Today is 45th anniversary of the launch of Voyager 1, one of humanity’s iconic twin messengers into the cosmos. (Her sister, Voyager 2, was launched a few weeks earlier.) Now in the dark, faraway regions of interstellar space — more than 10 billion miles from home, where our Sun looks like any other bright star — the two operate still science. They carry the gold discs with the sounds and symbols of the earth in case an extraterrestrial should ever meet one of the spaceships and become curious about its distant sender.
“I’ve followed Voyager’s arc throughout my career,” says Linda Spilker, associate Voyager project scientist at NASA’s Jet Propulsion Laboratory, who joined the agency in 1977, the year the probes were launched. “I’m amazed at how long these two spacecraft, Voyager 1 and Voyager 2, have been able to keep going and deliver unique science about new places that no spacecraft has visited before. And now they have become interstellar travelers. How cool is that?”
The two car-sized probes, each with a 12-foot antenna mounted on it, had one main purpose: to visit the gas giants in our own solar system. Voyagers parted ways after their launches, but both benefited from a rare planetary lineup, captured groundbreaking photos as they flew past Jupiter, Saturn, Uranus and Neptune, and revealed tantalizing details about the planets’ moons. By the end of 1989 they had completed that mission. In 1990, Voyager 1 topped it by turning around and taking what astronomer and science communicator Carl Sagan called a poignant picture of our own world Pale blue dot.
“Look at that point again. That is here. This is home. This is us. That’s what everyone you love, everyone you know, everyone you’ve ever heard of, every person that’s ever been has lived their lives on,” Sagan wrote. The image of Earth from a cosmic perspective—a mere “speck of dust floating in a moonbeam,” as he put it—became almost as memorable as that earthrise Photo taken by an Apollo 8 astronaut showing the planet as seen from the moon.
The two probes, powered by nuclear-powered systems called radioisotope thermoelectric generators (RTGs), continued to fly. Our solar system doesn’t have a clear boundary, but in the 2000s they crossed the “termination shock,” where solar wind particles abruptly decelerate below the speed of sound due to the pressure of the gas and magnetic fields in interstellar space. Then, in the 2010s, they broke the heliopause, the boundary between the solar wind and the interstellar wind.
With four instruments operating on Voyager 1 and five aboard Voyager 2, they now have a new task: measuring the strength of the magnetic field, the density of the plasma, and the energy and direction of charged particles in the environment through which they travel. “The purpose of the interstellar mission is to measure the effects of the Sun as we move farther and farther from Earth. We’re trying to figure out how the Sun’s heliosphere interacts with interstellar space,” says Suzanne Dodd, project leader of the interstellar mission Voyager at JPL. Voyager 1 is currently 14.6 billion miles from home and Voyager 2 is 12.1 billion miles away, but for perspective, the nearest star is about 25 trillion miles away. (NASA maintains a tracker of their voyages.) It’s a notable coda for their mission, decades after the probes achieved their primary goals.