Launched in 1972, Pioneer 10 crossed the orbit of Neptune on June 13, 1983, at the time considered the outermost planet, making it the first spacecraft to travel beyond the known planetary region of the solar system. The probe, which had already delivered the first close-up images of Jupiter, continued transmitting data until 2003 from a distance of billions of miles, a testament to its remarkably durable 1970s-era engineering.
Pioneer 10 was never designed for the journey it ended up making. Launched on March 2, 1972, its primary mission was to survive the asteroid belt and photograph Jupiter, tasks scientists were not even confident it could complete. It did both, delivering the first close-up images of the solar system's largest planet in 1973 and proving that a spacecraft could survive the belt's debris field without being shredded.
Having succeeded beyond expectations, Pioneer 10 kept going. It sailed onward in a straight trajectory out of the solar system, and on June 13, 1983, it crossed the orbital path of Neptune, which at that time, due to Pluto's eccentric orbit, was the outermost known planet. That milestone made Pioneer 10 the first human-made object to travel beyond the orbits of all the solar system's then-recognized planets, a headline that ran in newspapers worldwide.
The spacecraft carried a gold-anodized plaque designed by Carl Sagan and Frank Drake, depicting a man and woman and the location of Earth, a message in a bottle cast into the cosmic ocean on the off chance some other intelligence might one day find it. NASA maintained contact with Pioneer 10 until January 2003, when its last, faint signal arrived from more than 12 billion kilometers away, powered by a plutonium generator that had outlasted every expectation.
Key people: Carl Sagan