Starting at about the orbit of Neptune and stretching out to roughly 50 astronomical units (1 AU = Earth-Sun distance), or 7.5 billion kilometers from the Sun, is a region called the Kuiper Belt. It’s like a giant version of the more familiar asteroid belt between Mars and Jupiter—20 times as wide and between 20 to 200 times more massive. We need to learn more about it by having spacecraft fly into it and send back close-up images of some Kuiper Belt Objects (KBOs). The first of these long-distance robot explorers—the New Horizons mission—is well on its way. Astronomers have already discovered more than a thousand KBOs and believe there may be upwards of 100,000 with diameters exceeding 100 km (62 miles). The largest known of them is a world that until recently was considered one of the nine planets of the solar system—Pluto. But the realization that Pluto has an orbit, composition (mostly ices), and size (just 2,300 km across) that fit the KBO bill has led to its reclassification as a dwarf planet.
Other big KBOs include Quaoar, over 1,200 km across, and Makemake, with a diameter of about 1,400 km, and there may be undiscovered bodies in the Kuiper Belt that are larger than Pluto. In fact, Pluto is exceeded in size by an object called Eris, which has taken Pluto’s old slot as ninth biggest and most massive world in orbit around the Sun, but Eris is classified not as a KBO but as a Scattered Disk Object. Scientists also believe that Neptune’s largest moon, Triton, which is slightly larger than Pluto, may be a KBO that was captured long ago by the giant planet’s gravity.
Although four spacecraft from Earth—Voyagers 1 and 2, and Pioneers 10 and 11—have already flown farther from the Sun than the outer edge of the Kuiper Belt, none of these was on a path that allowed study of objects in the Belt. The first spacecraft to be specifically targeted at KBOs is New Horizons, which will encounter Pluto and its collection of moons on July 14, 2015. Pluto’s demotion to dwarf planet status occurred while New Horizons was en route. The spacecraft will send back our first clear views of this largest of KBOs, along with its big moon Charon—itself among KBO heavweights—before heading deeper into the Belt.
The hope is that, having made a thorough survey of the Pluto system, New Horizons will be put on course to make a close flyby of at least one other member of the Kuiper Belt, probably in the size range 50 to 100 km (31 to 62 miles) across—and chosen so that its composition is somewhat different than that of Pluto. Whereas Pluto has a reddish tinge to it, scientists will be looking for the next target to be more white or gray in color, indicative of a different substance on the surface.
New Horizons doesn’t have the ability to drastically alter its course after leaving Pluto, so potential targets will need to lie reasonably close to its flight path. Fortunately our knowledge of smaller KBOs and their whereabouts is set to grow dramatically through ground-based searches such as the Pan-STARRS survey project, which began operations in 2010 and will eventually involve an array of four 1.4 gigapixel digital cameras. Over the next few years Pan-STARRS is expected to increase the number of known KBOs 20-fold, enabling candidates to be chosen for New Horizons to explore.
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