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Rosetta's colour photo shows comet is many shades of grey

Paul Sutherland, Feature writer
Dec 16, 2014, 1:44 UTC

Sen—Rosetta scientists have finally come up with a colour picture of Comet 67P/Churyumov-Gerasimenko, and ironically the cosmic nomad turns out to be several shades of grey!

Many impatient space fans had complained that the team operating the European Space Agency probe’s OSIRIS instrument—it stands for Optical, Spectroscopic, and Infrared Remote Imaging System—were dragging their heels in issuing a colour image.

Though monochrome shots from another Rosetta camera, NAVCAM, have been regularly released, the clamour grew for a true-colour picture to show us what 67P would look like to the human eye.

Now the OSIRIS team has issued a colour photo by superimposing images that were taken in quick succession through red, green and blue filters with the instrument's Narrow Angle Camera (NAC). It was no mean achievement, because the comet is spinning rapidly and so moved slightly between the individual shots. The work needed to align features correctly took a considerable time.

But the resulting combined image shows that, just as the scientists anticipated, the comet is very, very grey with just the slightest subtle variations in colour.

OSIRIS’s Principal Investigator Holger Sierks, from the Max Planck Institute for Solar System Research (MPS) in Germany, told ESA’s Rosetta blog: “As it turns out, 67P/C-G looks dark grey, in reality almost as black as coal.”

Rosetta's changing orbit during the final three months of 2014. Credit: ESA

The team say that the intensity of the images has been enhanced to span the full range of shades between black and white. But they stress that the colours have not been enhanced, meaning the comet really is very grey, and covered with dark dust.

This confirms observations made by telescopes on Earth before Rosetta got to 67P. But the lack of any colour, even in small areas, does surprise the science team because it suggests the comet has a uniform composition.

Rosetta has been making a complex series of manouevres since it sent its companion probe Philae to an uncertain landing on the comet a month ago. It is currently orbiting at a height of 20 km (12.5 miles) but two engine burns will send it out to a 30 km (18.5 miles) high circular orbit later this month.

It will stay in that orbit until 3 February when it will begin a series of daring swoops over the comet, including the closest approach to a height of just 6 km (3.7 miles) on 14 November. The flybys are to help it taste and analyse the dust and gas now fizzing from the comet’s nucleus.

Rosetta Project Scientist Matt Taylor told the Rosetta blog: “Landing week marked an epoch where we began the 100 per cent science phase. From now on, that’s all we focus on with the mission. The measurements we make now set the tone for the entire mission. The comet’s activity will continue to increase and we’ll be watching.”