For spacecraft that zoom through the cosmos at thousands of miles per hour, calculating which one is traveling at the fastest speed is more complicated than simply clocking the first to cross the finish line.
When space agencies calculate and establish speed records, these numbers need to be defined and qualified, because there can be more than one frame of reference. In other words, the speed of a spacecraft can be calculated relative to the Earth, the sun, or some other body.
The record for the highest speed at which a spacecraft has launched and escaped from Earth's gravity is held by the New Horizons probe. This 1,054 pound (478 kg), piano-sized spacecraft, which launched in January 2006, sped away from the Earth at a blistering pace of 36,000 miles per hour (almost 58,000 kilometers per hour).
As the first mission to the distant Pluto, New Horizons is currently on a trajectory that will take it more than 3 billion miles away, toward the dwarf planet.
New Horizons' escape speed from Earth beat the previous record of 32,400 mph (about 52,000 km/h), set when Pioneer 10 set out for Jupiter in 1972.
After New Horizon encounters its primary science target, Pluto, and possibly a few of the asteroid-like objects in the Kuiper Belt that stretches beyond Pluto, the probe will leave our solar system. Here, it will join four other spacecraft, and could vie for yet another title: fastest interstellar spacecraft ever launched from Earth.
At present, the four interstellar spacecraft include Voyager 1, Voyager 2, Pioneer 10 and Pioneer 11. Voyager 1 is speeding away from the sun at the quickest pace, at a speed of more than 38,500 mph (more than 62,000 km/h).
New Horizons is expected to reach Pluto by 2015, and will approach the planet at a speed of more than 31,000 mph (about 50,000 km/h). It is, however, unlikely that the probe will surpass Voyager 1's speed, as the spacecraft won't be able to get much of a speed boost from the relatively weak gravitational effect of Pluto and other small Kuiper Belt bodies.
For spacecraft that have re-entered Earth's atmosphere, the highest speed was set by the comet-catching Stardust spacecraft, which plunged back toward Earth at a speed of 29,000 mph (more than 46,600 km/h).
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