Seeing The Invisible: The Spitzer Space Telescope
NASA's great infrared observatory
On August 25, 2003, NASA's Spitzer Space Telescope was launched from Cape Canaveral aboard one of the largest ever Delta rockets. The muscle was needed because Spitzer is the largest infrared telescope ever launched. Spitzer orbits in a much colder, more distant region of space than does the Hubble Space Telescope: it orbits the Sun rather than Earth and sees much farther out into space. Among its objectives is to observe the vast, dense clouds of gas and dust that fill many regions of space. Scientists believe these interstellar clouds are key to understanding how stars and planets form.
The Old, the Cold, the Dusty, the Dirty
Unlike the Hubble Space Telescope, which views the cosmos in a wide range of spectral wavelengths, Spitzer looks completely in the infrared-light range. Infrared light has longer wavelengths and lower energies than visible light. In astronomy, the infrared range is particularly useful for studying not hot but warmish matter, such as interstellar dust clouds and the dusty disks that surround some stars. Hotter objects, including some stars, emit more energetic light; they can be seen in visible light but are largely invisible in the infrared. As Michelle Thaller, a senior staff scientist at Caltech and manager of the SIRTF education program puts it, "Infrared probes the old, the cold, the dusty, or the dirty."
What's So Special About Infrared?
Light exists in many different wavelengths depending on its energy. Humans perceive only a small segment of that spectrum. Thaller compares viewing the Universe in only optical light—the wavelengths visible to the human eye—to listening to Beethoven's Ninth Symphony and hearing only the three notes around middle C. "We wouldn't be able to understand the structure, the beauty, any of the composition," she says.
Spitzer surveys all kinds of celestial objects that produce little or no visible light, everything from submicron-size interstellar dust grains to small stars too dim to be detected by their visible light to dusty disks that might be planetary systems in the making. Viewed through Spitzer, the visible-light stars appear very dim, while cooler stars or stars surrounded by dust appear brighter and are easier to detect.
Detecting Dusty Disks
Stars are thought to form within giant clouds of gas and dust. Planets originate in gaseous disks around young stars. As these protoplanetary disks cool, grains of dust collide and accrete into rocks, boulders, and eventually planets. Spitzer is detecting protoplanetary disks of dust by measuring their infrared brightness. In effect, the space telescope is taking a census of possible nearby planetary systems in the making.
Finding these nurseries is an important first step toward the larger goal of understanding how planets form. In addition, interstellar dust may hold clues as to how life evolved. The dusty disks are thought to contain organic molecules fundamental to humans and all forms of life. "We're going to answer the 'Where did we come from?' question in a very real way," Thaller says.
Because infrared is primarily heat radiation, Spitzer must be cooled to within a few degrees of absolute zero (-459 degrees Fahrenheit or -273 degrees Celsius), the lowest temperature possible, which is characterized by a complete lack of heat. If the telescope is not kept this cold, its own heat will interfere with its ability to observe the faint heat signals coming from space. If you go outside on a sunny day and close your eyes, it is easy to feel the heat from the Sun on your face. "But imagine going out at night and trying to feel the actual heat from the stars," says Thaller. In the past, large tanks of liquid helium kept telescopes cool, functioning rather like a big thermos bottle. These tanks made telescopes very heavy and expensive to launch. In contrast, Spitzer uses only a small amount of liquid helium as a coolant. Reflective paint and a heat shield do the rest of the work of keeping the telescope at the correct low temperature.
Along with the telescope, Spitzer carries an array camera that takes detailed images of the infrared sky and precisely measures the amount of incoming light. Spitzer also has a spectrometer that analyzes light to see what chemical elements are present in it.
Spitzer's instruments have revealed landmark information about protoplanetary disks and extrasolar planets in the telescope's first three years of operation. Spitzer will continue to see the invisible until the helium runs out—perhaps until 2009. To learn about some of the discoveries scientists have made using the Spitzer Space Telescope, click on the Astro Bulletins in the Related Links for this feature.