As NASA's New Horizons deep space probe heads for its July rendezvous with Pluto, it's not only revealing the secrets of the dwarf planet, but of its moons as well. On the 85th anniversary of Pluto's discovery, the unmanned spacecraft sent back its first look at the small moons Nix and Hydra. Taken by New Horizons’ Long-Range Reconnaissance Imager (LORRI), the images will help space scientist better understand their orbits.

Nix and Hydra are two of five known Plutonian moons, and were discovered by the New Horizons team in 2005 using images from the Hubble Space Telescope. The new long-exposure images sent back on February 18 were captured by the LORRI from January 27 to February 8 of this year at a distance of 125 million to 115 million miles (201 million to 186 million km) using a special mode that NASA says combines pixels in a way that increases sensitivity over resolution.

The result was a seven-frame movie with each frame made of up five 10-second images. These were released in two sets seen in the main image above. The left-hand images shows the moons against the glare of Pluto, its largest moon Charon and a backdrop of stars. The right-hand image has been enhanced to make the small moons more visible. There is also a bright and dark streak caused by the imager suffering overexposure by Pluto and Charon.

So far, Pluto is known to have five moons, but astronomers believe that there may be as many as 10, plus ring systems. Hydra is the outermost at a distance of about 40,200 miles (64,700 km) and a period of 38 days, while Nix is 30,360 miles (48,700 km) away with a period of 25 days. Both moons are very small with a diameter of between 25 and 95 miles (40 to 150 km), though the moons Styx and Kerberos are smaller still and are not yet visible to the robotic explorer.

The US$650 million New Horizons mission was launched January 19, 2006 atop an Atlas V rocket from Cape Canaveral, Florida and has so far traveled 3 billion miles (4.8 billion km). The 1,054 lb (478 kg) nuclear-powered probe is on a 9.5-year mission to fly by Pluto and then on to study selected objects in the Kuiper Belt. Sent on a slingshot trajectory using the gravitational pull of Jupiter, New Horizons passed the orbit of Neptune on August 24 last year and will rendezvous with Pluto on July 14 2015, which it will pass at a distance of 8,000 miles (13,000 km).

NASA says that the moon images are the first in a series of long-exposure pictures that the space agency will collect through March to help scientists gain a better understand of the Plutonian moons' orbits.

Source: NASA

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