Repairing Voyager 1 after 48 years in space

As the Voyager 1 spacecraft's thrusters get fixed, we take a look at the long-lived mission, which brought worldwide excitement and wonder when launched in 1977. In particular, because on top of the 11 scientific instruments designed to study celestial bodies, interplanetary space, and eventually interstellar space, both spacecraft also carried the famous Golden Record. This record, curated by a team led by Carl Sagan, is a symbolic gesture trying to communicate humanity's existence, diversity, and curiosity about potential extraterrestrial civilization along the path of the Voyagers.

Voyager 1 is one of a pair of spacecraft that NASA launched in the summer of 1977 from Cape Canaveral, Florida. As initially designed, Voyager1 and Voyager2 were to conduct close-up studies of Jupiter and Saturn, Saturn's rings, and the larger moons of the two planets in five years.

The primary mission objective aimed to explore two planets became four planets, and their five-year lifetimes stretched to 48 years. Within the first 12 years, Voyager 1 and 2 explored all the giant outer planets of our solar system and 48 of their moons. After completing these objectives, both spacecraft continued their journey towards their next phase, the Sun's termination shock region, the boundary of the Sun's influence, moving then to the heliosheath and finally to interstellar space, more than 30 years after launch. Since then, NASA JPL has collected large amounts of unprecedented information that helps us corroborate some of the theories about the material that permeates the galaxy and between stars.

How was this possible?

Thanks to its robust design powered by the Radioisotope Thermoelectric Generator (RTG), and the management of the thrusters by NASA's JPL team, the mission achieved its primary objectives and additional goals that the team considered optimistic. Although some instruments, like the Ultraviolet Spectrometer Subsystem (UVS), stopped working, NASA JPL manages power usage by turning off instruments, like the Plasma Science (PLS) on Voyager 1 in 2007, after it showed degraded performance in 2007. Other instruments like the Planetary Radio Astronomy Investigation (PRA) on Voyager 1 and 2 and the Plasma Science (PLS) system on Voyager 2 were turned off in early 2008 and September 26, 2024, respectively, to save power. On top of this, NASA JPL reprogrammed the Voyagers to give them more capabilities, allowing the mission to extend beyond its original objectives.

The first years of the mission

The Voyagers' main objectives were to conduct intensive close flyby studies of Jupiter and its large moon, Io, and Saturn and its large moon, Titan. In the case of Voyager 2, the chosen flight path also preserved the option to continue to Uranus and Neptune. In this case, the extended planetary mission was possible due to an outer planet's rare geometric arrangement happening in the late 70's and 80's. Occurring about every 175 years, the particular positions of the planets provided the needed slingshot effect to go from one planet to the other with Voyager's small propulsion system, minimum propellant, and short trip time.

Both Voyagers launched from the NASA Kennedy Space Center at Cape Canaveral, Florida, aboard Titan-Centaur expendable rockets. Voyager 2 launched first on August 20, 1977, while Voyager 1 launched on a faster but shorter trajectory on September 5, 1977. Voyager 1 reached Jupiter on March 5, 1979, and Saturn on November 12, 1980, followed by Voyager 2 to Jupiter on July 9, 1979, and Saturn on August 25, 1981.

Studying Jupiter

Together, the spacecraft took about 52,000 pictures of Jupiter and its moon. From these images, astronomers understood critical physical, geological, and atmospheric processes in the planet, its satellites, and its magnetosphere. They also discovered active volcanism on the satellite Io, the first time we saw active volcanoes on another body in the solar system.

Studying Saturn

Saturn's encounters occurred nine months apart, with Voyager 1 in November 1980 and Voyager 2 in August 1981. From them, we learned that the atmosphere of this planet is almost entirely hydrogen and helium and has winds blowing at high speeds. Voyager 2 used its radio beam to penetrate the upper atmosphere, measuring minimum temperatures of 82 Kelvins (-312 degrees Fahrenheit) to 143 Kelvins (-202 degrees Fahrenheit) at the deepest levels. Near the north pole, temperatures were about 10 degrees Celsius (18 degrees Fahrenheit). The Voyagers also found aurora-like ultraviolet emissions of hydrogen at mid-latitudes and auroras at polar latitudes (above 65 degrees). Both Voyagers measured Saturn's rotation (the length of a day) to be 10 hours, 39 minutes, and 24 seconds.

The mission beyond 5 years

After Voyager 1 visited Saturn's moon, Titan, its path bent 35 degrees northward, out of the ecliptic, the plane where the planets in the Solar System orbit. This path took the spacecraft on a direct course outside the solar system at a rate of about 320 million miles a year.

Visiting Neptune and Uranus

Voyager 2, on the other hand, continued its journey to Uranus and Neptune. Voyager 2 encountered Uranus on January 24, 1986, and Neptune on August 25, 1989. The images of the five largest moons around Uranus taken by Voyager 2 revealed complex surfaces indicative of varying geologic pasts. It also detected 11 previously unseen moons. It uncovered fine details of the previously known and two newly detected rings. It also showed that the planet has a magnetic field that is both large and unusual. Finally, it measured that the planet has a rotation rate of 17 hours and 14 minutes.

After visiting Neptune, Voyager 2 is also headed out of the solar system, diving below the ecliptic plane at an angle of about 48 degrees and a rate of about 292 million miles per year.

Studying the interstellar medium

Once the planetary flybys were done, the Voyager Interstellar Mission (VIM) began. Using the four remaining operating instruments aboard the Voyager 1 and Voyager 2 spacecraft, the science teams started collecting data focused on understanding the strength and orientation of the Sun's magnetic field

• The composition, direction, and energy spectra of the solar wind particles and interstellar cosmic rays

• The strength of radio emissions that are thought to be originating at the heliopause and

• The distribution of hydrogen within the outer heliosphere

The VIM was executed in three phases; the first one was completed by Voyager 1 when it passed through the Termination Rhock in December 2004 at 94 AU (1 AU is the distance between Sun-Earth). Voyager 2 passed this boundary at 84 AU in August 2007. Next, the mission began the phase "Exploring the Heliosheath," or the outer layer of the heliosphere that is still dominated by the Sun's magnetic field and the particles in the solar wind. Finally, on August 25, 2012, Voyager 1 entered interstellar space (ISM) at about 122 AU, or about 11 billion miles from the Sun, becoming the first human-made object to do so. Voyager 2 crossed this boundary on November 5, 2018.

After 48 years, Voyager 1 is about 15.46 billion miles, and Voyager 2 is about 12.94 billion miles from Earth. They continue gathering data from interstellar magnetic fields, particles, and plasma waves in interstellar space, sending back this information in real-time during 6-8 hour periods. With this, the JPL team has learned about the gas's composition, density, and temperature near the heliosphere. They keep measuring the strength and direction of the interstellar magnetic field, learning about the ISM's magnetic turbulence and the dynamic interactions occurring there. From the Voyagers, they also leaned how the solar influence extends further into the ISM than expected.

Credit NASA

The Voyagers in the coming years

The Voyager mission will continue studying the ISM as long as RTG, powered by plutonium-238 radiation) produces enough energy to run the instruments and there is enough fuel (hydrazine) available to keep the thrusters working. The thrusters are critical to keeping Voyager's antennas aligned with NASA's Deep Space Network (DSN), which downloads the data and uploads commands to the spacecraft. The DSN is a set of 3 large antennas in the USA, Spain, and Australia to gather data and send commands to each spacecraft.

Unfortunately, fuel is not the only concern with the thrusters. These clog with time, becoming unusable and threatening the end of the mission. Each Voyager has 12 thrusters: two primary attitude control thrusters used to maintain the spacecraft's orientation and their backups. Two thrusters control the spacecraft's roll motion, keeping the Voyager pointed at a guide star it uses to orient itself; these thrusters also have a backup. A set of thrusters assisting in the early years with the spacecrafts' trajectory change during the flybys of the outer planets and their backup.

Thrusters’ clogging

On Voyager 1, the primary roll thrusters stopped working in 2004 after losing power in two small internal heaters, forcing engineers to rely solely on Voyager 1's backup roll thrusters to orient the star tracker. The trajectory thrusters came back to life in 2018 and 2019 --they went dormant in 1980 for Voyager 1 and 1989 for Voyager 2. Unfortunately, these cannot induce roll motion, which is critical to maintaining the spacecraft's orientation.

Since the Voyagers have used the backup roll thrusters for 20 years now, the NASA JPL team is concerned these will clog as early as 2025's fall. Without the ability to control the spacecraft's roll motion, various issues could threaten the mission.

Fixing the thrusters

This concern motivated the NASA JPL team to reexamine the 2004 thruster failure. Suspecting a flip of the heater switch to the wrong position, they decided to try switching it back to its original position and see if the heaters might work again, enabling them to reactivate the primary roll thrusters.

However, this had to happen before May 4, 2025, when the Deep Space Station 43 (DSS-43) antenna in Canberra, Australia, part of NASA's DSN, would be turned off for almost 11 months to undergo upgrades. DSS-43 is a 230-foot-wide antenna and the only dish of the three the DSN has, with enough signal power to send commands to the Voyagers.

That is why, on March 20, 2025, the team got the star tracker pointed as precisely as possible, sent commands to turn on the dormant roll thrusters, and then tried fixing and restarting the heaters. Due to the spacecraft's distance from Earth, the team had to wait 23 hours for the signal to travel from the spacecraft to Earth; however, within 20 minutes of data gathering, the team saw the temperature of the thruster heaters rise dramatically and knew they had succeeded.

With the roll thrusters bank online, we can expect the Voyager mission to continue gathering information about the ISM for many more years until the RTG cannot produce enough energy or the fuel gets depleted sometime in the 2030s.