Object: Neptune鈥檚 tiny moon, Naiad
Size: 100 kilometres across
Favourite game: Hide and seek
Neptune鈥檚 moon Naiad is a coy sprite. The small moon kept close to its parent planet to hide in the brighter body鈥檚 reflected glare, and it zipped around at erratic speeds singing 鈥淐atch me if you can!鈥� Only once did Naiad stick its head out to wave at a passing spacecraft. It has hidden ever since.
Now after 24 years, the Hubble Space Telescope has found Neptune鈥檚 long-lost ward 鈥� and its strange behaviour hints that the planet鈥檚 entire family is heading towards chaos.
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Naiad was discovered closely orbiting Neptune during a fly-by of the Voyager 2 spacecraft in 1989. Images beamed back to Earth showed the moon to be roughly 100 kilometres wide, with an orbit lasting just 7 hours. Then the probe departed. Despite efforts to find Naiad using ground-based telescopes and a 2004 search with Hubble, no sure sign of the moon turned up.
at the SETI Institute in California and his colleagues wondered if they could find Naiad using a technique that just recently helped them uncovered an even smaller, entirely new Neptunian moon. They overlaid eight Hubble shots from the 2004 search effort to mimic a much longer exposure and reveal features that were otherwise too faint to see. Sure enough, Naiad showed itself.
Wibbly wobbly
The image-stacking work, presented on 8 October at an , revealed that Naiad had been hidden for so long in part because its orbit is very different from what was predicted based on the Voyager data. Showalter thinks Neptune鈥檚 other moons are pushing and pulling on Naiad, making its orbit hard to predict.
鈥淭here鈥檚 probably some kind of a perturbation going on that is making its orbit kind of wobble,鈥� he says. 鈥淥ver periods of a decade or so, it speeds up and slows down in a way that is not entirely predictable.鈥� These wobbles could be the first indications that Neptune鈥檚 entire moon system is unstable, he adds, although it could take tens of billions of years for the effects to cause catastrophic collisions.
Now that the image-stacking technique has gained more ground, Showalter plans to turn it on other targets, such as Uranus鈥檚 missing moon Cordelia, which has only ever been spotted by Voyager 2. 鈥淭hat鈥檚 my remaining personal challenge,鈥� says Showalter. 鈥淚鈥檓 getting close.鈥�
