NASA Saturn Cassini VIMS Instrument re-assigned to Venus watching

This image shows the visual and infrared mapping spectrometer instrument (VIMS) just before it was attached to NASA's Cassini spacecraft. 

Cassini launched in 1997 and has been exploring the Saturn system since 2004.

Credit: NASA/JPL-Caltech/University of Arizona.

For seven years, a mini-fridge-sized instrument aboard NASA's Cassini spacecraft reliably investigated;

• weather patterns swirling around Saturn; 
• the hydrocarbon composition of the surface of Saturn's moon Titan;
• the aerosol layers of Titan's haze; and 
• dirt mixing with ice in Saturn's rings. 

But this year the instrument - the visual and infrared mapping spectrometer - has been testing out some new telescopic muscles.

This Friday, Dec. 21, the spectrometer will be tracking the path of Venus across the face of the sun from its perch in the Saturn system.

Earthlings saw such a transit earlier this year, from June 5 to 6, but the observation in December will be the first time a spacecraft has tracked a transit of a planet in our solar system from beyond Earth orbit.

Cassini will collect data on the molecules in Venus's atmosphere as sunlight shines through it.

But learning about Venus actually isn't the point of the observation. Scientists actually want to use the occasion to test the VIMS instrument's capacity for observing planets outside our solar system.

"Interest in infrared investigations of extra-solar planets has exploded in the years since Cassini launched, so we had no idea at the time that we'd ask VIMS to learn this new kind of trick," said Phil Nicholson, the VIMS team member based at Cornell University, Ithaca, N.Y., who is overseeing the transit observations. "But VIMS has worked so well at Saturn so far that we can start thinking about other things it can do."

VIMS will be able to complement exoplanet studies by space telescopes such as NASA and ESA's Hubble and Spitzer space telescopes. VIMS scientists are particularly interested in investigating atmospheric data - such as signatures of methane - from far-off star systems in near-infrared wavelengths.

The pointing has to be very accurate to get one of those extrasolar planets in VIMS's viewfinder, but the instrument has had lots of practice pointing at other stars.

Earlier this year, VIMS obtained its first successful observation of a transit by the exoplanet HD 189733b. Scientists want to improve these observations by reducing the amount of noise in the signal.