On this day in 1999 - September 22nd - NASA's Pioneer 10 reached the distance of 50 AU from the Sun.
The spacecraft was the first one to cross the Asteroid Belt, and it carried out the first mission to Jupiter. It was also the first to reach escape velocity from the Solar System. At 50 AU from the Sun, it was 50 times farther away from the Sun than Earth is, about 7.5 billion km (4.7 billion miles). It's now over 124 AU away.
Galle had received calculations from Urbain LeVerrier which showed where there might be a new planet, and Galle found the planet that very night. Eventually, it was named Neptune, after the Roman sea god.
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