On Aug. 17, Dione's thin, icy crescent filled Cassini's sky for the last time.
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On Aug. 17, Dione's thin, icy crescent filled Cassini's sky for the last time.

In Space, Every Goodbye Could Be Our Last

Getting to know a new place in space is kind of like becoming familiar with a new city: It reveals itself slowly, a patch of terrain or a new neighborhood here, a curious landform or hidden garden there. Over time, all those pieces assemble themselves into an image of somewhere we think we know.

But it’s necessarily superficial. For the most part, our glances are too fleeting to disentangle the threads of history, whether geologic or cultural, that really form the fibers of a place. And brief visits don’t offer much of a chance to dig beneath a world’s surface. Think about the richness of information that’s buried beneath cities, in bootleggers’ tunnels and abandoned train stations, in catacombed boneyards and the slices of life that tour operators don’t necessarily want you to see. It takes a bit of effort to really explore a metropolis, to crawl into its nooks and crannies and scoop up enough clues to say you’ve finally started getting to know it. How often have we visited somewhere new and thought, “I’ll have to check that out next time,” with a somewhat wistful pang?

In space, next times are few and far between. Think about Pluto, and all those features we’d take a closer look at if only we could send a second spacecraft zooming by. Or Uranus and Neptune, two giant planets that just now, more than a quarter-century after our first quick visit, might be inching back onto the itinerary. The same is true for distant galaxies, exploding stars, and ghostly nebulae, which often appear in the eyepiece only after competitive observing time is awarded.

Each goodbye could be our last.

Ten days ago, NASA’s Cassini spacecraft took its last close look at Dione, one of Saturn’s small icy moons (some photos from the flyby are in the gallery above). It flew just 474 kilometers above Dione’s surface, part of which is cut with chasms and cliffs, forming a “wispy terrain” that surprised scientists when Cassini brought it into focus in 2004. Dione, at just 1,123 kilometers across, is one of many intriguing worlds orbiting the spectacularly ringed planet; but once the Cassini mission ends in 2017, there’s no guarantee we’ll be going back.

Fortunately, we’ve got another year left to stare at Ceres, the largest world in the asteroid belt and a dwarf planet in its own right. NASA’s Dawn spacecraft has been orbiting the cratered sphere — at 950 kilometers across, it’s pretty close to Dione in size — since March, returning ever-closer and more confusing views of the planet’s enigmatic bright spots and strange, barnacle-shaped mountain. It’s the best kind of world to explore, a world that isn’t performing according to plan.

Rounding out the gallery above are a new Hubble image of an old friend (hooray!), and a composite image showing two colliding galaxy clusters far, far away. As these clusters slammed into one another, they excited a long-dead cloud of electrons that had originally been stirred up and energized by a nearby supermassive black hole. Over time, that bright cloud of electrons faded. But the collision re-energized the cloud, causing it to emit radio waves and producing what’s called a “radio phoenix,” resurrected from cold, cosmic ashes.

There must be a metaphor somewhere in there.