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Neptune's innermost moon revealed in Hubble images

Charles Black, Founder and CEO of Sen
Oct 18, 2013, 7:00 UTC

Sen—Astronomers have found images of Neptune's tiny innermost moon, Naiad, whilst analysing data obtained back in 2004 by the Hubble space telescope. 

Naiad was discovered by the Voyager 2 space probe as it sped past Neptune in 1989, but has not been seen since.

A team of astronomers led by Dr Mark Showalter, a senior research scientist at the SETI Institute, used new techniques to filter out the glare from bright Neptune to reveal Naiad moving across a sequence of eight images taken by Hubble in December 2004.

"Naiad has been an elusive target ever since Voyager left the Neptune system" explained Dr Showalter.


Voyager 2's image of Naiad captured on September 18, 1989. Its high orbital speed of about 43,000 km per hour caused faint streaks in this exposure. Image credit: NASA

The researchers were surprised to find that the small moon, which has a diameter of about 66 km, appeared to have veered off course significantly from its predicted orbital position. Further study will be required to explain its orbit which might be affected by gravitational interplay with one of Neptune's other moons. 

Naiad is named after a nymph who presided over fountains, wells, springs, streams and brooks in Greek mythology.

As reported by Sen in July, during their study of the archived Hubble data the team also discovered a fourteenth moon orbiting Neptune, designated S/2004 N 1.

Image of Neptune and its moons by Hubble

Neptune and some of its moons. S/2004 N 1 is visible as a faint dot. Only the third moon from the center -- Despina -- is not shown, because it was positioned behind the occulting mask, along with Neptune, at the time the images were taken. Image credit: M. Showalter/SETI Institute

As well as finding Naiad hidden amongst the Hubble images, the astronomers also studied Neptune's faint rings and ring-arcs (partial rings). Two of the four arcs imaged by Voyager 2 have been fading away and one appears completely absent from the Hubble images. The cause of this is not yet understood.

Neptune has 14 satellites, the largest being Triton, discovered in 1846 (just 17 days after the planet was found) by brewer and astronomer William Lassall. The second largest moon, Proteus, is dark and close to Neptune and was discovered along with five other satellites by Voyager 2. The third largest moon, Nereid, was discovered in 1949 by Dutch astronomer Gerard Kuiper. Further moons were observed from Earth in 2002 and 2003, whilst the fourteenth moon was discovered in 2013.

Neptune was discovered in 1846. It takes 165 years to complete one orbit around the Sun at an average distance of 4.5 billion kilometres (2.8 billion miles).

Neptune does not have a solid surface though is believed to have a small rocky core. Neptune's atmosphere consists primarily of hydrogen, helium and methane, the latter giving the gas giant its blue colour.

Dr Showalter's collaborators on the research were Dr Jack Lissauer of the NASA Ames Research Center, Dr Imke de Pater of UC Berkeley and Robert French of the SETI Institute.

So far 173 moons have been discovered around the eight planets in our Solar System (excluding the five moons around the dwarf planet Pluto). The scores so far are: Mercury - 0, Venus - 0, Earth - 1, Mars - 2, Jupiter - 67, Saturn - 62, Uranus - 27 and Neptune - 14.

Dr Showalter has discovered several moons in the Solar System. In 1990, he found Saturn’s innermost satellite Pan. Then in 2003 he was joint discoverer of Mab and Cupid orbiting Uranus. He also led the teams that found Kerberos and Styx orbiting Pluto.