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National Aeronautics and Space Administration
Pluto Moon Discovered
National Aeronautics and Space Administration
The full contents of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
Science Year
In Review
National Aeronautics and Space Administration
Hubble 21: science Year in review
Produced by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
Hubble 21: science Year in review
Taken from:
Hubble 2011: Science Year in Review
Science Year
In Review
New Pluto Moon Discovered
At this moment, an unmanned NASA spacecraft is hurtling toward the edge of the solar system at the speed of 35,000 miles
per hour. The craft, called New Horizons, will encounter the icy dwarf planet Pluto in 2015. What will New Horizons find
when it gets there? Recent Hubble observations have provided a partial but important answer to this question. In July 2011,
astronomers used Hubble to confirm the presence of a fourth moon in orbit around this distant world. Known simply (for
now) as P4, the small body joins Charon, Nix, and Hydra as the other known satellites of Pluto. The discovery indicates that
the moon system around Pluto is more complex than once thought—and potentially more hazardous to visiting spacecraft.
Clyde Tombaugh discovered Pluto in 1930 using the 13-inch refracting telescope at the Lowell Observatory in Arizona. Once
the planetary body’s orbit was determined, the new member of the solar system was found to be nearly 3.5 billion miles from
Earth. At the time, astronomers could only see Pluto as a tiny dot of light; no moons could be discerned. Based on their best
understanding of the orbital perturbations perceived in the other planets, astronomers had been expecting to find a body
with a mass similar to Earth’s. These initial estimates were decreased based on Pluto’s tiny diameter. It took nearly 50 years,
however, before scientists could actually measure its mass.
In 1978, astronomers using ground-based telescopes discovered that Pluto had a bulge on one side. The bulge was much
too big to be a mountainous surface feature, however. It also shifted position in accordance with Kepler’s laws of orbital
motion. This meant that the bulge must be an orbiting companion (albeit a relatively large one) located too close to Pluto
to be seen as a distinct object. In time, measurements of the companion’s orbital motion enabled the first accurate estimate
of Pluto’s mass—less than 1 percent of Earth’s, or about 17 percent of Earth’s Moon. Its newly discovered companion was
eventually named Charon.
The dwarf planet Pluto has more moons than any of the rocky, interior planets of the solar system: Mercury, Venus, Earth, and Mars. Located
approximately 3.6 billion miles from the Sun, (39 times the Earth–Sun distance) and visible only through reflected sunlight, the diminutive
Pluto and its even smaller moons (shown in this artist’s illustration) are extremely faint as seen from Earth.
June 28, 2011
July 3, 2011
These two Hubble images, taken about a week apart, show four moons orbiting the distant and icy dwarf planet, Pluto. The green circles mark the newly
discovered moon, P4, uncovered by Hubble in June 2011.
In 1990, astronomers used Hubble to make the first of many Pluto moon-system discoveries when they recorded visible-light
photographs that clearly showed a separation between Pluto and Charon. Hubble images of the pair allowed astronomers
to measure the diameters of Pluto and Charon directly. At 746 miles, the diameter of Charon is slightly more than one-half
the diameter of Pluto (1,485 miles). Pluto and Charon orbit about a common center of gravity that is located outside Pluto’s
physical radius. Consequently, astronomers sometimes refer to Pluto and Charon as an example of a binary (dwarf) planet
rather than a dwarf planet with its moon.
More news from Hubble came in 2005 when a team of astronomers from the New Horizons mission found two additional
moons orbiting Pluto. The team’s leaders were Hal Weaver of the Johns Hopkins University Applied Physics Laboratory in
Maryland and Alan Stern of the Southwest Research Institute in Colorado. These two tiny worlds, named Nix and Hydra,
reside beyond the orbit of Charon. The team conducted additional long exposures to search for more satellites, but none
were found.
It was in June 2011, while studying an even deeper Hubble exposure searching for a ring around Pluto that Mark Showalter
of the SETI Institute in California uncovered yet another faint moon. Nestled between the orbits of Nix and Hydra, it has
This illustration shows the relative sizes of the
orbits of Pluto’s moons: Charon, Nix, P4, and
Hydra. Their orbits are nearly circular but appear
slightly elongated, as they are tilted to our line of
sight. Pluto and its moons all revolve about a center
of mass (barycenter) that is located approximately
522 miles (840 km) outside the surface of Pluto.
been temporarily dubbed P4 while
it awaits an official name from the
International Astronomical Union. P4 is
Pluto’s smallest moon yet, with an estimated
diameter of only 8 to 21 miles. By comparison, Nix
and Hydra are roughly 20 to 70 miles wide. The moon was
first seen in photos taken with Hubble ’s Wide Field Camera 3 on
June 28, 2011. The sighting was quickly confirmed in follow-up
Hubble observations taken on July 3 and July 18.
Clues to the origin of Pluto’s moons are found in the shapes
and orientations of their orbits. All four appear to be co-planar
and nearly circular. If the moons were gravitationally captured
objects they would more likely have inclined and elliptical orbits,
like the outermost moons of Jupiter. They are also in nearly resonant
orbits, that is, the orbital period of one body is related to that of another
by a simple fraction—something that develops via small perturbations over time.
If an orbiting body’s mass (m) is sufficiently large relative to the primary’s mass (M) and it is located
close to the primary, then the two will revolve about a common center of mass located
outside either body. The mass of Pluto’s moon Charon is approximately
11.6 percent that of Pluto. With a mean distance of about
12,180 miles (19,600 kilometers) from the center of Pluto, the
system’s barycenter is located .1/10 the distance from the
center of Pluto to the center of Charon.
This evidence indicates that the moons had plenty of time to interact gravitationally with each other, and therefore were
probably born together. This would be the case if the moons formed in a single titanic collision between Pluto and another
Kuiper belt object billions of years ago. This collision scenario is supported by increasingly sophisticated computer
simulations that predict the aftermath of such planetary encounters. The simulations show that the orbits of Pluto’s moons
would have circularized quickly, perhaps in only a few hundred million years. The simulations have not explained, however,
why the moons’ orbits are so mysteriously close to exact resonance—but not precisely so—after all this time.
As more tiny moons are discovered around Pluto, scientists grow concerned about the possible existence of smaller, rocky
debris surrounding them. Such debris could pose a serious threat to New Horizons. The gravitational fields of the new
moons are so small that they cannot recapture material thrown into space from impacts with other bodies. Collisions would
This prelaunch photo, taken at NASA’s Kennedy Space Center, shows technicians inspecting the gold-colored thermal blankets that surround the
New Horizons spacecraft. The compact probe carries seven scientific instruments. Using these instruments, scientists hope to characterize the global
geology and geomorphology of Pluto and its moon Charon, map their surface compositions and temperatures, examine Pluto’s atmosphere, and
possibly image Pluto’s other small moons. If the spacecraft remains healthy, it may also conduct subsequent flybys of select Kuiper belt objects beyond
the Pluto system.
produce clouds of rocks and particulates, all traveling up to tens of thousands of miles per second through the Pluto system.
Entering such a residual debris cloud would be like encountering a minefield to New Horizons as it flies through the system
at high speed. Even particles weighing less than a milligram can penetrate the protective blankets on the probe and damage
its electronics and sensors if encountered at high velocity.
Ongoing Hubble imaging of the Pluto system will play a critical role in establishing the best strategy for the New Horizons
spacecraft to fly through and document these distant worlds. Hubble ’s views of this system will be the clearest ones
astronomers will have until early 2015 when New Horizons is within six months of encountering the system.
Further Reading
Andrews, B. “Astronomers Find a New Moon Around Pluto.” Astronomy 39, no. 11 (November 2011): 16.
Beal, T. “Hubble Discovers Dwarf Planet Pluto Has Fourth Moon.” The Arizona Daily Star, July 21, 2011, A1.
Lemonick, M. D. “Pass Out the Cigars! Pluto Is a Papa.” Time, July 25, 2011.,8599,2084606,00.html.
Overbye, D. “Despite Downgrade, Pluto Adds to Entourage.” New York Times, July, 21 2011, A16.
Wall, M. “Pluto Moon Discovery Hints at Future Surprises for NASA Probe.” Christian Science Monitor, July 21, 2011.
“Welcome to the Solar System, P4.” New Scientist 211, no. 2823 (July 30–August 5, 2011): 14.
Weaver, H. A., et al. “Discovery of Two New Satellites of Pluto.” Nature 439, no. 7079 (February 23, 2006): 943–945.
Dr. Mark Showalter is a senior research scientist at the Search for Extraterrestrial Intelligence (SETI) Institute in
Mountain View, California. His research focuses on the rings and inner satellites of the giant planets. He has
worked with Hubble extensively over the years, having targeted every planetary body from Mars to Pluto in
various observing programs. His studies of the Uranus system between 2003 and 2006 led to the discoveries of
two small moons and two faint rings orbiting that planet. Dr. Showalter is also a co-investigator on the Cassini
mission to Saturn and manages the Rings Node of NASA’s Planetary Data System. He received a bachelor’s
degree in physics and mathematics from Oberlin College, and his master’s and doctoral degrees in astronomy
from Cornell University.