'Rosetta measures comet's temperature'
Image credit: ESA
The European Space Agency (ESA)’s Rosetta spacecraft has used its visible, infrared and thermal imaging spectrometer - VIRTIS - to measure the temperature of comet 67P/Churyumov–Gerasimenko, revealing a surface temperature (they can’t probe internal temperatures without landing on it) of –70ºC!
Actually, that’s 20–30ºC warmer than predicted, for a comet 555 million kilometres from the Sun at the time, so the physicists have concluded that the comet must have a dusty surface.
The reasoning is that ice has a high albedo - it’s highly reflective of warming sunlight, which keeps the surface cool. Dust, however, is darker, and absorbs more heat, so a warmer surface will be dustier.
Here’s a video of Rosetta’s funky orbit. Amazing :-D