NASA Revives Voyager 1's thrusters

Maybe the better wording is NASA resurrected the thrusters since they've been considered dead since 2004.   

Engineers at NASA’s Jet Propulsion Laboratory in Southern California have revived a set of thrusters aboard the Voyager 1 spacecraft that had been considered inoperable since 2004. Fixing the thrusters required creativity and risk, but the team wants to have them available as a backup to a set of active thrusters whose fuel tubes are experiencing a buildup of residue that could cause them to stop working as early as this fall.

In addition, the mission needed to ensure the availability of the long-dormant thrusters before May 4, when the Earth-bound antenna that sends commands to Voyager 1 and its twin Voyager 2 went offline for months of upgrades.

As a refresher, the Voyagers were launched in 1977 for a "four year mission" to Jupiter and Saturn - the 48th anniversary of the first launch (Voyager 2, not 1) will be in August.  The flybys of Jupiter and Saturn were part of a "Grand Tour" of the outer planets that had been studied in the 1960s when a doctoral student named Gary Flandro was working at the Caltech JPL.  He was the guy who plotted the positions of all of the outer planets for the coming 20 years, (with pencil and paper) and realized that a trajectory was possible where a probe could use each planet in series as a gravitational slingshot to the next.  A complete tour of Jupiter, Saturn, Uranus and Neptune could be done in 10 or 12 years rather than the decades such a tour might require otherwise. The mission launch window would open for a matter of months in the late 1970s, and then the geometry would be gone - not appearing for another 175 years.   

Voyager 1 exited the solar system in August of 2012, followed by Voyager 2 in November of 2018, and are the farthest objects from Earth of anything made by humans.  They're getting farther at around 35,000 mph.  

Both spacecraft rely on a set of primary thrusters to gently pivot them up and down as well as to the right and left in order to keep their antennas pointed at Earth so they can send back data and receive commands. Within the primary set of thrusters are other thrusters that control the spacecraft’s roll motion. Seen from Earth, the roll motion rotates the antenna like a vinyl record to keep each Voyager pointed at a guide star it uses to orient itself. Both spacecraft have a primary and backup set for these roll movements.
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To manage the clogging tubes in the thrusters, engineers switch between the sets of primary, backup, and trajectory thrusters of both Voyagers. But on Voyager 1, the primary roll thrusters stopped working in 2004 after losing power in two small internal heaters. Engineers determined the broken heaters were likely unfixable and opted to rely solely on Voyager 1’s backup roll thrusters to orient the star tracker.

I've said before that I consider the Voyagers among the greatest accomplishments mankind has ever achieved, rivaling Apollo 11 as the Peak of Western Civilization, and things like this effort - coming up with clever tricks to keep these probes working as they're approaching the 48th year into their four year mission - are one of the reasons.  The big thing they were working around (and concerned about), that the Deep Space Network antenna they rely on would be going out of service, has happened before and the two Voyagers glided along just as predicted for 11 months until the dish was back operating.  The Voyagers are on the order of 18.5 radio (= light) hours away, so any communication that gets messed up could just open another batch of problems - or be the last communication we ever have with the Voyager.

While both Voyager spacecraft remain operational, however, their age and immense distance from Earth have brought about significant technical challenges. The radioisotope power generators that keep them running gradually weaken each year, forcing NASA to recently shut down instruments and heaters to conserve energy in order to push the spacecrafts' systems beyond their limits. Voyager 1 also experienced a recent data glitch caused by a faulty chip; engineers resolved this with a clever software workaround.

Yet despite these hurdles, the Voyagers continue to function — a testament to both their robust design and the ingenuity of the teams managing them.

This NASA graphic shows the locations of NASA's Voyager spacecraft in interstellar space.  At this scale, the distance that their 35,000 mph velocity adds is invisible. (Image credit: NASA/JPL-Caltech)