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Comet Conspiracy! Here's Why We Haven't Seen Colour Photos of 67P

An apparent "colour" photo of Rosetta's target comet has made the rounds online, but why is it so brown?
​Image: ​AGU

​There's a new mystery to add to the enigmatic depths of space: what colour is Comet 67P? And why haven't we seen colour pictures?

Spurred on by eager Reddit comet-spot​ters, the internet has seen the dissemination of what many purported to be the first colour photo of Comet 67P/Churyumov-Gerasimenko, the target of the European Space Agency's Rosetta mission.

The image comes from a poster for an upcomin​g paper to be presented at the fall meeting of the American Geophysical Union (AGU). Titled "Color Variegation on 67P/Churyumov-Gerasimenko," it shows what looks like a reddish-brown version of the rubber ducky-shaped space rock.


That's surprising; ESA has said before that the comet would be a dark charc​oal colour. So what gives?

I reached out to Stubbe Hviid, a co-investigator on Rosetta's OSIR​IS camera at the German Aerospace Center, and one of the researchers listed on the poster. He explained that the above is not really a true colour image.

A greyscale image of 67P from Rosetta's Navcam. Image: ESA/Rosetta/NAVCAM

The OSIRIS instrument is not like your regular colour digital camera. It uses colour filters—in this case orange, green, and blue—to look at the surface reflectance of the comet. Hviid wrote in an email that the comet would look like that in the image if you shone "a completely white light source" on it.

He added that the image shown "has been processed to emphasize any color variation present on the surface of the comet," and that some of the variation was down to compression artifacts. "The image is intended as an illustration to a presentation about color variation on the comet nucleus surface and it should not be seen as a true color image," he wrote. "If you looked at the comet with human eyes it would basically be black."

As one redditor s​urmised, the blurriness of the image is due to the comet moving between exposures; Osiris works by taking separate images with the different color filters deployed. Hviid said that to make a classical "colour image" therefore takes a lot of processing; you have to "reproject the images into a common reference frame using the 3D shape model of the comet." And as researchers are continuing to learn more about 67P's exact shape, getting it pixel-perfect is tough.


A true colour image OSIRIS took of Mars during a 2007 flyby. Image: ESA/2007 MPS for OSIRIS Team MPS/UPD/ LAM/IAA/ RSSD/ INTA/ UPM/ DASP/ IDA

One reason this "colour" image was shared so widely is because it's the first to seep into public view. Which begs the question, why have we only seen black-and-white images of 67P? ESA has literally hundreds of images from the Rosetta mission online, and those of the comet are invariably greyscale. But some shots taken by the Rosetta probe of other stuff—like M​ars and the ​Earth—are in glorious colour.

It's not a big conspiracy. As Business Ins​ider points out, most of the images we've seen of the comet so far have come from the Navcam instrument, which doesn't do colour.

As for OSIRIS, Hviid said they hadn't released colour images because OSIRIS isn't a colour camera in the conventional sense.

That said, there's some controversy over the delay in Rosetta data being provided to the public. Eric Hand at Science magazine wrote last month that colour images of 67P had already been shown to a conference in Arizona—but they're still not publicly available on ESA's website. According to Hand, even scientists at ESA's mission control hadn't seen the colour pictures at that point.

But as this incident shows, you can count on the Rosetta fans on the internet—of which, judging by Twitter followers and Reddit threads like this one, there are many—to unearth any tidbits that do make their way into the open. And for as long as they don't get any answers, to speculate profusely about them.