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SYFY WIRE Bad Astronomy

Voyager 1 turns on its thrusters after 37 years in the cold and dark

By Phil Plait
Voyager 1 heading away from the solar system. Credit: NASA’s Eyes on the Solar System

It was right around turning 37 years old when parts of me starting failing. My back started hurting, my neck started making weird clicking noises, and it became impossible to either sit down on or stand up from a couch without making some sort of "whoooof" or "ermf"-like sound.

And that's all happened while I've been on Earth, with air and water and temperate climes. Imagine how stiff and sore I'd be if I were, say, out in the hard vacuum of deep space, where the temperature is a handful of degrees above absolute zero.

Yet that's just where Voyager 1 is. Launched in 1977, the spacecraft was sent on a journey to the outer solar system, passing by Jupiter and Saturn and returning the best close-up photos we had yet seen. During the Saturn encounter the planet's gravity flung Voyager 1 up and away from the plane of the solar system.

Even after all these decades, it's still returning important data about conditions beyond the inner solar system as it heads into interstellar space. The longer it keeps going, the more we learn.

A schematic (not to scale) showing the locations of the two Pioneer and two Voyager spacecraft, currently heading out of the solar system Credit: NASA

Recently, though, engineers noticed a problem. Voyager 1 (and its sister spacecraft Voyager 2) have a series of small thrusters on them. Some of these are used to change the trajectory of the spacecraft, and are called the Trajectory Correction Maneuver thrusters, or TCMs. Others are the Attitude Control Thrusters, or ACTs. These are used to rotate the spacecraft, to make sure it stays correctly oriented as it flies through space (what engineers call its attitude).

The TCM thrusters haven't been used to change the course of the spacecraft since 1980, when it passed Saturn. Since then, it's been coasting, and they've been shut off. But the angle of the spacecraft needs to be adjusted sometimes to keep the main antenna pointed toward Earth, and the ACTs have been used to do that. If the spacecraft drifts in angle, the thrusters give off a little puff of gas in the right direction to counter it.

Recently, the ACTs have been degrading. Time has taken its toll on them, and they've been getting less efficient. There's not a lot of hydrazine fuel left for the thrusters, and engineers get stingy with consumables when a spacecraft is 21 billion kilometers from Earth.

So they decided to try something weird: Use the other set of thrusters, the TCM ones. These have been shut off since Saturn, for 37 years. Technically, in the vacuum of space, they should just be sitting there, ready to be switched back on. But you never know; after nearly four decades of ultra-freezing temperatures, all kinds of things can go wrong.

To be careful, engineers looked up old data and code to make sure they know what to do and what to expect when they turn them back on (including an old version of assembler language, which it's tempting to call "obsolete," but here they are using it). Everything checked out, so in late November they fired up the devices, sending a signal to the spacecraft telling the TCM thrusters to give off very short pulses of gas lasting just 0.01 seconds.

Then they waited. It takes the message, sent on a beam of light, a staggering 19.5 hours to get to the spacecraft. Then they had to wait another 19.5 hours for the signal from Voyager to get back.

And they worked! After 37 years, the TCM thrusters did their job. This means the small amount of fuel onboard will last that much longer, extending the mission lifetime by a few years. Amazing.

I'll note a similar thing was done with Voyager 2 in 2011. The spacecraft gets its power from a small amount of plutonium that generates electricity using a device called a radioisotope thermoelectric generator. Over time, the plutonium decays, and the power drops. Power was being used to warm the main thrusters, so engineers commanded the spacecraft to switch to a backup set of thrusters, saving about 12 Watts (out of less than 300 Watts being supplied by the RTG). That's a significant saving, and again will extend Voyager 2's life by a few years.

Why do this? Right now, both spacecraft (as well as the Pioneer 10 and 11 spacecraft, launched earlier but at a slower speed) are flying through a region of space where the Sun's influence wanes and things are becoming more interstellar. The Sun blows out a continuous wind of particles called the solar wind. As this wave pushes against the material floating between the stars it slows down and stops. There are lots of interesting magnetic effects in this region, including how zippy particles from different parts of the galaxy interact with the solar wind. There are lots of ways to measure these effects from Earth, but this part of space is a long way off, and it's really nice to have our machines right there, in situ, tasting the particles that hit them and seeing what's what. This is pretty nifty science that we get at a phenomenal discount, because the Voyagers are already there.

Space exploration is hard, and costly, and time-consuming. It makes me proud to know that men and women are working so hard to make sure that we squeeze every drop, every gram, every millisecond pulse of thrust out of these wonderful robots. They're our proxies to the galaxy, and it's lovely to know they still are out there doing their job.