A European spacecraft has taken the temperature of a comet that is its mission target, finding it too warm to support an icy crust, which suggests a dark and dusty covering instead, scientists say.
Rosetta, a comet-hunting spacecraft of the European Space Agency, took the temperature of the comet 67P/Churyumov-Gerasimenko from a distance of around 3,000 miles.
Using a sensor to record infrared light coming from the comet, the scientists estimate a surface temperature average of about minus 94 degrees Fahrenheit.
That's certainly cold, but is about 30 to 50 degrees warmer than would be expected of a comet with a surface that was exclusively ice, they say.
"This result is very interesting, since it gives us the first clues on the composition and physical properties of the comet's surface," says Fabrizio Capaccioni, principle investigator for the infrared instrument gathering the readings.
The scientists said they were not surprised to find that the comet did not have an entirely "clean" icy surface, since previous observations by ground-based telescopes had shown its surface to have a low reflectance.
The temperature data confirms that a lot of the comet's surface has to be dust, since darker material like dust heats up faster and throws off heat more easily than ice when exposed to the sunlight encountered as the comet approaches the sun.
"This doesn't exclude the presence of patches of relatively clean ice, however, and very soon, [the] VIRTIS [instrument] will be able to start generating maps showing the temperature of individual features," Capaccioni says.
Rosetta's journey to the comet began in 2004 when it was launched from Europe's Spaceport in Kourou, French Guiana.
Looping around the sun five times to pick up a gravity assist in its speed during flybys of Earth and then Mars, Rosetta has traveled some 3.9 billion miles on its way to a rendezvous with the comet, set for August 6.
Two short burns of the spacecraft's engines will put it in orbit around the comet for close-up scientists' examination, ESA scientists said.
When the comet moves closer to our sun in 2015, it will warm up and become more active, throwing off larger quantities of dust and gas that Rosetta will analyze.
"With only a few days until we arrive at just 100 kilometers (62 miles) distance from the comet, we are excited to start analyzing this fascinating little world in more and more detail," ESA Rosetta project scientist Matt Taylor says.
