Simulated image
of the Pluto-Charon system. The Pluto-Charon binary (represented by the
two largest white disks), the four small satellites -- P5, Nix, P4, and
Hydra (represented by the four small white disks), and three smaller
satellites (represented by the green disks) lie within an extended
ensemble of solid particles shown as small blue dots. This configuration
is the result after twenty years of a computer simulation with two
million dust particles surrounding the known and predicted moons. On
this short time scale, the small satellites clear out most of the dust
particles along their orbits. On much longer time scales, satellites
will clear dust from a larger fraction of their orbits. Credit: Scott Kenyon and Ben Bromley. Low Resolution Image (jpg)
In 2015, NASA's New Horizons spacecraft will
encounter the binary planet Pluto-Charon and its coterie of small
satellites. Discovered in June 2005, the satellites Nix and Hydra orbit
Pluto-Charon at distances roughly 40 times (Nix) and 55 times (Hydra)
larger than the radius of Pluto. Two other satellites, now known as P4
and P5, appeared on images from the Hubble Space Telescope in 2011-2012
and have similar orbits. With a planned closest approach to Pluto-Charon
of only 10 Pluto radii, the New Horizons spacecraft must have a
trajectory that avoids the satellites as it passes through the system.
New computer simulations by Scott Kenyon of SAO and Ben Bromley of the
University
of Utah suggest New Horizons will have to dodge a few other satellites
and may need to avoid a disk composed of small snowballs. Using a code
developed to simulate the formation of a planetary system around a star
like the Sun, Kenyon & Bromley explored whether satellites could
grow in a disk of small
particles surrounding Pluto-Charon. In their picture, the disk consists
of icy material captured from the Kuiper Belt or left over from the
giant impact thought to produce Pluto-Charon. Within this disk, small
particles collide and merge into larger and larger objects which
eventually become stable satellites orbiting Pluto-Charon.
The simulations typically form a few satellites with diameters of 10-30
km, similar to the sizes of Nix and Hydra, and many satellites with
diameters of a few kilometers. After 10 million years of simulation
time, all of the
satellites orbit within a tenuous disk of very small particles with
diameters of a centimeter or less. The accompanying image shows one
outcome with a more massive disk and three predicted satellites lying
outside the orbit of Hydra.
Although the small sizes of the predicted satellites preclude detection
with the Hubble Space Telescope, New Horizons can identify them. When
the spacecraft is roughly 70 days away from Pluto, its camera will begin
to search for satellites smaller than P5. A few weeks out, images might
reveal a tenuous disk or an ensemble of rings outside the orbit of
Hydra. The New Horizons observations will test Kenyon & Bromley's
model and give us new clues about the origin of Pluto and the solar
system.
