Last June, astronomers urged sky watchers to observe the transit of Venus. It was a once in a lifetime opportunity, they said. The black disk of the second planet wouldn't crawl across the face of the sun again for more than 100 years. In fact, it's happening again this week--not on Earth, but Saturn.

"On Friday, Dec. 21st, there will be a transit of Venus visible from Saturn, and we will be watching it using the Cassini spacecraft," says Phil Nicholson, a Cassini science team member from Cornell University. "This will be the first time a transit of Venus has been observed from deep space."

Because Saturn is 10 times farther from the sun than Earth, this transit of Venus won't be so easy to see.

The silhouette of the second planet will be just a tiny black speck on the shrunken disk of a sun 10 times farther from Saturn than Earth. Cassini won't be beaming back any "beauty shots." Nevertheless, the spacecraft will be conducting potentially ground-breaking science.

"As Venus crosses the face of the sun, we will see if we can detect chemical compounds in the planet's atmosphere by looking at the spectrum of sunlight filtered by Venus," explains Nicholson.

This is, essentially, an experiment in exoplanet studies. NASA's Kepler spacecraft routinely discovers new planets around distant stars by looking for the minuscule reduction in starlight that occurs during a planetary transit. Watching Venus transit the sun from the faraway orbit of Saturn is a good analog.

"We already know what Venus's atmosphere is made of," says Nicholson. "But this will give us a chance to see if we can pull this information out of a faint, distant planetary transit."

The research team will be using Cassini's VIMS instrument. VIMS is an infrared spectrometer designed to tease out the chemical composition of Saturn and its moons. It isn't designed for planetary transits, but with a little ingenuity Nicholson and colleagues have figured out how to gather useful data.

"VIMS has a heavily-filtered 'solar port' 20 degrees off the main axis of the spectrometer. We use it to occasionally observe the sun for calibration purposes--or to watch the sun set in the atmosphere of Saturn's moon's Titan," says Nicholson.

"On Dec. 21st we'll be using the solar port to monitor the transit of Venus."

The images won't be very impressive. Only a few pixels will fit across the entire solar disk. But the researchers aren't looking for images. "We want spectra," says Nicholson. "Carbon dioxide, the main constituent of Venus's atmosphere, has several absorption bands squarely inside our 1 to 5 micron observing window."

VIMS will gather data for the entire 9 hours of the transit--as well as many hours before and after for comparison. "Even with so much observing time, we still might not detect any chemical signatures," cautions Nicholson. "The signals are going to be faint--only a few parts in a million--so this is an extremely difficult observation."

Nevertheless, Nicholson is looking forward to Friday. "While most people have to wait a hundred years for the next transit of Venus, we get to experience one right away. And if we make any discoveries at the same time... so much the better."