Forty years after they left Earth, the Voyager twin spacecraft are still chugging along, logging 35,000 miles an hour as they zoom farther and farther into the cosmos.

“I’ve had people ask me, you mean the mission is still going on?” says Suzanne Dodd, the Voyager project manager at NASA’s Jet Propulsion Laboratory. “They assumed that it had stopped after it passed Neptune.”

Far from it. After the Voyagers completed their tours of the outer planets in the 1980s, giving humanity its first real look at Jupiter, Saturn, Uranus, and Neptune, they continued on to the outer reaches of the solar system. In August 2012, Voyager 1 left the system entirely, emerging from inside the protective bubble formed by the sun’s wind and exiting into interstellar space. Voyager 2 is on its way out; the spacecraft is currently coasting through the heliosheath, the outermost layer of the sun’s bubble. Voyagers 1 and 2 are currently about 13 billion and 10 billion miles from Earth, unfathomable distances that mean little more to us terrestrials than giant numbers on a page.

And they still call home. It takes a while, but they do.

The Voyagers transmit data to Earth every day. The spacecraft collect information about their surrounding environment in real time and then send it back through radio signals. Voyager 1 data takes about 19 hours to reach Earth, and signals from Voyager 2 about 16 hours. (For comparison, it takes the rovers on Mars 20 minutes on average to call home.) The signals get picked up by NASA’s Deep Space Network, a collection of powerful antennae around the world that communicate with dozens of missions.

The Voyagers break down their measurements into digital “bits” that can be shipped to Earth using radio waves, explains Jeff Berner, the Deep Space Network chief engineer, who has worked there for nearly 30 years. DSN collects these bits and then sends them over to the Voyager team, where scientists unpack and translate them back into readable measurements.

The DSN spends between four and seven hours per day listening for the faint pings of the spacecraft. The power of the transmitter aboard the spacecraft is similar in wattage to that of a refrigerator light bulb. But the DSN is sensitive enough to hear its messages, and, if necessary, can become even more sensitive. If Earth needs a bigger ear for Voyager in the future, the DSN can make its facilities work as a single array, with more collecting power.  

“That we’re able to get a signal and extract data from it, to me, is just an amazing thing,” Berner said. “It just goes to show how well the spacecraft was built and how good the equipment that we have in the DSN is.”

The Voyagers eventually will go quiet. The spacecrafts’ electric power, supplied by radioisotope thermoelectric generators, weakens each day. Dodd said scientists and engineers will likely begin shutting off instruments in 2020, a debate that she says is already underway. “These scientists have had their instruments on for 40 years,” she said. “Nobody wants to be the first one turned off.”

The spacecrafts’ transmitters will be the last to go. They will die on their own, in the late 2020s or perhaps in the 2030s. “One day we’ll be looking for the signal and we won’t hear it anymore,” Dodd said.

Until then, the Voyager mission will keep reporting back on magnetic fields and charged particles, a cross between a weather satellite and an explorer. There will be no more pictures; engineers turned off the spacecraft’s cameras, to save memory, in 1990, after Voyager 1 snapped the famous image of Earth as a “pale blue dot” in the darkness.

Out there in interstellar space, where Voyager 1 roams, there’s “nothing to take pictures of,” Dodd said.