More than a billion kilometers from here, a city-bus-sized robot has just rounded the corner on a close looping flight around Saturn. I’m talking about Cassini, a huge autonomous spacecraft that is now on the outbound leg of its 166th elliptical orbit of Saturn. It might seem pointless to count orbits, like counting up the school days in a year; but in fact it’s extremely important to count up Cassini’s orbits, because certain orbits, including the 166th, are special.
Cassini orbits Saturn, but it’s not the only thing circling out there; there are also Saturn’s rings, on their own circular orbits, as well as seven moons that are large enough to be somewhat spherical. Cassini’s orbit crosses the moons’, but most of the time, the moons are nowhere near Cassini when it crosses their orbits. Most of the time. Sometimes, though, a Cassini orbit is planned and timed just exactly so that Cassini and one of the moons wind up in very nearly (but not exactly) the same place at the same time. One of these encounters happened on Wednesday, May 2, 2012, when Cassini’s navigators placed it within 74 kilometers of the little moon Enceladus. That’s a pretty amazing accomplishment, when you realize that Enceladus’ orbital speed around Saturn is about 13 kilometers per second, and Cassini’s speed past Enceladus was about 7.5 kilometers per second. Miss the timing by just a few seconds, and you miss the close-encounter data collection, or worse, you crash into a moon.
Anyway, when Cassini flies close by Saturn’s moons, it can get very pretty photos like this one. This was taken after Cassini’s closest approach, when it had receded to a distance of 140,000 kilometers away. This is a color view of a skinny crescent Enceladus that shows why this particular little moon is so cool. It’s a tiny world with enormous geysers erupting from its south pole. These geysers have been erupting continuously, as far as we know, since Cassini first saw them in 2004, and probably for a great deal longer than that.
I want to point out how incredibly cool it is, not only that Cassini flies close enough to Saturn’s moons to take pictures like this, but that I, as a member of the unwashed public (and not a member of the Cassini mission team) can get my grubby hands on the images just hours after Cassini is done radioing them across 1,300,000,000 kilometers of empty space to a gigantic dish here on Earth.
You can get to these pictures, too. The Cassini mission is one of two right now that post their pictures directly to the Internet, as soon as they are received on Earth. (The other is the Mars Exploration Rover Opportunity.) For Cassini’s pictures, go to the Cassini Raw Images Website, and click the arrow to browse the most recent pictures, or select Saturn or one of its moons from the little dropdown menu, and browse data fresh from Saturn to your heart’s content.
Just a decade ago, you would have had to wait for months or years for these data, and even then there’d be a steep learning curve to figuring out how to even open the files, and probably a drive to some NASA library to get your hands on the data in the first place. Now, you can just sit at home and see views of Saturn and its moons that are just hours old. Depending on the time of day that you hit JPL’s website, you could even be among the first people in the world to see these brand-new images.
Cassini followed its stunt-flight past Enceladus with a close encounter with a larger moon, Dione. Here’s one pretty picture from that encounter. To see more, and an explanation of how I make pretty pictures like this from the raw image data, visit my blog at The Planetary Society’s website!