But Pioneer 10’s margin of survival was closer to the knife-edge than it seemed. It absorbed a thousand times the lethal dose of radiation for a human, suffering darkened optics and fried transistor circuits. Other unwanted side effects included the generation of false commands, which caused the craft to lose at least one image of the moon Io and several shots of Jupiter.
Fortunately, any changes to Pioneer 10’s systems induced by radiation disappeared in the following months, as it continued on its way. But they served as an acute reminder that future missions would need some serious beefing up to protect their equipment.
A lasting legacy
Pioneer 10’s whistle-stop tour of Jupiter officially ended Jan. 2, 1974. The brief visit nonetheless revealed much about the planet. We learned that Jupiter’s magnetic field is inclined 15° to its axis of rotation. Pioneer 10’s measurements allowed researchers to pinpoint the presence of helium in the atmosphere and confirmed long-held suspicions that this was a predominantly gaseous world with no discernible solid surface. And its data showed atmospheric temperatures were broadly the same during day and night.
The probe’s passage through the Jovian system included a pass behind the Galilean moon Io, which astronomers suspected might possess an ionosphere of its own and, by extension, a substantial atmosphere. Pioneer 10’s data confirmed the ionosphere’s existence and even hinted that sulfur might be a key chemical component within it. But it would be several more years before the Voyagers fully revealed Io to be a world of ubiquitous, sulfur-driven volcanism.
Pioneer 10’s encounter with Jupiter was a great success. While its slow-scan images were deemed “passable” by some observers, they certainly whetted appetites for future endeavors. The probe crossed the orbits of Saturn in 1976, Uranus in 1979, and Neptune — then the solar system’s most distant planet while Pluto swung interior to the ice giant’s orbit — in 1983. NASA received the spacecraft’s last feeble radio signal in January 2003 from a distance of 7.4 billion miles (11.9 billion km). Efforts to contact Pioneer 10 two weeks later were met with silence.
Five decades after its launch, Pioneer 10 continues silently at a velocity that could zip between New York and London in eight minutes. Each year, it crosses 230 million miles (370 million km), more than twice the gulf separating Earth from the Sun, heading generally in the direction of the red giant Aldebaran, the brightest star in the constellation Taurus. It may take a couple million years to get there.
For now, the probe flies inexorably onward. The Sun remains visible in its rear view, a bright star with a magnitude of –16.3. It serves as a reassuring companion and sole reminder of a long-ago home that Pioneer 10 will never see again.