The first far ultraviolet spectra of a cometary surface

NASA’s Alice ultraviolet (UV) spectrograph, located aboard the European Space Agency’s Rosetta comet orbiter, has made the first far ultraviolet spectra of a cometary surface. The Rosetta is currently in orbit around comet 67P/Churyumov-Gerasimenko and is the first spacecraft to study a comet up close.

Developed by the Southwest Research Institute (SwRI), Alice has more than 1000 times the data-gathering capability of instruments flown a generation ago, yet it weighs less than 4 kg and draws just 4 W of power. It is probing the origin, composition and workings of the comet, gaining sensitive, high-resolution compositional insights that cannot be obtained by either ground-based or Earth-orbital observations.

The shoebox-sized instrument is one-third to one-half the mass of comparable UV instruments, yet with more than 10,000 times as many imaging pixels as the spectrometer aboard Galileo. Image courtesy of Southwest Research Institute.

The ultraviolet wavelengths Alice observes contain information about the composition of the comet’s atmosphere and the properties of its surface. The comet is unusually dark at ultraviolet wavelengths, with a surface that has shown no large water-ice patches. Alice is also detecting both hydrogen and oxygen in the comet’s coma, or atmosphere.

“We’re a bit surprised at both just how very unreflective the comet’s surface is, and what little evidence of exposed water ice it shows,” said Dr Alan Stern, Alice principal investigator and associate vice president of the SwRI Space Science and Engineering Division.

To reach its comet target, the Rosetta spacecraft executed four gravity assists (three from Earth, one from Mars) and a nearly three-year period of deep space hibernation, waking up in January 2014 in time to prepare for its rendezvous with Churyumov-Gerasimenko. Rosetta also carries a lander, Philae, that will drop to the comet’s surface in November 2014, attempting the first-ever direct observations of a comet surface.

“As the mission progresses, we will continue to search for surface ice patches and ultraviolet colour and composition variations across the surface of the comet,” said Dr Lori Feaga, Alice co-investigator at the University of Maryland.

A sister Alice instrument was developed by SwRI and was launched aboard the New Horizons spacecraft to Pluto in January 2006 to study that distant world’s atmosphere. It will reach Pluto in July 2015.

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