NASA's Spitzer Space Telescope, one of the agency's original four "Great Observatories," will execute a final set of commands Thursday, quietly shutting down and putting itself to sleep after more than 16 years of trailblazing infrared observations. There are no plans to ever wake it up.

"Spitzer taught us how important infrared light is to understanding our universe, both in our own cosmic neighborhood and as far away as the most distant galaxies," Paul Hertz, director of astrophysics at NASA Headquarters, said in a statement. "The advances we make across many areas in astrophysics in the future will be because of Spitzer's extraordinary legacy."

Launched in 2003, Spitzer was equipped with a reservoir of liquid helium to chill the telescope's 33.5-inch primary mirror to within about five degrees of absolute zero, roughly minus 450 degrees Fahrenheit. At those temperatures, the telescope could pick up faint infrared emissions — heat — from deep space that that cannot be detected from the ground.

When the helium coolant ran out in 2009, two of its three instruments — an infrared spectrograph and multi-band imaging spectrometer — were no longer able to record the longest infrared wavelengths, bringing Spitzer's "cold" phase of operations to a close.

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But even without its cryogenic coolant, the observatory was still a frigid 28 degrees above absolute zero -- minus 410 degrees Fahrenheit — behind its protective solar panel. That was cold enough for the third instrument, the Infrared Array Camera, to keep "seeing" near- and mid-infrared light. And so, Spitzer's mission continued.

The long-range goal of the so-called "warm" mission was to keep the observatory operating until launch of the $9.7 billion James Webb Space Telescope, or JWST, an infrared successor to the Hubble Space Telescope with a segmented mirror 21 feet across.

But it was not to be.

"In 2016, we looked ahead and saw the upcoming launch of the James Webb Space Telescope, NASA's next great observatory, which is also an infrared observatory, and the decision was made that the Spitzer mission should end as the James Webb mission was beginning," Hertz told reporters last week.

"We extended the Spitzer mission all the way through the launch of James Webb in 2018. When the James Webb launch was delayed until 2021, we extended Spitzer until now, in 2020. But the time has come for the Spitzer mission to end as we move on to the launch of James Webb next year."

The Government Accountability Office, in a report released Tuesday, said ongoing technical issues with the JWST, and the time needed to resolve them, leave only a 12 percent chance of meeting the March 2021 launch target. But NASA had already decided to pull the plug on Spitzer.

"As the angle between the Earth, the sun and the telescope changes, (it) makes it more difficult to simultaneously keep the solar panels pointed at the sun, to keep the communications antenna pointed at the Earth, to keep the telescope pointed at the stars and planets we want to look at, and to keep the sun off of the telescope so that it stays very cold and is able to continue its infrared (observations)," Hertz said.

As a result, around 5 p.m. EST Thursday, flight controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., will transmit a final set of commands to Spitzer some 165 million miles from Earth, trailing the planet in a slightly larger orbit. A half hour later, the observatory's final response should confirm the shut-down command.

"We're going to put Spitzer in a hibernation mode," said Joseph Hunt, Spitzer's current project manager. "We're going to command from the ground to put it in safe mode. In safe mode, it goes into a (commanded orientation), and it would just stay in that attitude forever."

An artist's impression of the Spitzer Space Telescope. NASA

NASA's Great Observatories program was aimed at studying the universe across the electromagnetic spectrum, from relatively long-wavelength infrared radiation, visible and ultraviolet light to short-wavelength X-rays and higher-energy gamma rays.

Spitzer, named after the American astronomer Lyman Spitzer Jr., was optimized to study infrared light while the Hubble Space Telescope, marking its 30th year in space, is sensitive to visible and near ultraviolet wavelengths.

The Chandra X-Ray Observatory, now in its 20th year of operation, covers higher-energy processes, like the radiation emitted from gas being sucked into black holes. The Compton Gamma Ray Observatory, which ended its mission in 2000, was focused on the most energetic events in the universe like supernova blasts and shattering gamma ray bursts.

Like its counterparts, Spitzer chalked up a steady stream of discoveries, averaging 7,000 hours of observations per year over more than 6,000 days in space. The result: More than 8,700 science papers to date and many more expected in the years to come as astronomers work through the data archive. Total cost: $1.36 billion.

"It's quite amazing when you lay out everything that Spitzer has done in its lifetime, from detecting asteroids in our solar system no larger than a stretch limousine to learning about some of the most distant galaxies we know of," Michael Werner, Spitzer project scientist, said in a NASA statement.

Suzanne Dodd, a former Spitzer project manager, told reporters much the same, saying "in the infrared ... we can see through dust, we're lifting the cosmic veil on the universe, and there's just a cornucopia of astrophysics objects out there that we can observe."

One of her favorite Spitzer discoveries: finding a new ring around Saturn.

Spitzer detected fine particles in a vast, previously unseen ring around Saturn. NASA

"This was only made possible by observing it in infrared light," Dodd said. "The ring is 300 Saturns in diameter, so it's gigantic. But the particles are tiny, and it was only the cool radiation given off by the particles in the ring that Spitzer was able to detect it. It's a fun discovery, it's right in our backyard, so it's one of my favorites."

For Farisa Morales, a Spitzer astronomer, the telescope opened a new window on vast molecular clouds across the Milky Way and other galaxies where stars are being born.

"It has allowed us to see what our human eyes could not see," she said. "These huge molecular clouds in our galaxy, which are stellar nurseries, become transparent when you're observing in infrared radiation, and it allows us to see these baby stars forming.

"And the process of forming a star, some of the debris is left over to form planetary systems. Spitzer has been a pathfinder in allowing us to find out which ones are forming now for future exploration."

Werner said when the telescope was launched, only a handful of exoplanets were known. But since then, thanks to NASA's Kepler spacecraft and others, thousands of exoplanets have been found and Spitzer played a major role in understanding the nature of many of those worlds.

"We've talked about exoplanets, which are nearby, but Spitzer, particularly working with Hubble, has seen galaxies as they were when the universe was only a few percent of its current age," Werner said. "And they, like the exoplanets, will be well studied by the James Webb Space Telescope."

He told reporters Spitzer's legacy goes beyond the science. The team that designed, built, launched and operated the telescope for more than 16 years "is absolutely astounding."

"I think it's stands as a sterling example of what people are capable of doing," he said. "I think that's important in these troubled times, to realize that there's a genius and ability and a strength in the human spirit, which is capable of doing amazing things. So that's another part of the legacy."