Artist illustration of NASA's Spitzer Space Telescope. (NASA/JPL-Caltech)
When you hear the phrase space telescope, you probably think of NASA's venerable Hubble, which has brought us decades of unique and breathtaking images of the most distant reaches of space, cosmic discoveries of universe-shaking magnitude, and a nice gallery of stunning computer wallpaper selections.
Quietly working in tandem, NASA's Spitzer Space Telescope has been making equally compelling cosmic discoveries through observations of the infrared light emitted by celestial objects.
If you haven't heard about it, now may be a good time to take a look at its work. After over 15 years on the job, Spitzer is scheduled to retire next January, passing the torch to the next generation of eyes in the sky.
Spitzer was launched in 2003 on a Delta II rocket and placed into an Earth-trailing solar orbit, following behind the Earth at an ever-growing distance instead of orbiting around it like Hubble does.
With each year of operation, Spitzer has moved over 9 million miles farther away. Today, though it still shares the same orbit around the sun with Earth, it is over 160 million miles from home.
Keeping Earth at a distance has advantages for an infrared telescope like Spitzer.
For one, Spitzer's sensitive observations are not interfered with by Earth's heat glow. Spitzer's forte is sensing faint infrared emissions from distant celestial objects, so being located near an enormous glowing heat-ball like Earth would be like trying to see a landscape against the blinding glare of the sun.
Spitzer's list of accomplishments is long, but here are a few highlights of discoveries made possible by this space-based, infrared-only telescope.
Seeing the Light: In 2005 scientists using Spitzer announced the first detection of light from an extrasolar planet — a planet orbiting another star. Before this, exoplanets were detected indirectly, by the pull of their gravity on a star or their blocking of starlight. Visible light reflected by exoplanets is very faint compared to their stars, but Spitzer spotted two hot gas giant planets — dubbed "hot Jupiters" — orbiting so close to their stars that they glow brightly with infrared light.
Catching a Whiff of Hot Jupiters: In 2007 Spitzer's infrared spectrometer was used to make the first identification of chemicals in the atmosphere of an exoplanet — two different exoplanets, a pair of gas giants.
First Extrasolar Weather Report: In 2009, Spitzer produced the first "weather map" of an extrasolar planet. A heat-map of the gas giant exoplanet HD 189733b revealed variations in temperature across its surface, as well as conditions for extreme atmosphere winds.
The Most Earth-sized Exoplanets Orbiting a Star: Spitzer revealed a whopping seven Earth-sized exoplanets orbiting the same tiny star, TRAPPIST-1, only 40 light years away. Though we don't yet know much more about them than their sizes and distances from their star, we know that three of them orbit within the system's "habitable zone," where it is possible for liquid water to exist on their surfaces.
The Most Distant Exoplanet: While most exoplanets have been found orbiting stars within about a thousand light years of Earth (our local neighborhood in the Milky Way galaxy), Spitzer helped detect an exoplanet 13,000 light years away. The detection was made by a technique called "gravitational lensing," where the exoplanet's gravity bends and distorts the light of a more distant star.
Biggest, Farthest Black Holes
Spitzer detected two of the most distant supermassive black holes ever discovered, at the cores of a pair of young active galaxies located near the edge of the observable universe. These active galaxies, or quasars, are so far away that it took the light that Spitzer captured 13 billion years to reach us, showing us these objects as they were in the very early universe.
Map of the Hidden Reaches of the Milky Way
Much of the greater Milky Way galaxy is hidden from our eyes and telescopes by great clouds of interstellar dust. Infrared light, however, can penetrate clouds of dust that visible light cannot, affording Spitzer a view of objects and structures otherwise obscured, such as "baby" stars still enshrouded in the cocoons of gas and dust they were born from. Over two million infrared images captured by Spitzer were assembled in 2013 into the most extensive map of the Milky Way galaxy every created.
The Hubble Space Telescope has been operating for almost 30 years — so why is Spitzer being retired after only half that time? The answer, in part, is that Spitzer is losing its cool, so to speak.
To function as a detector of distant infrared radiation, Spitzer's sensitive instruments must be kept at very cold temperatures — close to absolute zero, in fact; almost -460 Fahrenheit. This is so the instrument's own heat emissions won't interfere with the detection of the faint infrared signals from distant celestial objects.
Imagine if you tried to find your way around a dark room with a spotlight shining in your eyes.
Liquid helium was used to supercool Spitzer's infrared detectors — however the helium supply was depleted in 2009. Since then Spitzer has operated without cryogenic cooling, relying only on the passive cooling of its "sun shade" and its distance from Earth.
Two of Spitzer's shorter wavelength instruments, however, can still be used, and in fact have made some of Spitzer's more pivotal discoveries.
Passing of the Torch
As the Spitzer Space Telescope gets ready for its final shutdown, and the much older Hubble faces an eventual end of mission and de-orbiting sometime in the next decade or so, the successor to the great space telescope dynasty will be the James Webb Space Telescope, a much larger, solar-orbiting observatory geared to observe the universe at infrared wavelengths of light.
So, as one era of unique cosmic perspective ends, another begins.
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