A paper appearing online appears to show a colour image of Comet 67P/Churyumov-Gerasimenko.
There has been a lot of discussion online as to why we’re only being shown monochrome images of the comet. A lot of the most stunning pics have been taken by Rosetta’s Navcam, which only takes monochrome images.
Rosetta has another camera, Osiris, which can capture images beyond visible light range. It doesn’t have colour sensors, but instead can use filters placed in front of the sensor. The individual photos can be combined by ESA to form a “true colour” image, which is what a lot of commentors are surmising about this image.
(Universe Today has a great explanation here of what is meant by “true colour” and why it’s a bit controversial.)
However, the precis accompanying the image says the contribution to the AGU meeting “will discuss the colour variegation observed on the comet surface and its relationship to surface morphology and cometary activity”. That suggests this image is a colour composite made to highlight the morphology of the comet surface, ie the blues and reds represent different outcrops and pits and the like.
To date, we’ve only been told that if the comet landed on Earth, it would be “black as coal”, so there is understandably some confusion about the colour image and why it hasn’t been seen before.
But there’s no doubt Rosetta is able to send back colour images. ESA have utilised Osiris throughout Rosetta’s 10-year journey, as you can see here in these beautiful snaps of Mars and the Orion nebula:
Why they haven’t shown any “colour” photos of 67P to date has been widely speculated, but having a colour-capable camera on board was considered essential to the mission’s success. A document outlining why Osiris was commissioned for the project says “mineral composition and colour could provide the most obvious clues to the size of building blocks”.
The most likely explanation is ESA simply isn’t willing to share colour pictures until it has a proper explanation of what they represent to accompany them.
That means there’s a lot more spectacular imagery to come, starting with the AGU meeting.
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