On Aug. 20, 1977, NASA launched the Voyager 2 probe on a mission to explore the outer planets of our solar system.
Twelve years later, having scanned Jupiter, Saturn, Uranus and Neptune, the 1,600-pound spacecraft completed its initial mission.
But it kept going. And going. And going. Nursed farther and farther into space by a small team at NASA’s Jet Propulsion Laboratory in California, Voyager 2 is now more than 11 billion miles from Earth, making it the second-most-distant man-made object from Earth after its speedier twin Voyager 1.
In late January, Voyager 2’s decades-long voyage almost came to an untimely end. The mishap underscores the delicacy of NASA’s super-long-range space missions, as well as the ingenuity of the space agency’s probe-operators.
After zipping past Neptune in 1989, Voyager 2 took on a new mission: working alongside Voyager 1 to investigate the “heliosphere,” which NASA spokeswoman Calla Cofield described as “the bubble of particles and magnetic fields created by the sun.”
On Jan. 25, Voyager 2 was supposed to execute a scheduled maneuver, turning 360 degrees in order to calibrate an on-board magnetic instrument. But the probe never made the turn.
Voyager 2’s operators in California found out about the missed turn 17 hours later— the time it takes for a radio signal to travel the billions of miles from the spacecraft to Earth.
“Analysis of the telemetry from the spacecraft indicated that an unexplained delay in the onboard execution of the maneuver commands inadvertently left two systems that consume relatively high levels of power operating at the same time,” NASA explained in a release. “This caused the spacecraft to overdraw its available power supply.”
The Voyager probes carry plutonium-powered generators that can produce 470 watts of power—the equivalent of a handful of light bulbs. Any system that consumes even a few extra watts could shave years off the probes’ lifespans.
To protect against a sudden battery drain, the Voyagers’ software automatically shuts off the probes’ science instruments whenever it detects a surge in power consumption. Of course, without instruments the spacecraft are just useless hunks of metal streaking 34,000 miles per hour into the cosmos.
The team in California got to work fixing Voyager 2. They’d done it before. In 2010 a single bit in Voyager 2’s memory flipped from zero to one, introducing a flaw into the data the probe was sending back to Earth.
To diagnose the error, engineers at the Jet Propulsion Laboratory ran simulations on a computer running the same software as the distant probe. It took a few weeks, but they were able to transmit instructions to Voyager 2 to flip the bit back to zero.
But the 2010 fix perhaps seemed deceptively easy. In fact, the teams managing the Voyagers as well as NASA’s other deep-space probe, New Horizons, can prevent or cause catastrophe with a single keystroke.
(New Horizons, which launched in 2006, is probing the Kuiper Belt, a region of the solar system beyond Neptune that scientists believe contains a large number of comets and asteroids.)
In 1982, a NASA team botched a software update on the Viking 1 Mars lander. The bad code caused the lander to point its antenna in the wrong direction, permanently cutting off all communication with Earth.
“They can never make a single mistake,” Alan Stern, the principal investigator for the New Horizons mission, told The Daily Beast, referring to NASA’s probe teams. And the teams have to maintain that flawless performance every day for decades as presidential administrations come and go, budgets rise and fall, old staff retire and new staff join up.
Fixing Voyager 2’s January malfunction ended up being fairly straightforward. The engineers essentially rebooted a few key systems. Then waited 17 hours to hear back from the probe.
On Feb. 5, NASA announced the good news. “Mission operators report that Voyager 2 continues to be stable, and communications between Earth and the spacecraft are good,” NASA’s Cofield told The Daily Beast. “The spacecraft has resumed taking science data, and the science teams are now evaluating the health of the instruments following their brief shut-off.”
Stern praised the engineers who saved the probe. “The Voyager guys, they were under the gun of time with high stakes,” he said. “They discovered a problem and the clock was ticking to save the day.”
With any luck, the Voyagers have another four or five years worth of power left. NASA plans for the probes to continue investigating the heliosphere until the day they finally power down, sometime in the mid-2020s. With its fresher batteries, New Horizons could last into the 2040s.