Together, these expired spacecraft add up to more than a pile of space junk. They are a floating record of years’ worth of scientific inquiry, launched from the minds of explorers who couldn’t fly there themselves. “Each one tells a story about the state of knowledge at the time it was launched,” Alice Gorman, an archaeologist who studies space exploration, told me.

Photos: Building the world’s most powerful telescope

Spitzer—named for Lyman Spitzer, the American astrophysicist who called for space telescopes long before anyone had even launched a satellite—is designed to detect sources of light in infrared wavelengths. In the 1960s, scientists curious about seeing the cosmos in this way attached infrared telescopes to balloons and lofted them into the sky. They needed to get past Earth’s atmosphere, which absorbs this kind of radiation. (“Trying to see faint infrared sources from the ground,” NASA explains, “is like trying to observe stars while the sun is up.”) A National Academy of Sciences report in 1979 encouraged the deployment of infrared telescopes, saying that in recent years “the sensitivity of instruments used for infrared astronomy has been improved by more than two orders of magnitude,” leading to exciting findings about neighboring planets and distant stars. By the 1990s, NASA was working on Spitzer.

Spitzer’s sensitivity to infrared light allows it to see very faint cosmic objects. Its discoveries stretch from our own solar system to the edges of the universe. Spitzer found a ring around Saturn made of dust particles too spread out for other telescopes to see, and caught the light that left galaxies billions of years ago, not long after the Big Bang. By the end of its life, Spitzer was contributing to an area of study that barely existed when engineers were first laying out its blueprints: exoplanets. Spitzer detected chemical elements and even weather patterns in the atmospheres of distant planets orbiting other stars.

Once decommissioned, space missions aren’t expected to come back from the dead, but it has happened: Last year, a NASA spacecraft launched to study Earth’s magnetosphere miraculously resurrected itself 13 years after it was presumed broken, but, sadly, it never returned to full operations. (It’s still orbiting Earth, though.)

Spitzer is expected to remain in orbit for many, many years, along with its silent brethren. Spitzer and Earth will remain in a strange little dance as they move around the sun. According to NASA, Earth will catch up to Spitzer in 2051, approaching from behind. The encounter will push Spitzer into an orbit closer to the sun, where it will travel faster, leaving Earth to trail after it until their next close approach.

The long afterlife of these spacecraft presents an intriguing question for Gorman. Years from now, there may be many more space archaeologists, and some of them might be curious about what their ancestors left behind in the space near and around Earth. “If a future observer was trying to get an idea of the state of terrestrial science, could they infer it from the age of the spacecraft and the nature of the instruments on board?” Gorman says.