Voyager at 47: The Most Distant Human Objects Are Still Sending Data

In the summer of 1977, NASA launched two spacecraft on a trajectory that would take them past Jupiter, Saturn, Uranus, and Neptune, exploiting a rare planetary alignment that won't recur for another 175 years. Voyager 1 and Voyager 2 were designed for a four-year mission. Forty-seven years later, both probes are still operating in interstellar space — the only human-made objects to have left the solar system.

The Grand Tour

The planetary alignment of the late 1970s allowed a single spacecraft to visit all four outer planets using gravitational assist maneuvers — each planet's gravity bending the spacecraft's trajectory and adding velocity for the next leg. Voyager 1 visited Jupiter and Saturn before being flung northward out of the ecliptic plane. Voyager 2 made the complete Grand Tour: Jupiter, Saturn, Uranus, and Neptune, returning the only close-up images we have of Uranus and Neptune to this day.

The data returned transformed planetary science. At Jupiter, Voyager discovered active volcanoes on Io — the first extraterrestrial volcanism ever observed. At Europa, the images that would eventually lead to the hypothesis of a subsurface ocean. At Saturn, the complex structure of the rings revealed in unprecedented detail. At Uranus and Neptune, entire weather systems and magnetic field configurations we'd never imagined.

Into Interstellar Space

Voyager 1 crossed the heliopause — the boundary where the solar wind gives way to the interstellar medium — in 2012, becoming the first human-made object in interstellar space. Voyager 2 followed in 2018 on a different trajectory. The heliopause turned out to be more complex and variable than models predicted, and the in-situ measurements from both probes have significantly refined our understanding of the heliosphere's structure.

At the time of writing, Voyager 1 is approximately 23 billion kilometers from Earth. Its radio signals, traveling at the speed of light, take over 22 hours to reach us. The Deep Space Network antennas that receive those signals are detecting power levels of roughly 10⁻¹⁶ watts — about 10 billion times less than the power required to light a single LED.

The Golden Record

Both Voyagers carry a golden record — a gold-plated copper disk containing sounds and images of Earth, selected by a committee chaired by Carl Sagan. Music from Bach to Chuck Berry, greetings in 55 languages, the sound of rain, of surf, of a mother's first words to a newborn. A message in a bottle thrown into the cosmic ocean, readable by any civilization with sufficient technology and curiosity to intercept it.

The Voyagers will never reach another star system on their current trajectories — the distances involved are simply too vast. But they will drift through the galaxy essentially forever, long after Earth and Sun are gone, carrying their record of a civilization that once existed and reached, however humbly, beyond its world.