A bright inner moon of Neptune that has been missing in action since 1989 has been spotted, astronomers said Tuesday.
Credit for the re-discovery of Naiad, the giant plant's closest moon, goes to Mark Showalter, with the SETI Institute in Mountain View, Calif., and colleagues who devised a new way to analyze archived Hubble Space Telescope images.
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Astronomers have been on the hunt for Naiad, which orbits about 30,000 miles away from Neptune, since it was discovered in photographs snapped by NASA's visiting Voyager 2 spacecraft in 1989.
"There's a reason we had so much trouble finding it. It's in the wrong place," Showalter told reporters during a webcast press conference at the American Astronomical Society meeting in Denver on Tuesday.
Based on analysis of Naiad's orbit extrapolated from Voyager 2 data, astronomers had expected to find the moon about 80 degrees away from where it actually turned up in archived 2004 Hubble images.
"That's a surprisingly large amount," Showalter said.
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Scientists aren't sure why their predictions were so far off. One theory is that Naiad's sister moons have been exercising their gravitational muscles and elbowed Naiad off the projected course.
Another idea is that the uncertainties in the Voyager data were large enough to account for the moon's unexpected location.
"It is possible, if you really push the limits, that we could actually just have orbital uncertainties here," Showalter said. "That has to be looked at a lot more closely."
Naiad is one of 14 known moons of Neptune.
Image: The location of Naiad, as re-discovered in Hubble data. Credit: NASA/SETI