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.
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.
