Voyager 1, NASA's deep space exploration probe, could soon become the first spacecraft to reach a historic milestone. In November 2026, the probe will be one light day away from Earth.
Launched in 1977, Voyager 1 is the furthest spacecraft from our planet and is currently exploring interstellar space at a distance of 15.8 billion miles.
The term “daylight hours” refers to the distance it would take 24 hours for a signal or command traveling at the speed of light to reach the spacecraft from Earth, said Susie Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory. One day of light is equal to 16 billion miles (26 billion kilometers).
So if the Voyager crew asks the spacecraft to do something once it reaches that point, Voyager will take another day to respond.
“If I send a command and say, 'Good morning, Voyager 1,' at 8 a.m. on Monday, I'll get Voyager 1's response back on Wednesday morning at about 8 a.m.,” Dodd said.
Voyager 1 and its twin Voyager 2 are the only spacecraft operating outside of space. heliospherea solar bubble of magnetic fields and particles extending far beyond the orbit of Pluto. After decades in space, the two have had to shut down several instruments, but they are using their remaining instruments to explore this uncharted territory and provide data that can help future missions.
Keeping up with such far-reaching research comes with many challenges, but Dodd and her team are taking the necessary steps to ensure their “senior citizens” reach age 50.th anniversary in 2027.
Staying connected across cosmic distances
Launched on a voyage to explore Jupiter and Saturn, Voyager 1 has been moving away from Earth along the same trajectory and at the same speed—38,000 miles per hour—since its flyby of Saturn in November 1980.
Knowing the Earth's location relative to Voyager 1, the spacecraft's speed and its trajectory, engineers can calculate how long it will take a signal to reach the probe.
Voyager 1 launched from Kennedy Space Center in Florida on September 5, 1977. – NASA/JPL-Caltech/KSC.
For example, Voyager 1's trajectory after its flyby of Saturn took it up and out of the planetary plane after it passed Saturn, while Voyager 2 passed over the top of Neptune after its flyby of the ice giant in 1989 and moved down and out of the planetary plane. Neither probe has made any trajectory adjustments since their last flybys, meaning both have been on continuous cruises for decades.
Voyager 2 is expected to reach within one light day of Earth before November 2035, and even the most ambitious estimates suggest that the spacecraft will not be operational then. But both studies continue to surprise the team.
Every day, as the oldest active spacecraft, the Voyager probes set records just by continuing to operate.
But it was not an easy process.
The probes send data back at a very low speed of 160 bits per second, which is about the speed of dial-up Internet, Dodd said.
“At the distance we are from Earth, it takes a lot longer to get a signal there, and the signal strength just dissipates,” Dodd said. “It takes multiple antenna arrays to put that signal back together.”
The slow data rate means Dodd and her team have little information about the health of each spacecraft, and if a problem occurs, they won't be able to respond quickly.
However, both Voyagers are designed to be self-sufficient and have enough onboard autonomy that they can escape if something goes wrong billions of miles from Earth.
“If something goes wrong, they can put themselves in a safe state and wait until we can talk to the spacecraft and figure out what the problem is and fix the problem,” Dodd said.
Why the Voyager probes stand the test of time
Over the years, the team has made difficult decisions to ensure the probes last as long as possible, Dodd said. This means turning off engineering systems and instruments to save energy and ensuring the spacecraft remains warm enough to function.
In order for the Voyager probes to continue communicating with Earth, their antennas must also be pointed at our planet.
If the fuel lines freeze on any of the probes, causing the antennas to point elsewhere, “we will lose the mission because we will no longer be able to transmit a signal to the spacecraft,” Dodd said.
And it's not just that the Voyager probes continue to fly – they need to work with scientific instruments.
Before their 50th anniversary in 2027, both spacecraft will likely need to shut down additional instruments and systems. The team hopes to keep the cosmic ray subsystem on Voyager 2 running, as well as the magnetometer and plasma wave subsystems on both spacecraft. These instruments will allow both probes to function as weather satellites in interstellar space, sensing the environment in which they travel, Dodd said.
Scientists are interested in understanding how the Sun's magnetic field changes and interacts at the heliopause, the boundary of the heliosphere where the hot solar wind coming from the Sun meets cold interstellar space.
“Think of the heliopause as an ocean shoreline,” Dodd said. Once in the water, you will find ripples, waves and other changing factors as you move further from shore, and at some point things become more stable. The Voyager probes measure the ripples, or interactions, between the heliopause, our Sun and interstellar space the further the spacecraft travels from the Sun.
“It's important to work with these scientific instruments for as long as possible to map what changes as you move away from the sun,” Dodd said.
Dodd is confident that at least one of the spacecraft can operate for another two to five years. According to her, every year the process of supporting unprecedented missions becomes more difficult.
But behind Voyager is a remarkable team, including NASA retirees in their 80s who advise on specific subsystems and crew members so young that their parents weren't even born when the probes launched.
“This kind of generational effort on Voyager is really great to see,” Dodd said. “I love these spaceships. They are ambassadors for us here on Earth.”
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