London: ESA`s Rosetta spacecraft has recently measured the temperature of its target comet for the first time.
It was found that it was too hot to be covered in ice and must instead have a dark, dusty crust.
The observations of comet 67P/Churyumov-Gerasimenko were made by Rosetta`s visible, infrared and thermal imaging spectrometer, VIRTIS, between 13 and 21 July, when Rosetta closed in from 14 000 km to the comet to just over 5000 km.
At these distances, the comet covered only a few pixels in the field of view and so it was not possible to determine the temperatures of individual features. But, using the sensor to collect infrared light emitted by the whole comet, scientists determined that its average surface temperature would be about 70 degrees Celsius.
The comet was roughly 555 million kilometres from the Sun at the time, more than three times further away than Earth, meaning that sunlight was only about a tenth as bright. Although 70 degrees Celsius might seem rather cold, importantly, it was some 20-30 degrees Celsius warmer than predicted for a comet at that distance covered exclusively in ice.
In addition to global measurements, the sensor would study the variation of the daily surface temperature of specific areas of the comet, in order to understand how quickly the surface reacts to solar illumination. In turn, this would provide insight into the thermal conductivity, density and porosity of the top tens of centimetres of the surface and this information would be important in selecting a target site for Rosetta`s lander, Philae.
It would also measure the changes in temperature as the comet flies closer to the Sun along its orbit, providing substantially more heating of the surface.