INFRARED
A FIELD GUIDE TO THE SPITZER SPACE TELESCOPE
Ten chapters on the last of NASA's Great Observatories: an infrared telescope chilled to a few degrees above absolute zero, flown into an orbit that slowly drifts away from Earth to escape its heat — and which, after its coolant ran out, quietly reinvented itself and kept working for another eleven years, long enough to help confirm seven planets around another star. Scroll to begin.
BEGIN — CHAPTER 01 →The cool, the dusty, and the very distant
Infrared light passes through the dust clouds that block visible light, so Spitzer could look inside the nurseries where stars and planets form. It also sees objects too cool to glow visibly — brown dwarfs, planets, dusty debris disks — and the light of the earliest galaxies, stretched into the infrared by the expansion of the universe.[1]
The catch: everything warm glows in the infrared, including the telescope itself. A room-temperature instrument trying to see faint cosmic heat is like trying to photograph a candle inside a furnace.
Named for the man who first argued for telescopes in space
Spitzer launched on a Delta II rocket on August 25, 2003, the fourth and final member of NASA's Great Observatories program.[1] It is named after Lyman Spitzer, the astrophysicist who proposed orbiting observatories in 1946 and spent decades championing what became the Hubble Space Telescope.
Following Earth around the Sun, falling a little further behind each year
Instead of orbiting Earth, Spitzer was placed in its own orbit around the Sun, trailing just behind the planet and drifting away at roughly 0.1 astronomical units per year.[2] Far from Earth's reflected heat and warm glow, the telescope could cool down mostly on its own, saving precious coolant.
The price of that trick showed up slowly: as the years passed, Spitzer fell so far behind Earth that pointing its antenna home and getting data back became steadily harder.
A small mirror, three instruments, and a tank of liquid helium
Spitzer's telescope is only 85 cm across — modest, because in the infrared a small cold mirror beats a large warm one. Almost all the spacecraft's cleverness is in keeping it cold. Select a component.
Superfluid helium, and a design that leaned on empty space
Spitzer carried about 360 litres of liquid helium, kept superfluid near 1.4 kelvin, to soak up any trace of heat reaching the instruments.[2] The Earth-trailing orbit and a Sun-facing shield did so much of the cooling passively that the helium lasted far longer than planners dared promise — nearly six years instead of the guaranteed two and a half.
"Warm" meant 28 degrees above absolute zero
The helium boiled away on May 15, 2009. The telescope drifted up to about 28 K — still bitterly cold, but too warm for the longer-wavelength instruments. The two shortest channels of the IRAC camera, however, still worked perfectly.[1] NASA began the "Spitzer Warm Mission," which ran for another decade and produced some of the observatory's most cited results.
The warm telescope stared at one red dwarf for 500 hours straight
Spitzer had already made history in 2005 as part of the first detection of light from a planet around another star. Its signature exoplanet result came in 2017: a continuous 500-hour watch of the dim star TRAPPIST-1 that confirmed seven roughly Earth-sized planets and pinned down their orbits precisely, several of them in the range where liquid water could exist.[3]
The same instrument that weighs cold dust finds it everywhere
In 2009 Spitzer discovered a vast, faint ring of cold dust around Saturn — far larger than all the bright rings combined, invisible except in the far infrared.[1] At the other extreme of distance, Spitzer joined Hubble to survey the deep fields, catching the infrared glow of galaxies as they were within the first billion years after the Big Bang.
Switched off while still working, because pointing home had become too hard
By 2020 Spitzer trailed so far behind Earth that returning data required tipping the spacecraft to an attitude that overheated its own electronics and pointed its solar panels away from the Sun. On January 30, 2020, after more than sixteen years — against a primary mission of two and a half — NASA commanded it into permanent safe mode.[1] It still coasts around the Sun, silent, slowly lapping the Earth.
A link in a chain of colder and colder eyes
Infrared astronomy from space is a relay: each mission is limited by how cold it can stay and how long. Spitzer ran the middle leg, and handed the deepest infrared questions to a far larger successor.
Sources cited throughout this guide
Bracketed numbers in the text above correspond to the sources below.