Of all the spacecraft humans have launched, there have been some impressively fast movers. But which holds the record? Apart from the wow factor, it's an interesting yardstick for gauging our capacity to explore the cosmos, from familiar planets to the icy depths of space. But it's not always an easy quantity to evaluate. For one thing, launch velocities differ from eventual cruise velocities. They also depend on what you measure velocity relative to. Far away from Earth, it makes more sense to work with heliocentric (sun-relative) measurements.
The record holder for launch velocity is the New Horizons probe to Pluto and the Kuiper belt. Launched by NASA in 2006, it shot directly to a solar system escape velocity, which consisted of an Earth-relative launch of 36,000 miles per hour, plus a velocity component from Earth's orbital motion. Altogether these factors set New Horizons barreling off into the solar system with an impressive heliocentric speed of about 100,000 mph.
In terms of pure heliocentric velocity, the current champions are two probes called Helios I and II that were launched in 1974 and 1976. They entered orbits that took them closer to the sun than the planet Mercury. The nearer you orbit to a huge mass like the sun, the faster you have to move, and both Helios crafts hit orbital velocities in excess of 150,000 mph.
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They are not going to hold on to pole position for much longer, however. First, NASA's Juno probe to Jupiter will be arriving in the Jovian system in 2016 and will enter a polar orbit around the gas giant. But Jupiter weighs in at 317 times the mass of Earth. Falling deep into its gravity well will accelerate Juno to a velocity of about 160,000 mph relative to the planet before it can swing by, drop speed and get into its mission orbit.
In 2018 a new NASA mission—Solar Probe Plus—will be launched. Designed to come within 3.7 million miles of the sun, it will hit orbital velocities as high as 450,000 mph. To put that incredible figure into perspective, going this fast would get you from Earth to the moon in about half an hour. It is also about 0.067 percent the speed of light.
Adapted from Life Unbounded at blogs.ScientificAmerican.com/life-unbounded