English: A colour image of Comet 67P/Churyumov-Gerasimenko composed of three images taken with the Narrow Angle Camera (NAC) of the scientific imaging system OSIRIS in red (centred at 744 nm wavelength), green (536 nm), and blue (481 nm) filters on 6 August 2014 from a distance of 120 kilometres. The image covers roughly 4 x 4 km at a resolution of about 3.9 metres per pixel.

---

Rosetta’s OSIRIS team have produced a colour image of Comet 67P/Churyumov-Gerasimenko as it would be seen by the human eye. As anticipated, the comet turns out to be very grey indeed, with only slight, subtle colour variations seen across its surface.

To create an image revealing 67P’s “true” colours, the scientists superposed images taken sequentially through filters centred on red, green, and blue wavelengths.

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

As explained in earlier blog posts for the NAVCAM images, the intensity of the images has been enhanced to span the full range from black to white, in order to make surface details visible. But the colours have not been enhanced: the comet really is very grey.

A more detailed first analysis nevertheless reveals that the comet reflects red light slightly more efficiently than other wavelengths. This is a well-known phenomenon observed at many other small bodies in the Solar System and is due to the small size of the surface grains. That does not, however, mean that the comet would look red to the human eye. Natural sunlight peaks in the green part of the spectrum and the response of the human eye is similarly matched. Thus, overall, the comet would look rather grey to the human eye, as seen here.