New Horizons got a great shot of Neptune’s moon Triton last fall, as it was trucking toward Pluto and the Kuiper Belt.
The mission was 2.33 billion miles (3.75 billion kilometers) from Neptune on Oct. 16, when its Long Range Reconnaissance Imager (LORRI) locked onto the planet and snapped away. The craft was following a programmed sequence of commands as part of its annual checkout. NASA released the image Thursday afternoon.
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Mission scientists say the shot was good practice for imaging Pluto, which New Horizons will do in 2015. Neptune’s moon Triton and Pluto — the former planet retitled in 2006 as the ambassador to the Kuiper Belt — have much in common.
“Among the objects visited by spacecraft so far, Triton is by far the best analog of Pluto,” said New Horizons Principal Investigator Alan Stern.
Triton is only slightly larger than Pluto, boasting a 1,700-mile (2,700-kilometers) diameter compared to Pluto’s 1,500-mile (2,400-kilometer) girth. Both objects have atmospheres primarily composed of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures. Temperatures average -390 degrees F (-199 degrees C) on Triton and -370 degrees F (-188 degrees C) on Pluto.
Triton is widely believed to have once been a member of the Kuiper Belt that was captured into orbit around Neptune, probably during a collision early in the solar system’s history. Pluto was the first Kuiper Belt object to be discovered.
Furthermore, “We wanted to test LORRI’s ability to measure a faint object near a much brighter one using a special tracking mode,” said New Horizons Project Scientist Hal Weaver, of Johns Hopkins University, “and the Neptune-Triton pair perfectly fit the bill.”
LORRI was operated in 4-by-4 format (the original pixels are binned in groups of 16), and the spacecraft was put into a special tracking mode to allow for longer exposure times to maximize its sensitivity.
Mission scientists also wanted to measure Triton itself, to follow up on observations made by the Voyager 2 spacecraft during its flyby of Neptune in 1989. Those images revealed evidence of cryovolcanic activity and cantaloupe-like terrain. New Horizons can observe Neptune and Triton at solar phase angles (the Sun-object-spacecraft angle) that are not possible to achieve from Earth-based facilities, yielding new insight into the properties of Titan’s surface and Neptune’s atmosphere.
New Horizons is currently in electronic hibernation, 1.2 billion miles (1.93 billion kilometers) from home, speeding away from the Sun at 38,520 miles (61,991 kilometers) per hour. LORRI will continue to observe the Neptune-Triton pair during annual checkouts until the Pluto encounter in 2015.
LEAD IMAGE CAPTION: The top frame is a composite, full-frame (0.29° by 0.29°) LORRI image of Neptune taken Oct. 16, 2008, using an exposure time of 10 seconds and 4-by-4 pixel re-binning to achieve its highest possible sensitivity. The bottom frame is a twice-magnified view that more clearly shows the detection of Triton, Neptune’s largest moon. Neptune is the brightest object in the field and is saturated (on purpose) in this long exposure. Triton, which is about 16 arcsec east (celestial north is up, east is to the left) of Neptune, is approximately 180 times fainter. All the other objects in the image are background field stars. The dark “tails” on the brightest objects are artifacts of the LORRI charge-coupled device (CCD); the effect is small but easily seen in this logarithmic intensity stretch. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)