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For more than forty years, the two Voyager probes have delighted us with information and pictures from across our solar system. But now, they've embarked on a new mission.

As of 2018, both probes had left the solar system and begun an interstellar journey — a quest to drift between the stars and take measurements wherever they go. But for as beautiful and poetic as that is, we can't expect this mission to go on forever. Both Voyagers' clocks are ticking, and these days, they're running out of juice.

Since they launched in 1977, these spacecraft have really earned their names, and so far, each has taken an epic journey of more than 17 billion kilometers. On that journey, Voyager 1 visited Jupiter and Saturn, then crossed into interstellar space in 2012. Meanwhile, Voyager 2 took a more scenic route that also passed Uranus and Neptune, so it only moved beyond the Sun's influence in 2018.

Right now, both craft are still transmitting data, and are measuring the interstellar environment with a suite of instruments. They're detecting magnetic fields, cosmic rays, and plasma waves. But this isn't likely to last much longer, as the two sources of fuel the probes launched with are slowly running out.

One of those fuels is hydrazine. This simple compound is typically used as a propellant for thrusters, since it's cheap and has a very low freezing point. But in this case, it's not used to push the Voyagers along.

They got their speed from their initial launch, plus gravity-assisted slingshots around the planets they visited. So instead, their hydrazine is used for trajectory correction and as an attitude control propellant to control the spacecrafts' orientation. This helps correct for any rolls or tumbles, and keeps the probes pointed toward Earth, so they can beam back results and receive commands from mission control.

These kinds of corrections only need to be made every once in a while, so the hydrazine isn't used up very quickly. In fact, NASA estimates they're running at a fuel economy of about 13,000 kilometers per liter, which puts pretty much every other vehicle to shame! But still… it has been 40 years since these things launched.

So even though both Voyagers left Earth with 104 kilograms of hydrazine each, supplies are running low. NASA reckons Voyager 1's hydrazine will last until 2040, but Voyager 2's will run out in 2034. That's because it used up more of its supply maneuvering around Uranus and Neptune.

After that, the craft will keep speeding through interstellar space, but they might not be pointing in the right direction to send back results. So we'll likely have to say goodbye. 2034 and 2040 might seem like a long time from now, but sadly, the Voyagers will probably go silent before then. Because as well as liquid fuel, the probes have another power source, one that makes electricity for its onboard systems.

It's called a radioisotope thermoelectric generator, or RTG. And the ones in the Voyagers are on their last legs. RTGs work by generating heat.

And they do this thanks to their four and a half kilograms of plutonium dioxide. Like other radioactive compounds, this stuff is unstable. So over time, the plutonium atoms decay, breaking down into smaller particles and more stable uranium atoms.

This decay process gives off heat, which the RTGs are able to convert into electricity. Both Voyagers were fitted with three of these generators, which together provided about 470 watts of power at launch. It doesn't sound like much — your average toaster uses more than 1000 watts — but it was enough to power the central computer, communications equipment, and a suite of ten instruments.

Unfortunately, at this point, a large portion of the plutonium has decayed away. What's left is only able to provide about 250 watts. So, to conserve that power for the probe's more critical functions, scientists have — one by one — started shutting down the probes' instruments.

For Voyager 1, they started with the cameras and heaters in 1990. Then, 20 more components followed over the years. Voyager 2 has suffered the same fate, although its shutdowns started in 1991.

And it's now lost its radio astronomy setup, among other functions. These days, Voyager 1 has just four working instruments, and Voyager 2 has five. That's enough to take good data about things like magnetic fields.

But soon, they won't have enough power for that, either. Starting in 2020, engineers will have to likely start shutting down the remaining systems — or switch them on and off sequentially to make the most of the remaining power. Either way, as time goes on, the Voyagers will have less and less to say to us, and eventually, they'll fall silent forever as the central computer shuts down.

Of course, we should remember that these probes are flying at incredible speeds through totally uncharted territory between stars. They're passing through unknown fields and particles, so they could also encounter an entirely unexpected problem. One that could shut down their power systems or take out instruments ahead of time.

So, whether their missions last for days, years, or decades more, they will eventually come to an end. But honestly, even if something happens to them tomorrow, the probes have already achieved great things. They've given us unprecedented insights into the giant planets of the solar system and their moons.

And they've taught us about everything from giant storms on Jupiter to active volcanoes on Io to shepherding moons in Saturn's rings. Plus, they've been in space longer, and traveled farther, than any equipment we've ever made. Which by itself, is definitely worth celebrating.

Thanks for watching this episode of SciShow Space! Somehow, it's the last episode of 2019, which is kind of amazing! We've talked about so much cool stuff this year, and we couldn't have done it without you.

So to everyone who watched the show, left us comments, subscribed, shared a video, became a patron, or anything in-between — thanks for being part of the SciShow Space family. We'll see you next year! [♪ OUTRO].