Alice UV Spectrograph Unlocks A Comet's Secrets
By Jof Enriquez,
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The Rosetta spacecraft's onboard ultraviolet (UV) imaging spectrograph Alice has discovered valuable scientific insights into the origin, composition, and workings of comet 67P/Churyumov-Gerasimenko.
Rosetta wrapped up its two-year orbital observation of the comet when it executed a controlled descent and impact onto the comet on Sept. 30, 2016. The European Space Agency (ESA) launched the aircraft in 2004 to rendezvous with the comet as it orbited around the sun. The spacecraft had a suite of 11 total science instruments aboard Rosetta to conduct its observations.
Among these is Southwest Research Institute's (SWRI) Alice spectrograph, the first instrument to obtain far-ultraviolet observations at a comet.
Among Alice's findings during Rosetta's mission: an unexpectedly porous, “fluffy” dark surface; a surprising lack of exposed water ice on the comet’s surface; and an extremely volatile, unexpected gas in the comet’s atmosphere — molecular oxygen.
Not much bigger than a shoebox, and weighing less than four kilograms, Alice gathered more than 1,000 times the data of instruments flown just a generation ago, relying on only four watts of power.
“Alice did its job perfectly, taking over 70,000 spectra in two years, providing a gold mine of data for comet scientists to study for years to come,” said Dr. Alan Stern, Alice principal investigator and an associate VP of SwRI’s Space Science and Engineering Division, in a news release.
Data collected by Alice also revealed that electrons close to the surface of comet 67P/Churyumov-Gerasimenko – not photons from the sun, as had been believed – cause the rapid breakup of water and carbon dioxide molecules spewing from the comet's surface.
SwRI's other UV spectrometers are operating aboard the New Horizons spacecraft that explored Pluto last year, the Lunar Reconnaissance Orbiter that has been examining the moon since 2009, and the Juno spacecraft now orbiting Jupiter, according to the nonprofit applied research and development (R&D) organization, which is based in San Antonio, Texas.
Another SwRI instrument, the Ion and Electron Spectrometer (IES), was used aboard Rosetta to collect data about the interaction of the solar wind with the comet’s expanding atmosphere, or coma. Specifically, it gathered the first measurements of negatively charged, submicron-sized dust or ice grains in a cometary environment and discovered previously unobserved negative hydrogen ions produced through the solar wind/coma interaction.
Rosetta is an ESA mission with contributions from its member states and NASA. It is the first spacecraft to witness at close proximity how a comet changes as it is subjected to the increasing intensity of the sun's radiation, according to NASA's Jet Propulsion Laboratory. Data collected from the mission will help scientists learn more the role comets may have played in the formation of planets and the solar system.
“The Rosetta mission has provided an unprecedented window into the origin of comets and the way comets work,” says SwRI’s Dr. Joel Parker, Alice’s deputy principal investigator.