On Thursday afternoon, NASA operators will send a farewell missive to the Spitzer Space Telescope from the big antenna at the Goldstone Deep Space Communication Complex in Southern California. It will take the radio signal about 10 minutes to make the 165-million-mile journey across interplanetary space. When Spitzer receives the radio command, it will boot into safe mode and relay the command back to Goldstone. That transmission will officially end Spitzer’s 16-year mission and allow the telescope to drift peacefully through the void.

“It’s going to be a bittersweet moment,” says Michael Werner, a NASA astronomer who has worked as Spitzer’s project scientist since 1983, when the telescope was little more than an idea. “I’m sure it’s going to be somewhat painful, but I take great pride in my role in making it happen.”

Spitzer left Earth a little after midnight on August 25, 2003, atop a Delta II Heavy rocket launching from Cape Canaveral in Florida. Standing 13 feet tall and weighing 2,000 pounds, the tube-shaped telescope’s arrival in space marked the beginning of an exciting new era for astronomy. The scope was designed to study stars, exoplanets, and galaxies in the infrared, a region of the electromagnetic spectrum just on the other side of visible light.

Infrared radiation is the heat given off by all objects warmer than absolute zero, but the filtering effect of Earth’s atmosphere makes it difficult to record infrared observations from the ground. Historically, this meant that a lot of awesome astro phenomena that only registered as subtle changes in the temperature of the cosmos was inaccessible to landbound astronomers. Spitzer promised to reveal this invisible side of the universe.

Although Spitzer wasn’t NASA’s first infrared space telescope, it was far more powerful than its predecessors and was expected to operate for years, rather than months. Only a week after it arrived in space, Spitzer opened its eye for the first time. The first image it sent back to Earth showed a massive stellar nursery, where stars form in turbulent clouds of gas and dust, known as the Elephant’s Trunk nebula. “We could see right away that we were in a new regime as far as infrared astronomy was concerned,” says Werner.

Since then, Spitzer revolutionized our understanding of the universe and our place within it. It revealed the violent world of stellar nurseries by allowing scientists to see with unprecedented resolution the newborn stars that would otherwise be hidden behind clouds of dust. Spitzer teamed up with Hubble to spot the most distant galaxy ever, it discovered hundreds of dwarf galaxies tens of millions of light years away, and gave us a clear window into the heart of our own galaxy, which is also usually obscured by dust. It revealed that the bar-shaped collection of ancient stars at the center of the Milky Way was longer than previously known, and helped scientists more accurately determine the shape of our home galaxy. Although the first exoplanets had only been discovered a few years before Spitzer was launched, it quickly revealed our first glimpse of the weather on alien planets, revealing circling winds and hotspots, and helped uncover four new planets around TRAPPIST-1, making it the biggest known planetary system outside our own.

“Spitzer has been transformative for studying exoplanets in detail,” says Nikole Lewis, an astrophysicist at Cornell University and an expert in exoplanet atmospheres. “For many of us in the astronomy community, saying goodbye to Spitzer is saying goodbye to a dear friend that journeyed with us through the cosmos.”