[meteorite-list] New Horizons Spots Pluto's Faintest Known Moons

[Ron Baalke]

http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150512


New Horizons Spots Pluto's Faintest Known Moons
May 12, 2015

It's a complete Pluto family photo -- or at least a photo of the family 
members we've already met.

For the first time, NASA's New Horizons spacecraft has photographed Kerberos 
and Styx -- the smallest and faintest of Pluto's five known moons. Following 
the spacecraft's detection of Pluto's giant moon Charon in July 2013, 
and Pluto's smaller moons Hydra and Nix in July 2014 and January 2015, 
respectively, New Horizons is now within sight of all the known members 
of the Pluto system.

"New Horizons is now on the threshold of discovery," said mission science 
team member John Spencer, of the Southwest Research Institute in Boulder, 
Colorado. "If the spacecraft observes any additional moons as we get closer 
to Pluto, they will be worlds that no one has ever seen before."

Drawing even closer to Pluto in mid-May, New Horizons will begin its first 
search for new moons or rings that might threaten the spacecraft on its 
passage through the Pluto system. The images of faint Styx and Kerberos 
shown here are allowing the search team to refine the techniques they 
will use to analyze those data, which will push the sensitivity limits 
even deeper.

Kerberos and Styx were discovered in 2011 and 2012, respectively, by New 
Horizons team members using the Hubble Space Telescope. Styx, circling 
Pluto every 20 days between the orbits of Charon and Nix, is likely just 
4 to 13 miles (approximately 7 to 21 kilometers) in diameter, and Kerberos, 
orbiting between Nix and Hydra with a 32-day period, is just 6 to 20 miles 
(approximately 10 to 30 kilometers) in diameter. Each is 20 to 30 times 
fainter than Nix and Hydra.

[Movie]

The images detecting Kerberos and Styx shown here were taken with New 
Horizons' most sensitive camera, the Long Range Reconnaissance Imager 
(LORRI), from April 25-May 1. Each observation consists of five 10-second 
exposures that have been added together to make the image in the left 
panel, and extensively processed to reduce the bright glare of Pluto and 
Charon and largely remove the dense field of background stars (center 
and right panels), in order to reveal the faint satellites, whose positions 
and orbits, along with those of the brighter moons Nix and Hydra, are 
given in the right panel. 

"Detecting these tiny moons from distance of over 55 million miles is 
amazing, and a credit to the team that built our LORRI long-range camera 
and John Spencer's team of moon and ring hunters," added New Horizons 
Principal Investigator Alan Stern, of the Southwest Research Institute.

Kerberos is visible in all of the images, though is partially obscured 
in the second image. Styx is not visible in the first image, only in subsequent 
ones; on April 25 it was obscured by electronic artifacts in the camera 
-- the black and white streaks extending to the right of the extremely 
overexposed images of Pluto and Charon in the center of the frame. These 
artifacts point in different directions in different images due to the 
varying orientation of the spacecraft. Other unlabeled features in the 
processed images include the imperfectly removed images of background 
stars and other residual artifacts.

Although Styx and Kerberos are more visible in some frames than others, 
perhaps due to brightness fluctuations as they rotate on their axes, their 
identity is confirmed by their positions being exactly where they are 
predicted to be (in the center of the circles in the right panel).

The Johns Hopkins University Applied Physics Laboratory (APL) designed, 
built, and operates the New Horizons spacecraft, and manages the mission 
for NASA's Science Mission Directorate. SwRI leads the science team, payload 
operations and encounter science planning. New Horizons is part of the 
New Frontiers Program managed by NASA's Marshall Space Flight Center in 
Huntsville, Alabama.