NASA's New Horizons probe has entered the elite ranks of spacecraft operating beyond 50 astronomical units (AU) from the Sun. Remarkably, it remains fully operational and is now hunting for its next target.
The New Horizons spacecraft, launched by NASA, has crossed the 50 AU threshold from our Sun—equivalent to about 7.5 billion kilometers. At this vast distance in the Kuiper Belt, signals traveling at the speed of light take over 6.5 hours to reach Earth.
Just four spacecraft have reached this cosmic milestone. Pioneer 10, launched in 1972 as the first to traverse the asteroid belt and fly by Jupiter, passed 50 AU on September 22, 1990. Today, it drifts about 129 AU from Earth.
Its twin, Pioneer 11, launched in 1973 and the first to observe Saturn up close, hit 50 AU in 1991. It now sails roughly 105 AU away.
The Voyager probes complete the quartet. Voyager 1, launched September 5, 1977, explored Jupiter and Saturn; Voyager 2, 16 days later, also visited Uranus and Neptune. Today, Voyager 1 is 152 AU out, Voyager 2 at 127 AU—both still active in interstellar space, unlike the silent Pioneers.
This achievement marks more than a distance—New Horizons has outlasted its designed lifespan.
“One of the first things you do when designing a spacecraft is set requirements. We defined the maximum distance for reliable operations at 50 AU,” explains Alan Stern, New Horizons principal investigator at the Southwest Research Institute in Boulder, Colorado. The probe is now in extended mission mode.
After its historic Pluto flyby in 2015 and the farthest-ever close-up of a Kuiper Belt object, Arrokoth, on January 1, 2019 (at 43.4 AU from the Sun), New Horizons presses on, seeking new targets before fuel runs low.
Powered by a radioisotope thermoelectric generator (RTG) using plutonium, its output drops by 33 watts every decade. By the late 2030s, nearing 100 AU, it may lack sufficient power to continue.
