Pluto’s got five moons, not bad for a dwarf planet only 68% the size of our moon. Its largest, Charon, was recently captured in a video by NASA’s New Horizons spacecraft, due to encounter the once-planet on July 14 this year. New photos from the probe taken between Jan. 27 and Feb. 8, at distances ranging from about 125 million to 115 million miles (201 million to 186 million km) now show two more – Nix and Hydra.
Assembled into a seven-frame movie, the new images provide the spacecraft’s first extended look at Hydra (yellow diamond) and its first-ever view of Nix (orange diamond). NASA released the photos on a very special day, the 85th anniversary of Pluto’s discovery in 1930 by Clyde Tombaugh working at Lowell Observatory in Flagstaff, Arizona.
“This first good view of Nix and Hydra marks another major milestone, and a perfect way to celebrate the anniversary of Pluto’s discovery,” said New Horizons science team member John Spencer.
Nix and Hydra were discovered by New Horizons team members in Hubble Space Telescope images taken in 2005. Hydra, Pluto’s outermost known moon, orbits Pluto every 38 days at a distance of approximately 40,200 miles (64,700 km), while Nix orbits every 25 days at a distance of 30,260 miles (48,700 km). Each moon is probably between 25-95 miles (approximately 40- 150 kilometers) in diameter, but that’s just an estimate. We won’t nail down precise sizes until the probe gets there this summer.
Pluto’s two other small moons, Styx and Kerberos, are still smaller and too faint to be seen by New Horizons at its current distance from Pluto; they’ll become visible in the months to come.
Tombaugh built his first telescope in 1926 at the age of 20. Just four years later, using a 13-inch telescope at Lowell Observatory, he discovered Pluto. The young astronomer was looking for the so-called “Planet X”, a postulated planet beyond Neptune thought to be responsible for irregularities in the orbital motion of Uranus.
Later calculations showed that there was no need for Planet X once Neptune’s mass and its effects on Uranus were accurately calculated. No matter, Tombaugh turned up this extremely distant object just the same. From 1930 till 2006 we all thought of it as the 9th planet until it was reclassified as a dwarf planet, a slight step up from an asteroid.
Hunts for additional remote planets have increased in recent years. None has yet to be discovered, but in their searches astronomers have discovered additional dwarf planets far beyond the realm of Neptune.