Neptune moon: Tiny, dark, whizzing space ball captured on film (barely)
Neptune moon: Astronomer Mark Showalter used over 150 pictures of Neptune to find an almost-invisible moon of Neptune, bringing the total number of Neptune moons to 14.
Finding moons should be easy, right? Look how bright ours is, glowing away up there.Skip to next paragraph
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Except that our moon is (1) ridiculously close by, which makes it look bright, (2) ridiculously big, and (3) slow. Other moons like this – big, slow, bright – are relatively easy to find. Galileo spotted the four biggest moons of Jupiter back in the 17th century, using a telescope that worked about as well as a $50 pair of binoculars.
It's harder to find moons that don't follow this pattern. If they're small, or dark, it's much, much harder. Throw in fast – ridiculously fast – and it's no wonder that the most recently discovered moon of Neptune took years to find. This little planet, like a fidgety toddler, "never sits still long enough to have its picture taken," wrote Mark Showalter, the SETI researcher who discovered the tiny moon on July 1.
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In fact, it travels over 350,000 miles in less than one Earth day, giving it a speed of about 15,000 miles per hour. It's not the fastest moon in the solar system – Metis, zipping around Jupiter three times a day, goes more than twice as fast – but it's still too fast to photograph easily.
The peppy little moon, currently identified only as S/2004 N 1, appeared in pictures taken by the Hubble Space Telescope between 2004 and 2009. But that's using "appeared" pretty generously. As Dr. Showalter wrote today, "We estimate that it is no more than 20 km across and as dark as if it were paved with dirty asphalt. Naturally, taking its picture requires long exposures. But there’s the rub. If you expose it for too long, the moon vanishes in a blur..." The size and color are both calculations, not measurements, so it's possible that the moon is brighter than asphalt, but that would mean that it's also smaller. So how do you find something small, dark, and fast?
Well, you don't depend on the human eye's ability to spot things. Our eyes are good at seeing color and motion, but they're terrible at seeing tiny, dark smudges against a black sky. So Showalter, a research scientist working on several NASA missions to the outer planets, wrote a computer program to help him comb through Hubble's 150 or so pictures of Neptune. "The procedure I devised predicts where any given moon ought to move from one image to the next," he writes, "and then combines the images with a 'twist' that compensates for the expected motion." He wasn't looking for a moon – after all, he didn't know there was a moon to look for – but developed the technique to examine some arcs in Neptune's rings, which are also nearly invisible.