Voyager 2 left the solar system and it's still

Voyager 2 is 18.5 billion kilometers from Earth.

I’m going to try to make that number real, because our brains aren’t built for distances like this. Light, the fastest thing in the universe, takes about 16.5 hours to travel from Voyager 2 to Earth. If you could drive a car at highway speed, 24 hours a day, it would take you about 20,000 years to reach it. It’s so far away that it’s left the heliosphere, the bubble of solar wind that surrounds our entire solar system. It’s in interstellar space. Between the stars.

And it’s still talking to us.

The machine

Voyager 2 launched on August 20, 1977. Jimmy Carter was president. Star Wars had just come out. The Apple II had been on sale for four months. Punk rock was happening.

The spacecraft has less computing power than a modern car key fob. Its main computer has about 69.63 kilobytes of memory. Its transmitter broadcasts at 23 watts, roughly the power of a refrigerator light bulb.

Twenty-three watts, from 18.5 billion kilometers away.

By the time that signal reaches the Deep Space Network antennas on Earth, it’s so faint that the power received is about 10^-16 watts. That’s 0.0000000000000001 watts. To pick that up, NASA uses dish antennas 70 meters across, along with some of the most sensitive receivers ever built. They can detect a signal that is twenty billion times weaker than the power required to run a digital watch.

I’m not going to pretend I fully understand the signal processing involved. I’ve read about it and the math is beyond me. But the concept is this: a machine the size of a small car, with the processing power of a pocket calculator, is whispering across a distance that took it 42 years to cross, and we’re listening.

The grand tour

I should back up. Voyager 2 wasn’t supposed to last this long.

The original mission was a four-year flyby of Jupiter and Saturn. That’s it. But the spacecraft was built well (better than anyone expected), and a rare alignment of the outer planets that happens only once every 175 years made an extended tour possible. So NASA kept going. Jupiter. Saturn. Uranus. Neptune. Four planets in twelve years.

The images from those flybys changed everything we knew about the outer solar system. Jupiter’s moons were geologically active (Io has volcanoes, Europa has a liquid ocean under its ice). Saturn’s rings were far more complex than anyone imagined. Uranus was tilted on its side and shrouded in haze. Neptune had a storm the size of Earth and winds faster than the speed of sound.

All of this from a machine launched before the Apple II existed. Photographed with cameras less sophisticated than what’s in your phone. Transmitted across billions of miles at the speed of light on 23 watts.

Voyager 2 remains the only spacecraft to have visited Uranus and Neptune. Forty-two years later, we’ve never been back. I find that both remarkable and a little bit sad.

Why it’s still going

Voyager 2 runs on radioisotope thermoelectric generators (RTGs). Plutonium-238 decays and produces heat. Thermocouples convert the heat into electricity. No moving parts. No fuel to run out. Just atoms falling apart, slowly, reliably, for decades.

The power is declining, though. The plutonium decays at a predictable rate, and the RTGs produce about 4 watts less each year. Voyager 2 currently generates about 289 watts, down from 470 watts at launch. NASA has been managing this decline by turning off instruments one at a time. Heaters first. Then cameras (Voyager’s cameras were turned off in 1989 after the Neptune flyby because there was nothing left to photograph and the power was needed elsewhere). Eventually, sometime around 2025, there won’t be enough power to run any instruments at all.

And then Voyager 2 will go quiet.

But it won’t stop traveling. It’ll keep moving through interstellar space at about 55,000 kilometers per hour, carrying the Golden Record on its side. A gold-plated copper disk containing sounds and images from Earth, placed there by a committee chaired by Carl Sagan. Greetings in 55 languages. The sound of wind, and surf, and a kiss. Bach, Mozart, Chuck Berry. A mother’s first words to her newborn child.

A message in a bottle thrown into an ocean so vast that the odds of anyone finding it are essentially zero.

What I think about at 2am

I think about this more than is healthy. The idea of Voyager 2, dead and silent, drifting through the space between stars for millions of years, carrying a record that nobody will ever play. The loneliness of it. The hope of it. Both at the same time.

Sagan knew the odds. He knew the Golden Record would almost certainly never be found. He put it on anyway. Because the act of sending a message mattered, even if the message was never received. The act of saying “we were here, we existed, we thought these thoughts and heard these sounds and loved these things” was worth doing for its own sake.

I think that’s the most human thing about Voyager. Not the engineering (though the engineering is remarkable). Not the science (though the science is extraordinary). The most human thing is the stubborn, irrational insistence on reaching out. On sending a voice into the void even when the void doesn’t answer.

Eighteen point five billion kilometers away, a machine from 1977 is still whispering to us. And we’re still listening.

I find that unbearably beautiful.

There’s a moment on the Golden Record where someone says “hello from the children of planet Earth.” I think about those children. They’re in their fifties now. They probably don’t think about Voyager very often. But their greeting is farther from home than any human voice has ever been.

Farther than any human voice will ever be, probably, at least in our lifetimes.

Still going. Still whispering.

Still out there.

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