Published: February 4, 2020 11:05:41 am
Written by Adam Mann
On Thursday, NASA’s Spitzer space telescope signed off and went silent. But even during its final week of operation, the spacecraft was making one-of-a-kind observations.
The telescope, the size of a family sedan, follows Earth in its orbit around the sun but trails 158 million miles behind. Lately, it has gazed out with its infrared eyes, taking sensitive measurements of fine cosmic dust that pervades the space between planets in the solar system. The resulting imagery will enable researchers to better understand our celestial neighborhood while informing models of worlds circling other stars and giving insight into the early universe.
Since it launched on August 25, 2003, Spitzer has provided unique contributions to science. It gave us new views of distant galaxies, newborn stars and nearby exoplanets, as well as of asteroids, comets and other objects in our solar system. Its infrared cameras have observed the universe in a light imperceptible to human senses, providing otherwise unattainable visions of the sky.
“There is no field of investigation that has not been touched by Spitzer,” said Daniela Calzetti, an astronomer at the University of Massachusetts, Amherst, who has used the telescope to study galactic evolution.
Among the highlights of Spitzer’s 16-plus years of discovery:
— Spotting a never-before-seen ring around Saturn;
— Determining the point in cosmic history — 10 billion years ago — when star formation peaked;
— And, as part of its most famous finding, discovering four of the seven Earth-size planets spinning close around the star known as Trappist-1.
“It’s really the end of an era, particularly for me,” said Heather Knutson, an astrophysicist at the California Institute of Technology who, as a graduate student, used the telescope to map high-speed winds on a hot Jupiter-size exoplanet. “Spitzer has been around for as long as I’ve been doing science. I don’t remember a time without it.”
The end of a spacecraft’s mission always provides a moment for reflection. But Spitzer’s conclusion is particularly challenging for infrared astronomers, and many wish it weren’t yet time to say goodbye.
“From a purely technical point of view, we could continue to operate it,” said George Helou, an astronomer at Caltech who was part of a NASA review into whether the telescope should continue to operate. “The decision was taken at a time when it seemed the rational thing to do was to terminate the mission now.”
The telescope has been part of NASA’s Great Observatories program, which includes the well-known Hubble Space Telescope, the still-orbiting Chandra X-ray Observatory and the retired Compton Gamma Ray Observatory.
Although Spitzer’s components are aging, some scientists have suggested that the observatory is in overall great condition and that it could be kept around for at least another year.
The decision to shutter Spitzer was first made in 2016, when NASA assessed the scientific output versus the cost of its various undertakings. The telescope was ranked near the bottom of this review. Administrators resolved to conclude the spacecraft’s mission in 2019, around a year after the giant James Webb Space Telescope, which will be capable of infrared observations, was set to launch.
But technological issues have repeatedly delayed Webb, whose current launch date is March 2021, although it could face further postponement. In response, Spitzer received an extension to January 2020. Nevertheless, NASA declined to extend it further, citing the complexity of communicating with the spacecraft. That will leave a gap should a heavenly event occur that would benefit from Spitzer’s superior, spaced-based infrared eyes.
Spitzer cost NASA less than $14 million each year, and its overall lifetime expenditure has been calculated to be around $1.3 billion, a bargain relative to Hubble’s estimated $8 billion cumulative cost. Information from the infrared telescope has been used in more than 8,700 research papers and, adjusted for scientific impact per year of operation, Spitzer stands head and shoulders above other space telescopes.
“I like to think of Spitzer as the little engine that could,” said Nikole Lewis, an astrophysicist at Cornell University in Ithaca, New York.
During its career, the telescope continually exceeded expectations. Its initial mission, which required liquid helium coolant to keep its mirrors at a frigid -459 degrees Fahrenheit, was supposed to last 2 1/2 years. But clever engineering stretched this out to 5 1/2 years. Although the refrigerant ran out in 2009, operators found ingenious ways to keep using two of its three cameras and continue the mission.
But Spitzer’s position in space was getting the better of it. The spacecraft was placed far from our planet so that Earth’s heat wouldn’t interfere with its observations. But, as time goes on, it drifts ever farther away. To talk to controllers at NASA’s Jet Propulsion Laboratory in Pasadena, California, Spitzer had to tilt and point its antenna, preventing some solar energy collection and exposing its bottom to the sun, which caused distortions in its images.
At some point in the near future, the geometry of the sun, the Earth and the telescope would make communication and management of the observatory too difficult, said Paul Hertz, NASA’s astrophysics division director.
The spacecraft’s operators on Earth downloaded leftover science and engineering data from the observatory, then transmitted their final shutdown commands just before 5 p.m. ET Thursday. About 15 minutes later, according to NASA, the commands reached the probe and put it into hibernation, or safe mode, which was confirmed at 5:30 p.m.
“Everyone who has worked on this mission should be extremely proud today,” said Joseph Hunt, the Spitzer project manager. “There are literally hundreds of people who contributed directly to Spitzer’s success and thousands who used its scientific capabilities to explore the universe.”
The special controlling hardware in Pasadena will be dismantled, making it unlikely Spitzer will ever be roused again. Researchers understand the rationale behind the spacecraft’s end, but there remains a sense of sadness in the community.
“There’s a lot of reasons I would argue that we should continue to operate Spitzer,” said Sean Carey, senior staff astronomer at the Spitzer Science Center at Caltech. “But I respect the process.”
Were they not busy planning for the upcoming Webb telescope, infrared astronomers might be more dejected. With a 21-foot mirror, Webb will dwarf the light-collecting power of Spitzer’s 33-inch mirror, helping answer many questions the older observatory left open.
Spitzer was able to spot the dynamics of some of the most distant galaxies in the universe, but Webb, once it gets off the ground, will look even farther and help pinpoint how the first stars and galaxies came to be. And although Spitzer could identify a few molecules in exoplanetary atmospheres, its successor will probe many more planets and search them for the chemical building blocks of living organisms.
Nevertheless, Spitzer has been transformative, and it leaves behind an extensive data archive that will be mined for future discoveries. The telescope will spend the rest of its days quietly drifting through space. Here on Earth, its impact will continue.
“Spitzer has fundamentally changed the way that we as a society have looked at the universe,” said Carey of Caltech, adding that, next year, his son will enter sixth grade, where the science curriculum will include a section on the seven-planet Trappist-1 system.