New Horizons Spots Triton
12 Mar 2009
(Source: Johns Hopkins University Applied Physics Laboratory)
Add another moon to the New Horizons photo gallery: the spacecraft's
Long Range Reconnaissance Imager detected Triton, the largest of
Neptune's 13 known moons, during the annual spacecraft checkout last fall.
New Horizons was 2.33 billion miles (3.75 billion kilometers) from
Neptune on Oct. 16, when LORRI, following a programmed sequence of
commands, locked onto the planet and snapped away.
"We wanted to test LORRI's ability to measure a faint object near a much
brighter one using a special tracking mode," says New Horizons Project
Scientist Hal Weaver, of the Johns Hopkins University Applied Physics
Laboratory, "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. "We needed to achieve the highest possible
sensitivity," Weaver adds.
Mission scientists also wanted to measure Triton itself. "Among the
objects visited by spacecraft so far, Triton is by far the best analog
of Pluto," says New Horizons Principal Investigator Alan Stern. The
Voyager 2 spacecraft took spectacular images of Triton during its flyby
of Neptune in 1989, showing evidence of cryovolcanic activity and cantaloupe-like
Triton is only slightly larger than Pluto (1,700 miles [2,700
kilometers] in diameter compared to Pluto's 1,500 miles [2,400
kilometers]). 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 (-390? F on Triton and -370? F on
Pluto). Triton is widely believed to have once been a member of the
Kuiper Belt (as Pluto still is) that was captured into orbit around
Neptune, probably during a collision early in the solar system's history.
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, and this unique perspective can provide insight
into the properties of Triton's surface and Neptune's atmosphere.
LORRI will continue to observe the Neptune-Triton pair during annual
checkouts until the Pluto encounter in 2015.
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.
What other moons has New Horizons photographed? Check out the collection
from the 2007 Jupiter flyby: http://pluto.jhuapl.edu/gallery/sciencePhotos/view.php?gallery_id=2&page=1&bytopic=38