The infrared Spitzer Space Telescope, considered one of NASA’s four “great observatories,” will be switched off on 31 January after a 16-year career. It probed some of the earliest galaxies ever seen, charted how they evolved and formed stars, and picked apart the constituents of exoplanet atmospheres. And in a late tour-de-force, it discovered a clutch of Earth-size planets around a nearby star. “It’s going out on a high note, producing great science to the end,” says Lisa Storrie-Lombardi, who worked on the mission for 20 years and now directs the Las Cumbres Observatory.
Spitzer is sensitive to infrared light, the photons emitted by the glow of warm objects. Stars do not dominate in Spitzer images. Instead, the telescope sees the glow of galaxies and the clouds of gas that coalesce into stars. It is also suited to finding the universe’s most distant objects, those whose light has been stretched to infrared wavelengths by the expansion of the universe. Earth’s atmosphere blocks most infrared light, so space telescopes are essential. A couple of infrared satellites preceded it, but Spitzer had the biggest mirror (85 centimeters), more sophisticated instruments, and state-of-the-art infrared sensors.
It didn’t have an easy journey into orbit, however. Originally, the Space Shuttle was supposed to carry it aloft for monthlong observing campaigns, before the 1986 Challenger disaster prompted a rethink. After several redesigns, it was finally launched in 2003 on a Delta II rocket. It was the last of the great observatories to launch, following the Hubble Space Telescope, the Chandra X-ray Observatory, and the Compton Gamma-Ray Observatory.
One of Spitzer’s innovations was passive cooling. Any warm object glows brightly in infrared—including the sunlit telescope itself—so it must be cooled. Earlier missions were entirely chilled with limited supplies of a cryogenic liquid. But Spitzer used passive methods—reflective materials and radiators to shed heat into space—to cool most of the spacecraft to 40 kelvins. It kept a smaller supply of liquid helium to chill the mirror and instruments to 12 kelvins or 5 kelvins, depending on which instrument was being used. This transformed the mission from “fundamentally unaffordable to very cost effective,” says Thomas Soifer of the California Institute of Technology and director of the Spitzer Science Center.
Another innovation was putting Spitzer in an orbit around the Sun, trailing behind Earth. Earth and the Moon are bright sources of infrared light, and so the distant vantage made it easier to cool the spacecraft. Moreover, Earth blocked less of the sky. “It makes operations much simpler,” Soifer says.
With a wide field of view and fast mapping speed, Spitzer was soon imaging entire galaxies and whole star forming regions. It built up a groundbreaking 360° panorama of the plane of the Milky Way, which took thousands of hours to piece together. One unanticipated capability was the study of exoplanets, which hadn’t been discovered when Spitzer was designed. “The creative [astronomy] community said let’s try it, and it was incredibly successful,” Soifer says.
Spitzer pioneered the study of hot Jupiters, gas giants orbiting very close to their stars. By observing starlight as such a planet passes in front and then behind its star, Spitzer researchers could gauge the planet’s temperature and even map the distribution of temperature across its surface. It was also able to discover the compositions of some exoplanet atmospheres by the telltale frequencies of light that those molecules absorb when starlight passes through the gases.
When the helium coolant ran dry in 2009, the Spitzer team realized one of its three instruments could still do valuable science at the elevated temperature of 28 kelvins. “It was a whole new mission,” Storrie-Lombardi says.
During this “warm mission,” Spitzer excelled at studying distant objects. Teaming up with Hubble (approaching its 30th birthday), Spitzer researchers have gathered light from galaxies that existed less than half a billion years after the big bang. How they could have formed so soon remains a mystery. “No one would have guessed these moderately sized telescopes could measure light and deduce properties of galaxies that far back in the history of the universe,” Soifer says.
But an unexpected exoplanet discovery became the highlight of Spitzer’s later years. TRAPPIST-1 is a star not much larger than Jupiter about 39 light-years from Earth. It is an ultracool red dwarf—shining at the perfect wavelengths for Spitzer. In 2016, a small ground-based telescope detected brightness dips caused by three small planets transiting in front of the star, but astronomers suspected more. Michaël Gillon of Liège University asked for observing time on Spitzer and was given 25 hours that revealed a tightly packed system of seven planets. “It’s a really, really special example of what you can do with an orbit that can stare for a very long time,” Soifer says.
And now it’s time to say goodbye. A 2016 NASA review of operating missions said Spitzer should at least keep operating until the James Webb Space Telescope (JWST), also observing in infrared, is up and running. But JWST kept getting delayed. When its launch was pushed back to 2021, NASA called time on Spitzer. Soifer thinks Spitzer could do more, “but I’ve made my peace with it,” he says.
Storrie-Lombardi says everyone will be blown away by the spectrographic abilities of JWST, but says it lacks the wide-view mapping abilities of Spitzer. In the meantime, infrared astronomers will have to be patient. “There’s going to be a gap,” she says. “JWST will fill some of it.”