About 1 billion years from now, the sun will begin to die, blow off its outer atmosphere, and engulf our tiny planet in hot plasma.

Luckily, the galaxy will have NASA's twin Voyager spacecraft to remember us by.

The two nuclear-powered probes launched 40 years ago and became the first and only robots to take close-up photographs of Uranus and Neptune, the planets' moons and rings, and other objects in the outer solar system.

The Voyagers also carried with them a golden record of soundsimages, and other information about life on Earth — a basic human catalog that aliens might one day discover and decode.

The mission is now detailed in a remarkable PBS documentary called "The Farthest", which premiered on August 23 and will re-air on September 13 at 10 p.m. ET.

"Fifty years from now, Voyager will be the science project of the 20th century," Brad Smith, a Voyager imaging scientist, said in the movie.

Here's why many scientists and engineers not only hail Voyager as the farthest, fastest, and longest-lived space mission, but also one of humanity's greatest endeavors.

NASA began working on the Voyager mission in 1972 with a budget of $865 million, or roughly $5 billion in 2017-adjusted dollars.

An illustration of NASA's Voyager spacecraft drifting through space.NASA
Image Source: NASA/JPL-Caltech, Bureau of Labor Statistics

The goal was to tour the outer solar system using a planetary alignment that happens just once every 176 years. The gravity of the planets would speed up the spacecraft, allowing at least one probe to visit Uranus and Neptune for the first time.

NASA worried that Jupiter's radiation fields might short-circuit the Voyagers. So engineers shielded and grounded cables of the probes with kitchen-grade aluminum foil. (It worked.)

Kitchen-grade aluminum foil was used to insulate and ground the outer cabling of the Voyager spacecraft.Image Source: NASA/JPL-Caltech

Voyager 2 launched into space on August 20, 1977. To the confusion of many people, Voyager 1 launched several weeks later, on September 5, 1977.

The Voyager 2 spacecraft launching from NASA's Kennedy Space Center at Cape Canaveral, Florida, on August 20, 1977. Image Source: NASA/JPL

But as scientists featured in "The Farthest" explain, they were thinking long-term: Voyager 1 would move faster and eventually overtake Voyager 2.

Both launches left scientists' nerves frayed. For example, unforeseen rocket vibrations made Voyager 2's computer go on the fritz and didn't let people control it for days. Fortunately, a remote software patch fixed the problem.

Part of the Voyager spacecraft's computing system. Image Source: Smithsonian National Air and Space Museum

"That was a cliffhanger," Fred Locatell, an engineer on the Voyager mission, said in the film. "That could have been the end of the mission."

Meanwhile, Voyager 1's liquid-fueled rocket sprung a leak during launch. The rocket corrected for this by burning extra fuel — leaving just 3.5 seconds' worth of thrust.

Had the rocket leaked any more fuel than it did, Voyager 1 could have failed.

"Instead of getting to Jupiter, you know, we would have gotten almost to Jupiter — then it would have come back toward the sun. Which would not have been good," John Casani, the mission's project manager, said in the film.

Both Voyagers explored Jupiter, catching a speed boost along the way. Before the probes, no one had ever seen such detailed images of Jupiter...

The Great Red Spot, as seen by the Voyager 1 probe in 1979.Image Source: NASA/JPL; Björn Jónsson/IAAA

...Or those of the gas giant's many moons, including Europa. This ice world hides a subsurface ocean that's estimated to contain more water than exists on all of Earth.

A photo mosaic of Jupiter's icy moon Europa, as photographed by NASA's Voyager 2 probe.Image Source: NASA/JPL

Source: Business Insider

Both probes also took extraordinarily detailed photos of Saturn...

Image Source: A false-color image of Saturn taken by Voyager 1 in 1980. Image Source: NASA/JPL-Caltech

...And its moons, such as Titan. Many scientists think of this world as a "proto-Earth" due to its similar size, hazy atmosphere, and abundance of carbon.

Saturn's hazy moon Titan, as seen by NASA's Voyager 1 spacecraft.Image Source: NASA/JPL

Voyager 2 was the first and so far only robot to fly by and photograph Uranus. It discovered the planet is inexplicably tipped on its side...

A true-color photo of Uranus (left) and a false color image (right) taken at the turn of 1986 by NASA's Voyager 2 probe. Image Source: NASA/JPL-Caltech

...Has dozens of moons...

A photo of Uranus' moon Miranda taken by the Voyager 2 spacecraft. Image Source: NASA/JPL-Caltech

...And has rings of ice and dust.

A photo of Uranus taken by Voyager 2 in 1986 that's edited to show its moon and rings. Image Source: NASA/JPL-Caltech

Voyager 2 is still the only probe ever to fly by the outermost planet, Neptune...

A false-color view of Neptune from Voyager 2 that reveals a haze of methane gas in red. Image Source: NASA

...See the gas giant's enormous Dark Spot storm...

...And photograph its large and icy moon Triton, which was caught spewing geysers of nitrogen from its surface.

A mosaic of images of Neptune's moon Triton, as photographed by NASA's Voyager 1 probe in 1989. Image Source: NASA/JPL/USGS

Although both probes finished their main missions decades ago, they've moved beyond the arguable edge of the solar system — and into a region between stars.

An illustration of a Voyager probe leaving the solar system. Image Source: NASA/ESA/G. Bacon (STScI)

The spacecraft still regularly contact Earth from more than 10.6 trillion miles away, reporting what data they can with the instruments that still work and are turned on.

The Canberra antennas of the Deep Space Network regularly listen for signals from the Voyager probes. Image Source: NASA

They Voyagers still work more than 40 years after launch because they're powered by plutonium-238 — a radioactive byproduct of Cold War nuclear weapons production.

A puck of plutonium-238 dioxide glowing under its own warmth.Department of Energy, via Wikipedia

Special materials surround the plutonium to convert its escaping heat into electricity. Each Voyager has three nuclear power supplies, and together they generated 420 watts at launch — about half the power of a microwave oven.

Because half of any amount of plutonium-238 decays in 87.7 years, these radioisotope power sources now generate about three-quarters of that wattage.

Even after these nuclear batteries die, however, the probes will continue carrying golden records loaded up with the sights and sounds of Earth.

The golden record and American flag attached to each of NASA's Voyager probes. Image Source: NASA/JPL-Caltech

The cover to each record includes careful instructions on how to use a stylus to read the data. These directions aren't in English, though, but rather in mathematical terms an intelligent alien might understand.

Image Source: NASA

"This, in the long run, may be the only evidence that we ever existed," Frank Drake, who worked on the golden record project (and also came up with the Drake Equation), said in the film.

A view of Africa taken by Apollo 11 astronauts on July 20, 1969. Image Source: NASA/Flickr

President Jimmy Carter left what is perhaps the most prescient message to anyone (or anything) who discovers the probes: "This is a present from a small distant world, a token of our sounds, our science, our images, our music, our thoughts and our feelings. We are attempting to survive our time so we may live into yours."

President Jimmy Carter in a photograph taken on January 31, 1977. Image Source: Wikipedia; Library of Congress

Source for all images: "The Farthest"/PBS and Business Insider

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