Pluto is a little larger than astronomers previously believed, the New Horizons spacecraft has revealed. This could reveal new information about the distant icy dwarf.
New Horizons is the first observatory to ever visit the distant body. When the vehicle first launched, in 2006, Pluto was still considered a major planet. Within months after liftoff, astronomers downgraded the status of the body to that of a dwarf planet.
The Kuiper Belt is a massive collection of rocks and other debris orbiting the sun far beyond the path of Neptune, the most distant planet. Another body in that system, Eris, was discovered in 2005, accompanied by at least one satellite, Dysnomia. Since the time of its discovery, astronomers have debated how large Eris is compared with Pluto. For a decade, the two icy worlds have competed against each other in the minds of astronomers for the title of the largest body in the Kuiper Belt. With this new finding, Pluto, once the smallest planet in the Solar System, is believed to be the largest object in this distant collection of debris.
Pluto has now been shown to measure 1,473 miles in diameter, plus or minus 12 miles. This measurement is 30 miles greater than the estimated size of Eris, astronomers report. The world was first seen by astronomer Clyde Tombaugh in 1930, but astronomers were unable to determine the exact size of the body due to an atmosphere of nitrogen, which made observations challenging.
"We knew from the time we designed our flyby that we would only be able to study the small moons in detail for just a few days before closest approach. Now, deep inside Pluto's sphere of influence, that time has come," said Alan Stern, New Horizons Principal Investigator at the Southwest Research Institute.
Despite Pluto's apparent larger size compared with Eris, the former planet is less massive than its brethren, implying the compositions of the two objects are likely significantly different from each other. The larger-than-expected size of Pluto also suggests the dwarf planet likely contains a greater concentration of ice than traditionally believed.
As the spacecraft closes in on its primary target, it has detected the release of nitrogen from Pluto's atmosphere five days before the effect was expected to be seen. Mission planners were also able to confirm that a light-colored formation resembling a polar cap is, indeed, a polar cap composed of methane and nitrogen.
Mission planners are also beginning to see detail on two of Pluto's five known moons, Nix and Hydra. The distant observatory was able to determine the diameter of these bodies for the first time, utilizing the Lorri camera aboard the vehicle. Astronomers found Nix stretches 20 miles from side to side, two-thirds the size of Hydra, reaching 30 miles in diameter.
New Horizons will make its closest approach to Pluto on July 14, 2015 at 7:50 a.m. EDT. At that time, the far-flung observatory will pass just 8,000 miles away from the frozen dwarf planet.
