Pluto Moon Discovered

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1 National Aeronautics and Space Administration 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: Hubble 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 Hubble Science Year In Review NP GSFC NP GSFC

2 New 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 in 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, Nix, and Hydra as the other known satellites of. The discovery indicates that the moon system around is more complex than once thought and potentially more hazardous to visiting spacecraft. Clyde Tombaugh discovered 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 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 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 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 to be seen as a distinct object. In time, measurements of the companion s orbital motion enabled the first accurate estimate of s mass less than 1 percent of Earth s, or about 17 percent of Earth s Moon. Its newly discovered companion was eventually named. The dwarf planet 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 and its even smaller moons (shown in this artist s illustration) are extremely faint as seen from Earth. 49

3 P4 Hydra P4 June 28, 2011 Nix Hydra Nix July 3, 2011 These two Hubble images, taken about a week apart, show four moons orbiting the distant and icy dwarf planet,. The green circles mark the newly discovered moon, P4, uncovered by Hubble in June In 1990, astronomers used Hubble to make the first of many moon-system discoveries when they recorded visible-light photographs that clearly showed a separation between and. Hubble images of the pair allowed astronomers to measure the diameters of and directly. At 746 miles, the diameter of is slightly more than one-half the diameter of (1,485 miles). and orbit about a common center of gravity that is located outside s physical radius. Consequently, astronomers sometimes refer to and 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. 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. 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 that Mark Showalter of the SETI Institute in California uncovered yet another faint moon. Nestled between the orbits of Nix and Hydra, it has 50

4 This illustration shows the relative sizes of the orbits of s moons:, Nix, P4, and Hydra. Their orbits are nearly circular but appear slightly elongated, as they are tilted to our line of sight. and its moons all revolve about a center of mass (barycenter) that is located approximately 522 miles (840 km) outside the surface of. - barycenter P4 been temporarily dubbed P4 while it awaits an official name from the International Astronomical Union. P4 is s smallest moon yet, with an estimated Nix 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, The sighting was quickly confirmed in follow-up Hubble observations taken on July 3 and July 18. Hydra Clues to the origin of 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 s moon is approximately 11.6 percent that of. With a mean distance of about 12,180 miles (19,600 kilometers) from the center of, the m system s barycenter is located.1/10 the distance from the center of to the center of. M Barycenter 51

5 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 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 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, 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 and its moon, map their surface compositions and temperatures, examine s atmosphere, and possibly image s other small moons. If the spacecraft remains healthy, it may also conduct subsequent flybys of select Kuiper belt objects beyond the system. 52

6 produce clouds of rocks and particulates, all traveling up to tens of thousands of miles per second through the 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 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. Astronomy 39, no. 11 (November 2011): 16. Beal, T. Hubble Discovers Dwarf Planet Has Fourth Moon. The Arizona Daily Star, July 21, 2011, A1. Lemonick, M. D. Pass Out the Cigars! Is a Papa. Time, July 25, Overbye, D. Despite Downgrade, Adds to Entourage. New York Times, July, , A16. Wall, M. Moon Discovery Hints at Future Surprises for NASA Probe. Christian Science Monitor, July 21, Welcome to the Solar System, P4. New Scientist 211, no (July 30 August 5, 2011): 14. Weaver, H. A., et al. Discovery of Two New Satellites of. Nature 439, no (February 23, 2006): 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 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. 53