Something odd happened a few months after NASA’s deep space probe New Horizons was launched Jan. 19, 2006—its target got demoted from a planet to a “dwarf planet.” In September 2006, astronomers voted to downgrade Pluto so that it now falls into the same category as Ceres, previously considered the largest asteroid in our solar system. New Horizons will be the first spacecraft to conduct a flyby of what used to be the ninth planet in the solar system and its system of five known moons.
Not that Pluto’s reclassification makes it any less interesting. Even though it’s smaller than Earth’s Moon—with a diameter of only about 2,300 kilometers—it does have five moons of its own, including one called Charon, which is two-thirds Pluto’s size. Pluto also has a fascinating past, which scientists hope New Horizons will help shed some light upon.
It used to be thought that Pluto might be an escaped moon of the eighth planet, Neptune, which got kicked out when the gas giant captured a much bigger world, Triton. But there were always problems with that theory because Pluto’s orbit, although very elongated and at times bringing it closer to the Sun than Neptune, never carries it close to Neptune itself. Nowadays, astronomers recognize Pluto to be the largest member of the Kuiper Belt—a ring of objects located between 30 and 50 AU (1AU = the average distance from the Earth to the Sun) from the Sun. In some respects, Pluto is like a giant comet nucleus, and if it were brought close to the Sun it would develop a tail like that of a comet.
At the time of writing, New Horizons is 26 AU from the Sun, having left Earth more than seven years ago with the highest-ever launch speed for a man-made object—58,536 km/h (16.26 km/s). It will sweep past Pluto and its moons July 14, 2015, coming, on its present planned course (though that may change), within 10,000 kilometers of Pluto and 27,000 kilometers of Charon.
Even our best telescopes on Earth and in space, including the Hubble Space Telescope, have never shown details on Pluto’s surface, other than bright and dark smudges several hundred kilometers across, so New Horizons will give us our first clear view of the dwarf planet and its outsized moon. Intriguingly, Pluto shows dramatic variations in brightness and color, ranging from charcoal black, through dark orange, to white. No one yet knows why.
Other questions are related to Pluto’s thin atmosphere of nitrogen, methane, and carbon monoxide—similar in composition to the ice coating on the surface—and to changes in the surface that have been observed since the mid-1990s. Pluto’s northern polar region has been seen to brighten and the southern hemisphere to darken, a seasonal effect no doubt related to the dwarf planet’s rapidly changing distance from the Sun as it tracks around its very elliptical orbit. Pluto’s color—notably its redness—has also changed.
Are there volcanoes spewing ice on Pluto? Are there rings around this lonely world, discovered less than a century ago? Are there more moons waiting to be found? In less than two years’ time, as New Horizons begins the Pluto-Charon observing phase of its mission, six months ahead of arrival, we’ll begin to discover the answers.Missions » New Horizons »