeuvres.

These will likely start in September, when Rosetta will cut the separation distance to about 30km to make a detailed map of the comet's surface. It is this information that will drive the choice of landing zone for Philae - Rosetta's little contact platform. Its historic attempt to make the first soft touch-down on a comet is scheduled for 11 November.