Rosetta's target comet too warm to be an icy world

Initial observations from an imaging spectrometer aboard Europe’s Rosetta spacecraft show the comet it is chasing has a dark, dusty surface instead of one covered in ice, scientists said Friday.

The observations agree with earlier research from ground-based observatories, which showed the comet did not reflect enough sunlight to have a surface entirely made of ice. 

“This result is very interesting, since it gives us the first clues on the composition and physical properties of the comet’s surface,” said Fabrizio Capaccioni, principal investigator for Rosetta’s visible, infrared and thermal imaging spectrometer, or VIRTIS. 

Capaccioni is based at the Astronomical Observatory of Rome. The VIRTIS instrument also includes contributions from institutions in Germany and France. 

VIRTIS collected the temperature data from July 13 and July 21 as Rosetta closed in on comet 67P/Churyumov-Gerasimenko. The comet was between 3,000 miles and 8,700 miles from the comet when VIRTIS made the observations. 

The comet and Rosetta were 345 million miles from the sun at the time, nearly four times the distance between the Earth and the sun. 

According to a European Space Agency press release, the comet’s distance from Rosetta mean the spectrometer collected infrared light from the whole comet. The target was too far away to study individual features. 

Data detected by the imaging spectrometer showed the comet’s average surface temperature is about minus 94 degrees Fahrenheit, warmer than would be expected if the comet was covered entirely by ice.

 
More detailed data will be gathered by VIRTIS — and Rosetta’s 10 other experiments — after the spacecraft arrives close up to the comet Aug. 6. 

“This doesn’t exclude the presence of patches of relatively clean ice, however, and very soon, VIRTIS will be able to start generating maps showing the temperature of individual features,” Capaccioni said. 

Rosetta’s navigation camera and main science camera have collected more detailed imagery of the comet throughout the approach phase, which began in January when engineers awakened the probe from a nearly three-year deep space power-saving hibernation. 

The images show the comet is made up of two distinct lobes connected with a narrow collar-like neck. 

Ground controllers plan to fire Rosetta’s rocket thrusters Wednesday to stop its approach to the comet, which the mission has pursued since it launched in March 2004. 

Rosetta will stay near the comet for more than a year. The Rosetta orbiter will drop a small daughter satellite named Philae to the surface in November for the first-ever planned landing on a comet.
“Combined with observations from the other 10 science experiments on Rosetta and those on the lander, VIRTIS will provide a thorough description of the surface physical properties and the gases in the comet’s coma, watching as conditions change on a daily basis and as the comet loops around the Sun over the course of the next year,” said Matt Taylor, ESA’s Rosetta project scientist, in a press release. 

“With only a few days until we arrive at just 100 km (62 miles) distance from the comet, we are excited to start analyzing this fascinating little world in more and more detail,” Taylor said.