Beyond Earth and its bubble of satellites; past Mars, where rovers explore; past Jupiter and its circling orbiter, two spacecraft are gliding across interstellar space. They have crossed over the invisible boundary that separates our solar system from everything else, into territory nearly untouched by the influence of the sun. People have seen much deeper into the universe, thanks to powerful telescopes that catch the light of distant stars. But this is the farthest a human invention has ever traveled. These hunks of gleaming metal and circuitry—they are the furthermost tangible proof of our existence.
The twin Voyager spacecraft took off in 1977, carrying scientific instruments and golden records stuffed with information. Millions of miles away, they still communicate with Earth. They still collect data. But they are aging.
The spacecraft, traveling in slightly different directions, weaken every year. Their thrusters, which keep them steady, are degrading. Their power generators produce about 40 percent less electricity than they did at launch.
To keep the Voyagers going, engineers make some tough decisions from afar. They tell the Voyagers, in commands carried over radio waves, to shut down systems or switch to backups, rationing every single watt. They prepare for what may be the mission’s final years. “Someday we’re going to have to say goodbye,” says Candy Hansen, a scientist at the Planetary Science Institute who worked on the Voyager mission in the 1970s and 1980s.