As New Horizons cruised toward the edge of the asteroid belt and a date with Jupiter, the spacecraft that revolutionized our knowledge of that giant planet more than 25 years ago made history again this week. On Aug. 15, NASA's enduring Voyager 1, already Earth's farthest-flung robotic ambassador, became the first spacecraft to reach a distance of 100 astronomical units (AU) from the Sun.

That's 100 times farther from the Sun than the Earth is, equivalent to 9.3 billion miles or 15 billion kilometers. Though New Horizons will also reach 100 AU, it will never pass Voyager 1, because Voyager was boosted by multiple gravity assists that make its speed faster than New Horizons will travel. Voyager 1 is escaping the solar system at 17 kilometers per second. When New Horizons reaches that same distance 32 years from now, propelled by a single planetary swingby, it will be moving about 13 kilometers per second.

"Voyager blazed an historic trail of exploration across the giant planets and out into the distant heliosphere. Now New Horizons follows, almost 30 years later, exploring the Sun's population of Kuiper Belt ice dwarfs for the first time," says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute. "From the generation of New Horizons explorers to the Voyager generation of explorers, we say congratulations on reaching 100 AU, you showed us all the way, keep exploring, onward, ever onward!"

New Horizons will reach 100 AU in December 2038, long after the probe passes through the Pluto system and enters the Kuiper Belt. But another milestone occurs much sooner: next week New Horizons crosses the outer boundary of the main asteroid belt at 3.3 AU (nearly 307 million miles, or 494 million kilometers) from the Sun, just seven months after launch.

Read more about Voyagers 1 and 2: http://www.jpl.nasa.gov/news/features.cfm?feature=1150. Follow New Horizons on its own voyage: http://pluto.jhuapl.edu/mission/whereis_nh.php.

Pluto-Charon: A True Double Planet

Under proposed International Astronomical Union definitions, two planets that orbit each other around a barycenter (or center of mass) between them are a binary planet. Those same definitions would expand the "family" of planets to include Charon, promoting Pluto's large companion from moon to planet and securing the pair's status as the first and (so far) only binary planet in the solar system.

Read more about binary worlds in the New Horizons Science section: http://pluto.jhuapl.edu/science/everything_pluto/10_binary_planet.html

Read the IAU press release on proposed planet definitions: http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_release.html

New Horizons is the first mission to Pluto and the Kuiper Belt of rocky, icy objects beyond. Principal Investigator Alan Stern, of the Southwest Research Institute (SwRI), leads a mission team that includes the Johns Hopkins University Applied Physics Laboratory, Ball Aerospace Corporation, the Boeing Company, NASA Goddard Space Flight Center, NASA Jet Propulsion Laboratory, Stanford University, KinetX, Inc., Lockheed Martin Corporation, University of Colorado, the U.S. Department of Energy, and a number of other firms, NASA centers and university partners. For more information on the mission, visit http://pluto.jhuapl.edu.