From the June 2020 issue

A look back on Spitzer’s legacy

The Spitzer Space Telescope ceased operations in January, but not before giving scientists keen insight into star birth, exoplanets, and the shape of our galaxy.
By | Published: June 23, 2020
The universe has a different look at infrared wavelengths. In visible light, dark dust blocks the glow of more distant gas to create the outline of the North America Nebula (NGC 7000) in Cygnus. But the continent disappeared to Spitzer’s eye because it peered through this dust to reveal the cocoons of embryonic stars within the nebula. Many of the stars seen here are only about a million years old. 
NASA/JPL-Caltech/L. Rebull (SSC/Caltech)
On January 30, NASA turned out the lights on the Spitzer Space Telescope. The infrared-sensitive observatory had far exceeded initial expectations. When the spacecraft blasted off from Florida in August 2003, scientists hoped the mission would last at least two and a half years and, with any luck, live to see its fifth birthday. But Spitzer survived, and even thrived, until a few months after it turned sweet 16.

Spitzer was the fourth and final component of NASA’s Great Observatories program, which also included the Hubble Space Telescope, the Chandra X-ray Observatory, and the Compton Gamma Ray Observatory. This quartet was designed to deliver super-sharp views of the heavens across a broad span of wavelengths. Originally christened the Space Infrared Telescope Facility (SIRTF), Spitzer was renamed for American physicist Lyman Spitzer Jr., an early and vocal advocate for space-based telescopes, once the observatory opened its eye to the universe.

Spitzer’s primary mirror measured 33 inches (0.85 meter) across, and its three science instruments recorded infrared radiation, or heat, with wavelengths between 3 and 180 micrometers. Needless to say, heat from nearby objects would have swamped the telescope’s sensitive detectors. The biggest source of heat in near-Earth space is Earth itself, so NASA launched Spitzer into an orbit around the Sun that trails our planet and drifts slowly away.

Yet spacecraft generate their own heat. To combat this source of infrared radiation, Spitzer carried a tank of liquid helium that kept its instruments chilled to –449 degrees Fahrenheit (–267 degrees Celsius). When the helium ran out in May 2009, the spacecraft warmed to a toasty –408 F (–244 C). Although two of Spitzer’s instruments could no longer operate, the two shortest-wavelength channels on its infrared camera continued to function with little change in sensitivity for another decade.

Spitzer’s window on the universe allowed astronomers to study regions largely hidden from optical telescopes. Because infrared radiation penetrates dust, the observatory could peer inside stellar nurseries to watch stars being born. It also looked through the thick dust lanes that clog the Milky Way’s disk and confirmed that our galaxy is a barred spiral.

The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) used Spitzer to survey the Milky Way Galaxy’s disk and confirm that we live in a barred spiral galaxy. The whole survey required stitching together more than 800,000 individual frames; this segment represents just 6 percent of the entire project.
NASA/JPL-Caltech/University of Wisconsin
The observatory’s ability to detect heat signatures exposed the inner workings of giant molecular clouds, revealed failed stars known as brown dwarfs, and discovered a slew of exoplanets as well as the constituents in the atmospheres of many of these worlds. Notably, Spitzer discovered four of the seven known planets in the TRAPPIST-1 system, at least three of which appear to lie in the star’s habitable zone where liquid water could exist on a rocky planet’s surface.

Although Spitzer is now gone, its impact will be felt for years to come. That impact will endure in part because NASA’s 6.5-meter James Webb Space Telescope, currently scheduled for launch in March 2021, will also probe the universe at infrared wavelengths and follow up on many of Spitzer’s discoveries. Not a bad legacy for an observatory astronomers once hoped would last two and a half years.