Today, NASA is shutting down one of its long-lasting space observatories, a telescope that’s been wandering through space for nearly two decades, snapping images of some of the coldest and most ancient objects in the Universe.

The soon-to-be sleeping spacecraft is the Spitzer Space Telescope, launched in 2003. The observatory was originally designed to last between two-and-a-half and five years, but thanks to clever engineering, the scientists operating the observatory were able to squeeze another decade of operations out of it. “I can assert with confidence that no one expected that the Observatory would still be operating and doing exciting science in 2019, the tenth year of the extended mission,” Lisa Storrie-Lombardi, the project manager for Spitzer, wrote in a blog post. Spitzer has proven to be a remarkable tool for learning more about the cosmos, helping us to discover newly forming stars, new rings of Saturn, and even an entire solar system around 40 light-years away.

But now the telescope’s time of gathering data is coming to an end. NASA has run out of money to fund the spacecraft. In 2017, NASA started searching for private organizations to take over Spitzer, which cost roughly $11 million to operate in 2018. That search was unsuccessful. “Two institutions responded to the request for information, but these proposals could not provide enough resources to operate Spitzer,” NASA said in a statement to The Verge. So the decision was made to put Spitzer into hibernation, meaning the telescope will no longer collect any more data, and send it back to Earth.

Despite being offline, Spitzer could still help scientists make more discoveries in the future. The entire archive of observations made by the telescope will be available to anyone who wants to use it. It’s possible that even more great finds are hiding in these records. “We’re going to be learning things out of the Spitzer archive for decades to come,” Luisa Rebull, an associate research scientist at Caltech working on Spitzer, tells The Verge.

Image: NASA/JPL-Caltech/L. Rebull (SSC/Caltech)/D. De Martin

Spitzer is one of four space telescopes operated by NASA known as the Great Observatories. Its fellow “greats” are the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the famous Hubble Space Telescope. Combined, the four telescopes were meant to observe the Universe in as many wavelengths of light as possible — ranging from the visible light that we can see, to the kinds of light our eyes cannot register. Spitzer’s charge has been to observe infrared light, a type of light that humans can’t see but can sense as heat. Objects that are faint and super cold can still be seen by the infrared light that they produce, so Spitzer can show us things that might otherwise seem invisible.

But seeing the Universe in infrared is tough. Anything that emits a lot of heat also emits a lot of infrared light, which means there are plenty of sources that can muck up observations. The Earth’s atmosphere is a source of infrared light, which makes it difficult to observe the Universe in infrared from the ground. And if your telescope gets too warm, the infrared light it emits can also be a source of confusion. That’s why a telescope like Spitzer is ideal. “If you want to do the most sensitive infrared work, you want to use a cold telescope in space,” Michael Werner, the project scientist for Spitzer, tells The Verge.

To make sure Spitzer stays extra cold, NASA decided to put the telescope in what’s known as an “Earth-trailing orbit.” It’s exactly like it sounds: the spacecraft is in more or less the same orbit as Earth, lagging behind the planet as it travels around the Sun. In this location, Spitzer gets farther and farther from Earth each year and doesn’t warm up from the heat coming off our planet. That way it stays nice and cool. For an extra layer of coolness, Spitzer also launched to space with a special liquid helium coolant, which helped to keep the spacecraft and its instruments at a frigid temperature of -459 degrees Fahrenheit (or -273 degrees Celsius).

When it was extra frosty, Spitzer did some incredible infrared work. “The somewhat snarky phrase is that Spitzer studies the old, the cold, and the dirty,” Rebull says. Old stars and galaxies were Spitzer’s specialty, as well as extra cold objects that may be cold by human standards, but are actually warmer than the backdrop of our extra cold Universe. Spitzer could spot baby stars in the middle of forming, which are too faint to see but observable in infrared.

Image: NASA/JPL-Caltech/Keck

The telescope could also “see” through giant clouds of dust to observe objects that might be hiding within — hence the “dirty” part of the phrase. And one of Spitzer’s greatest discoveries was a dusty one. It found rings of Saturn we hadn’t been able to detect before. “This ring is so tenuous that even if you were standing in it, you would be hard pressed to see it in the optical,” says Rebull. “But because it’s dusty, Spitzer could see it.”

Spitzer operated in this “cold mission” for 5.5 years, well beyond the 2.5 years that NASA had hoped. But eventually, the spacecraft ran out of the liquid helium coolant, which prompted Spitzer’s warm mission. “The telescope warmed up, but it didn’t warm up very much,” says Werner. Because of how it was built and its location in space, Spitzer was able to radiate much of its heat out into the cold blackness of space. Still, it wasn’t able to observe as much as it could before. But Spitzer made perhaps its biggest find during its warm phase: the discovery of an entire solar system of seven Earth-sized planets, orbiting a star called TRAPPIST-1. “It was our greatest hit, which got us above the fold on The New York Times,” says Werner.

The team that’s been running Spitzer can easily boast a successful mission with the telescope, even as it’s coming to what might be perceived as a premature end. This afternoon, NASA scientists will send a command to Spitzer, telling it to stop taking observations and no longer send pings back to Earth. Conceivably, NASA could wake Spitzer back up again someday, but as more time passes, the tougher that will be. Spitzer will no longer be pointing its antenna back at Earth and sending a strong signal that NASA can pick up. So making a link with the spacecraft will become harder and harder.

But even as Spitzer hangs up its hat, there are still other infrared telescopes in the works — notably, NASA’s next great space observatory, the James Webb Space Telescope. Designed to study the Universe in infrared, the James Webb will be the most powerful space telescope ever when it launches, and will be able see back in time to the beginning of the Universe.

Still, it’s a bittersweet day for many at NASA who have been working on Spitzer for most of its lifetime. “I know this is just a space robot, but he’s our space robot, right?” Rebull says. “It is sad. It is hard to say goodbye. But basically my primary feeling is just profound gratitude for being allowed to come to work for Spitzer for as long as I have.”