Voyager 1 and 2: Humanity's Farthest Travelers

In the late summer of 1977,

two robotic spacecraft left Earth a little over two weeks apart. They were built for a planned five-year mission to Jupiter and Saturn.

Instead, they became the most far-traveled machines humans have ever made.

The Voyagers flew past all four giant planets, took the famous Pale Blue Dot photograph,

carried a golden message for anyone who might find them and eventually crossed into interstellar space.

Here is how they did it and why their journey still inspires scientists and stargazers alike.

A Once-in-175-Years Opportunity

In the 1960s, mission planners noticed that Jupiter, Saturn,

Uranus and Neptune would line up in the late 1970s in a way that happens only about once every 175 years.

A spacecraft launched at the right time could swing from one planet to the next,

using each one's gravity to bend its path and pick up speed.

This technique, called a gravity assist,

works a bit like a ball bouncing off the front of a moving truck: as the spacecraft swings around a planet moving along its orbit,

it steals a tiny bit of the planet's orbital energy.

For the Voyagers, gravity assists turned a journey that would have taken decades into one that reached Neptune in 12 years.

Oddly, Voyager 2 launched first, on August 20, 1977.

Voyager 1 followed on September 5, but on a faster path that let it overtake its twin and reach Jupiter first.

Built to Last

Each Voyager carries a dish antenna 3.7 meters (12 feet) across, pointed back toward Earth, along with cameras,

magnetic field sensors and instruments that measure charged particles, radio waves and plasma.

Their onboard computers had a tiny fraction of the memory found in a modern phone.

So far from the Sun, solar panels would have been useless.

Instead, each spacecraft is powered by radioisotope thermoelectric generators, devices that turn the heat from decaying plutonium into electricity.

Their output drops a little every year,

so over the decades engineers have switched off heaters and instruments one by one to stretch the remaining power.

The radio transmitter on each probe uses only about 23 watts, roughly the power of a refrigerator light bulb.

The Grand Tour: What the Voyagers Found

The encounters with the outer planets rewrote textbooks. Among the highlights:

• Jupiter, 1979: The probes revealed active volcanoes on the moon Io, the first ever seen beyond Earth, plus a faint ring around Jupiter and a cracked, icy surface on Europa that hinted at hidden water beneath.
• Saturn, 1980 and 1981: The Voyagers showed that Saturn's rings contain countless narrow ringlets, spotted shadowy spokes sweeping across them and studied the thick, hazy atmosphere of the giant moon Titan.
• Uranus, 1986: Voyager 2 found ten previously unknown moons, a magnetic field tilted far from the planet's spin axis and a bizarre patchwork landscape on the small moon Miranda.
• Neptune, 1989: Voyager 2 discovered a huge storm called the Great Dark Spot, measured winds faster than any others known in the solar system and saw dark plumes erupting from the icy surface of the moon Triton.

Voyager 1 skipped Uranus and Neptune because scientists chose to send it close to Titan.

That flyby bent its path up and out of the plane of the planets, ending its planetary tour.

Voyager 2 carried on alone, becoming the first spacecraft to visit Uranus and Neptune.

The Pale Blue Dot

On February 14, 1990, Voyager 1 turned its camera back toward home from about 6 billion kilometers (3.7 billion miles) away.

As part of a series of images capturing the planets of the solar system,

it photographed Earth as a tiny speck, less than a single pixel, floating in a band of scattered sunlight.

The picture was suggested by astronomer Carl Sagan, who later wrote movingly about how everyone who has ever lived existed on that small dot.

Soon afterward, the cameras were switched off for good to save power and memory for the long cruise ahead.

The Golden Record

Each Voyager carries a 12-inch gold-plated copper phonograph record, a message in a bottle for any intelligent beings who might someday find it.

A committee chaired by Carl Sagan chose its contents:

• Greetings spoken in 55 languages.
• Sounds of Earth, including wind, thunder, surf, birdsong, whales and a human heartbeat.
• Music from many cultures and eras, from classical pieces to folk songs and rock and roll.
• More than 100 images encoded in the record's grooves, showing people, animals, landscapes and science.

The record's cover is engraved with instructions for playing it and a map that uses the positions of pulsars to show where the Sun is located. The chances that anyone will find it are extremely slim,

but the record is a thoughtful portrait of humanity as it saw itself in the 1970s.

Into Interstellar Space

The Sun blows a constant stream of charged particles called the solar wind,

which inflates a vast bubble around the solar system known as the heliosphere.

Its outer boundary, the heliopause, is where the solar wind finally gives way to the thin gas between the stars.

After their planetary encounters, the mission was renamed the Voyager Interstellar Mission, with a new goal of reaching that boundary.

Along the way, each probe first crossed the termination shock,

the zone where the solar wind abruptly slows from supersonic speed: Voyager 1 in 2004 and Voyager 2 in 2007.

Voyager 1 became the most distant human-made object in 1998 and crossed the heliopause in August 2012, becoming the first spacecraft to enter interstellar space.

Voyager 2 followed in November 2018. Because an instrument that measures plasma still worked on Voyager 2,

while the one on Voyager 1 had failed back in 1980,

the second crossing gave scientists a much more direct look at the boundary itself.

Both probes move at tremendous speeds.

Voyager 1 travels at about 17 kilometers per second (38,000 miles per hour) relative to the Sun,

fast enough to cross the United States in a few minutes.

Talking to a Spacecraft Billions of Miles Away

Staying in touch with the Voyagers relies on NASA's Deep Space Network, a set of giant radio antennas in California,

Spain and Australia, spaced so that at least one can always see a given spacecraft.

Neptune's gravity flung Voyager 2 steeply south of the plane of the planets, so after that encounter only the antennas in Australia could see it.

By the early 2020s, a radio signal needed more than 20 hours to travel one way between Earth and Voyager 1.

By the time it arrives,

the probe's whisper of a transmission has spread out and weakened to an almost unimaginably faint signal,

which only the largest dishes can pick up.

A Message in a Bottle

The Voyagers were meant to last five years.

Instead, they sent back science from the outer planets and the edge of the heliosphere for decades,

turning a rare planetary alignment into one of the greatest voyages of exploration in history.

Whenever their radios finally go quiet, the two probes will keep drifting among the stars,

carrying their golden records and a small reminder that the people of a pale blue dot once reached out to the cosmos.