RIP Spitzer, the Coolest Heat Telescope in the Solar System

For 16 years, NASA's trusty scope revealed a hidden infrared universe. Now it’s up to the James Webb Space Telescope to pick up where it left off.

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.”

Spitzer fundamentally changed our view of the universe, but the telescope’s days were numbered from the start. To measure ultra-faint temperature variations across light years of space, Spitzer itself had to be kept very cold. It traveled to space with a six-year supply of liquid helium that would keep the scope just a few degrees above absolute zero. When the helium ran out, Spitzer’s mission would come to an end. Even if NASA had packed more helium for the trip, the telescope’s unusual Earth-trailing solar orbit means that it drifts about 10 million miles away from home each year, which makes data transfer increasingly challenging.

But, as Werner says, “nature was kind to us.” When Spitzer’s helium supply ran out in 2009, the freezing temperatures of interplanetary space kept the telescope cool enough to continue operating some of its instruments. This marked the beginning of Spitzer’s so-called “warm” mission, when the telescope was operating at a balmy –408 degrees Fahrenheit. The bonus mission extended the telescope’s life for another decade. Although it lost the ability to monitor some of the coolest objects in the universe, it could still observe relatively hot objects like distant galaxies, dust shrouded stars, and near-Earth asteroids.

Wednesday will mark Spitzer’s last day of data collection before NASA sends the command for the telescope to enter safe mode. Its final assignment is measuring the zodiacal light, a glow produced by sunlight that's been scattered by dust. The last image Spitzer will deliver to Earth will be of a nebula lurking at the edge of our galaxy that is associated with the formation of massive stars.

“The observatory really feels like a member of the family, and it’s so sad to see its mission end,” says Lisa Storrie-Lombardi, who worked on the Spitzer team at Caltech for 20 years. “At the same time, there is so much to celebrate, as its accomplishments surpassed our wildest dreams.”

Illustration: NASA

According to Werner, there were no technical limitations preventing Spitzer from continuing observations for at least another year. Instead, NASA retired Spitzer so its resources could be allocated to the agency’s next generation infrared observatory, the James Webb Space Telescope, which is about 1,000 times more powerful.

In many ways, Spitzer was a pathfinding mission for Webb, says Eric Smith, NASA’s James Webb Space Telescope program scientist. Webb has a more narrow view of the universe than Spitzer, but its improved suite of sensors means it will be able to conduct detailed observations of known exoplanets, many of which were discovered by Spitzer. Webb’s larger mirror means it will also be able to probe the early universe for signs of the first galaxies, a region of time and space that is beyond the reach of Spitzer’s instruments.

NASA expects the $10 billion Webb to launch in March 2021, although a recent Government Accountability Office report says there is only a 12 percent chance of meeting this time frame. In the meantime, Werner says astronomers will continue to probe Spitzer's vast repository of data to learn more about the multitude of stars, planets, and galaxies that it observed over the past 16 years. Spitzer may be dead, but its scientific legacy will live on.