Infrared astronomy is the study of astronomical objects at infrared wavelengths. Many objects in the universe that are too cool and/or faint to be detected in visible light, can be seen in the infrared. These include cool stars, infrared galaxies, clouds of particles around stars, nebulae, interstellar molecules, brown dwarfs, and exoplanets. In the case of dust, infrared astronomers can either see through it or focus on it, depending on the wavelengths they choose. At near infrared wavelengths, dust is essentially transparent, enabling far-flung views along the dusty plane of the Milky Way that are impossible in visible light. The galactic bulge, for example, shows up particularly well in near infrared. On the other hand, at mid infrared wavelengths astronomers pick up radiation generated by dust itself, which is valuable in studying such phenomena as protoplanetary disks and possible extrasolar planets. Work in cosmology also depends crucially on infrared observations. The expansion of the universe means that all of the ultraviolet and much of the visible light from distant sources is shifted into the infrared part of the spectrum.
Infrared astronomy timeline
1800: William Herschel discovers infrared radiation. He detects infrared light by measuring the temperatures in the colors of the solar spectrum created by passing sunlight through a prism. The area just beyond the visible red, in a region where there was no visible light, registered the highest temperature. This discovery shows for the first time that there are forms of light which we can't see with our eyes.
1856: Infrared radiation is detected from the Moon by Charles Piazzi Smyth from the peak of Guajara on Tenerife. He uses a thermocouple (a device which converts heat into electric current) to detect infrared light from the full moon. Piazzi also tests observations at different altitudes and shows that better observations are obtained at higher altitudes. This is the first indication that our atmosphere absorbs some of the infrared radiation from space.
1870: The 4th Earl of Rosse detects infrared radiation from the Moon during several of its phases. He uses his infrared measurements to try to estimate the Moon's surface temperature.
1878: The bolometer is developed by Samuel Pierpoint Langley. This instrument is an electrical detector of radiant heat which can detect a broader range of infrared wavelengths (far past the region of Herschel's discovery). Langley's bolometer is sensitive to differences in temperature of one hundred-thousandth of a degree Celsius (0.00001 C). The new bolometer is used to study the intensity of infrared radiation from the Sun.
Early 1900s: Infrared radiation is detected from Jupiter and Saturn and from some bright stars, such as Vega and Arcturus. In 1915, William Coblentz at the US National Bureau of Standards develops thermopile detectors which he uses to measure the infrared radiation from 110 stars. He also studies planets and nebulae in infrared light. William Coblentz would become the founder of modern infrared spectroscopy.
1920s: The first systematic infrared observations of celestial objects are made by Seth Nicholson, Edison Pettit, and other American astronomers. They use a vacuum thermocouple to measure the infrared radiation from the Moon, planets, sunspots, and stars. Their infrared studies allow them to make some of the first measurements of the diameters of giant stars.
1948: Infrared studies of the Moon reveal that its surface is covered with a fine powder (more than 20 years before the Moon landings).
1950s: Astronomers begin to use lead sulphide (PbS) detectors to study infrared radiation. When infrared radiation in this range falls on a PbS cell it changes the resistance of the cell. This change in resistance can be measured and is related to the amount of infrared radiation which falls upon the cell. To increase the sensitivity of the PbS cell, it is cooled to a temperature of 77 K by placing it in a flask filled with liquid nitrogen. The development of PbS detectors allows astronomers to study the infrared radiation from space out to a wavelength of about 3 microns (millionths of a meter).
1959-1961: Harold Johnson builds the first near-infrared photometers covering the R, I, J, K and L bands. This extends infrared research out to a wavelength of 4 microns. Johnson and his team measure thousands of stars in these new near infrared bands, providing useful information on the radiation from cool stars. Johnson defines the first infrared magnitude system.
1961: The germanium bolometer is developed by Frank Low. This instrument is hundreds of times more sensitive than previous detectors and capable of detecting far-infrared radiation. When infrared radiation hits the germanium, it warms the metal and changes its conductivity. The change in conductivity can be measured and is related to the amount of infrared radiation entering the container. The germanium bolometer works best at an extremely low temperature (much lower than liquid nitrogen). It is placed in a metal container (Dewar) filled with liquid helium which cools the bolometer to 4 K (only a few degrees above absolute zero).
1960s: Balloons carry infrared telescopes up to altitudes as high as 25 miles. In 1963, a germanium bolometer is attached to a balloon to make infrared observations of Mars. Beginning in 1966, the Goddard Institute of Space Sciences uses balloons to survey the sky at 100 microns. Their program leads to the discovery of about 120 bright infrared sources near the plane of our Milky Way Galaxy.
1967: Cooled infrared telescopes are placed on rockets which can observe the sky for several minutes before reentry. The first infrared all-sky map results from a series of rocket flights by the Air Force Cambridge Research Laboratory. This project, called Hi Star, surveys the cosmos at wavelengths of 4, 10 and 20 microns. Although the total observation time accumulated by these flights is only about 30 min, they successfully detect 2363 reliable infrared sources which are published in the AFGRL Infrared Sky Survey. The Hi Star project continues until 1975.
Mauna Kea Observatories is founded – soon to become a leading site for ground based infrared astronomy. New observatories, specializing in infrared astronomy, become possible in the 1960s due to advances in infrared detectors. At an elevation of 13,796 ft, the Mauna Kea Observatories are located above much of the infrared absorbing water vapor in the atmosphere.
1968: Robert Leighton and Gerry Neugebauer make the first infrared survey of the sky from the Mount Wilson Observatory by detecting infrared radiation in the 2.2 micron region. This survey covers approximately 75% of the sky and finds about 20,000 infrared sources, which include star-forming regions, galaxies, our galactic center and numerous stars. The brightest 5,500 of these sources make up the first catalog of infrared stars.
1970: Mount Lemmon Infrared Observatory established in the Catalina Mountains of Arizona. This observatory would become a leading site for infrared astronomy.
Early 1970s: Most galaxies, including our own, are found to emit strongly in the infrared. Quasars and other active galaxies are also found to be strong infrared emitters. This infrared image of the Andromeda Galaxy was taken several years later (in 1983) by the IRAS satellite.
1974: The Kuiper Airborne Observatory (KAO) begins research operations. The KAO is a C-141A jet transport aircraft which carries an infrared telescope up to altitudes of 41,000 ft – above 99 percent of the Earth's infrared- absorbing water vapor. The KAO, which would be used to gather infrared data for the next 20 years, was used to discover the rings of Uranus in 1977 and the presence of water in the atmospheres of Jupiter and Saturn.
Mid-1970's: A far-infrared balloon borne spectrometer is used for three flights in the mid-1970 to test the Big Bang theory. To increase its sensitivity, the instrument is cooled to one degree above absolute zero by immersing it in a cryostat cooled with superfluid helium. This is the first time that such a low temperature is used for infrared observations. The observations provide the most widely accepted support for the Big Bang theory until the launch of the COBE satellite in 1989.
1980's: The development of infrared array detectors causes another giant leap in observational capability. A detector array is essentially a combination of several single detectors. This development greatly increases the efficiency of infrared observations and leads to the development of infrared cameras which can produce images much more quickly compared to using a single element detector. Infrared detector technology continues to advance at a rapid rate.
1983: IRAS (Infrared Astronomical Satellite) is launched. For 10 months IRAS scans more than 96% of the sky four times, providing the first high sensitivity all-sky map at wavelengths of 12, 25, 60 and 100 microns. IRAS doubles the number of cataloged astronomical sources by detecting about 500,000 infrared sources. IRAS discoveries included a disk of dust grains around the star Vega, six new comets, and very strong infrared emission from interacting galaxies, as well as wisps of warm dust called infrared cirrus which could be found in almost every direction of space. IRAS also reveals for the first time the central core of our galaxy, the Milky Way.
1985: During Jul and Aug 1985, an infrared telescope is flown onboard the Space Shuttle's Spacelab 2 to complement observations made by the IRAS mission. This mission produces a high quality map of about 60% of the plane of our Galaxy.
1989: NASA launches the Cosmic Background Explorer (COBE) in Nov 1989, to study both infrared and microwave characteristics of the cosmic background radiation (the remains of the extreme heat that was created by the Big Bang). Over the next four years, COBE maps the brightness of the entire sky at several infrared wavelengths and discovers that the cosmic background radiation is not entirely smooth, showing extremely small variations in temperature. These variations may have led to the formation of galaxies.
1990s: The development of adaptive optics removes the blurring of ground-based astronomical images. This distortion of the light from space is caused by turbulence in the Earth's atmosphere. Basically, adaptive optics takes a sample of light and calculates how much it has been distorted by the atmosphere. Deformable mirrors are then used to correct for this distortion and straighten the light. Adaptive optics works best at longer wavelength, producing excellent results for infrared telescopes. This system greatly improves the resolution of images taken by ground-based infrared observatories.
1993: The South Pole Infrared Explorer (SPIREX) begins operations taking advantage of the much lower thermal background at the very cold South Pole to gain better sensitivity. Another advantage is a very dark night sky, especially during the long, dark polar winter.
1995: IRTS (Infrared Telescope in Space), launched in Mar 1995, is Japan's first infrared satellite mission. During its 28 day mission, IRTS surveys about 7% of the sky. This data adds to our knowledge of cosmology, interstellar matter, late type stars and interplanetary dust.
The European Space Agency launches ISO (the Infrared Space Observatory) in November 1995. ISO observes a very wide range of infrared wavelengths between 2.5 and 240 microns (much wider than that covered by IRAS). ISO is also thousands of times more sensitive than IRAS and views infrared sources with much better resolution due to advances in infrared technology. ISO will collect data for about 2.5 years (3 times longer than IRAS) before its cooling liquid helium runs out in early 1998. This data leads to numerous new discoveries.
1996: DENIS (DEep Near Infrared Survey of the Southern Sky) begins operation. This ground based survey will survey the entire southern sky at 0.8, 1.25 and 2.12 microns using a 1-m telescope at La Silla, Chile. The survey will continue until 2001.
MSX (Midcourse Space Experiment) is launched in Apr 1996 and will last until its liquid helium coolant runs out in Feb 1997. During its 10 months of operation, MSX gathers a vast amount of data at 4.2-26 microns. MSX studies the infrared emission from the gas and dust which permeates the universe. It has 30 times the spatial resolution of IRAS and surveys areas of the sky which were not covered by the IRAS mission.
1997: 2MASS (Two Micron All-Sky Survey) begins operations. This highly uniform digital imaging survey of the entire sky uses two ground based telescopes to gather data at 1.25, 1.65 and 2.17 microns. Key science goals include probing the large-scale structure of the Milky Way and the local universe, performing an accurate census of stars in the solar neighborhood, and discovering brown dwarfs and active galactic nuclei. The survey continues until 2001, providing valuable data archives and spectacular infrared images.
NICMOS (Near Infra-Red Camera and Multi-Object Spectrometer) is attached to the Hubble Space Telescope in February 1997. This infrared instrument, consisting of three cameras and three spectrometers, provides spectra and high resolution images in the near-infrared.
2001: The Keck Interferometer began operations in 2001. The Keck Interferometer is part of NASA's overall effort to find planets and ultimately life beyond our solar system. It will combine the light from the twin Keck telescopes to measure the emission from dust orbiting nearby stars, directly detect the hottest gas giant planets, image disks around young stars and other objects of astrophysical interest, and survey hundreds of stars for the presence of planets the size of Uranus or larger.
2003: The Spitzer Space Telescope was launched in Aug 2003. It is the last of NASA's Great Observatories in space. Spitzer is be much more sensitive than prior infrared missions and will study the universe at a wide range of infrared wavelengths. Spitzer will concentrate on the study of brown dwarfs, super planets, protoplanetary and planetary debris disks, ultraluminous galaxies, active galaxies, and deep surveys of the early universe.
2004: SOFIA (Stratospheric Observatory For Infrared Astronomy) begins operations. A joint project between NASA and the German Space Agency, SOFIA incorporates a 2.5-m optical/infrared/sub-millimeter telescope mounted in a Boeing 747. Designed as a replacement for the successful Kuiper Airborne Observatory, SOFIA is the largest airborne telescope in the world.
IRIS (Infrared Imaging Surveyor) is planned for launch. IRIS is an infrared space mission planned by the Japanese space agency ISAS. It will have a near and mid-infrared camera and a far-infrared scanner. IRIS will be used to study the formation and evolution of galaxies, star formation, interstellar matter and extra-solar systems at wavelengths of 2-25 microns and 50-200 microns.
2007: The Herschel Space Observatory, planned for launch in 2007, is a European Space Agency infrared-submillimeter mission. Herschel will perform spectroscopy and photometry over a wide range of infrared wavelengths and will be used to study galaxy formation, interstellar matter, star formation and the atmospheres of comets and planets.
Planck Surveyor, also planned for launch in 2007, is a proposed European Space Agency far infrared-submillimeter mission. Planck will image the anisotropies of the cosmic background radiation over the entire sky with exceptional resolution and sensitivity, and will provide a uniform submillimeter map of the sky.
2021: The James Webb Space Telescope, planned for launch in 2021, is a visible/infrared space mission which will have extremely good sensitivity and resolution, giving us the best views yet of the sky in the near-mid infrared. JWST will be used to study the early universe and the formation of galaxies, stars and planets.