Neptune's Long Lost Moon Reappears

Neptune has a moon called Naiad that was discovered by Voyager 2 in 1989 when the space probe made its flyby — but it hasn't been heard of since. Now, some 24 years later, astronomers have finally been able to spot it from Earth, and they did so using archived photographs.

Naiad is an irregularly shaped moon that's about 60 miles (100 km) across and orbits 14,600 miles (23,500 km) above Neptune. Its orbit is decaying and it'll eventually smash into Neptune's atmosphere, or possibly break up into a planetary ring.

Neptune's Long Lost Moon Reappears

Simulated view of Naiad via JiFish.

After receiving the Voyager 2 image (below), scientists have desperately tried to relocate it. The Neptune system has since been studied from ground-based telescopes, the Hubble, and even adaptive optics. All four of Neptune's inner satellites have been re-detected, but Naiad remained out of sight.

Neptune's Long Lost Moon Reappears

But now, Mark Showalter at the SETI Institute in California and his colleagues have spotted it — and they did so by using a technique that recently helped them uncover an even smaller, entirely new Neptunian moon. By taking eight archived Hubble images from 2004, and laying them on top of each other, they created a virtual long-exposure shot of the Naiad — a glimpse of the elusive moon that would have otherwise been too faint to see.

And it turns out that scientists had been looking in the wrong place. They had miscalculated Naiad's location by a factor of 80 degrees. But it's not really their fault; Naiad has a funky orbit.

"There's probably some kind of a perturbation going on that is making its orbit kind of wobble," noted Showalter. "Over periods of a decade or so, it speeds up and slows down in a way that is not entirely predictable." These wobbles may indicate that Neptune's entire moon system is unstable — although it could take tens of billions of years for the effects to cause catastrophic collisions.

The findings were recently presented at an AAS meeting in Colorado.

Top image: M. Showalter/SETI Institute.