Pioneer 10 is an unmanned space probe, the first ever to cross the asteroid belt and study Jupiter and its moons. Launched on 2 March 1972, Pioneer 10 is also the first probe to reach the solar system's escape velocity. It is currently heading in the direction of the orange giant star Aldebaran, which is the "eye" of the bull that makes up constellation Taurus, located about 65 light years away. If our system and the Aldebaran system had zero velocity relative to each other, Pioneer 10 would reach it in about two million years.
Pioneer 10 took the first high-resolution, close-up images of Jupiter. Passing through the asteroid belt was not hard — although it is sometimes portrayed as being dense with asteroids, they are in fact very spread out. True to its name, Pioneer 10, and its sister craft, Pioneer 12, were the first true pioneers of outer solar system exploration.
Some of the scientific assignments given to Pioneer 10 were the study of interplanetary and planetary magnetic fields; solar wind parameters; cosmic rays; transition region of the heliosphere; neutral hydrogen abundance; distribution, size, mass, flux, and velocity of dust particles; Jovian aurorae; Jovian radio waves; and the observation of Jupiter's atmosphere, as well as its satellites.
The Pioneer 10 craft was 276 kg — small for an interplanetary probe — and was constructed as part of the Pioneer program out of NASA Ames Research Center. Like some of the other craft designed to exit the solar system, it included an informative plaque sent as a message to any extraterrestrials who might pick it up in the future.
More recently, Pioneer 10 has been observing to be accelerating very slightly back towards the solar system as it keeps moving out. This has been called the Pioneer anomaly. This deceleration will not prevent Pioneer 10 from leaving the solar system, but its presence is very confounding to physicists, who are used to being able to predict the physical behavior of such objects very precisely. Although the reason for this deceleration may be mundane, like gas leakage, entire new physics is also being considered.