"IRAS: Mapping the Infrared Sky"


Because our vision is tuned to the particular lighting conditions of our own planet Earth, we see only a few of the many "colors" of the universe. Most of the electromagnetic spectrum is invisible to the human eye‹radio waves, ultraviolet, x-rays, gamma rays and the infrared wavelengths just below the threshold of sight.

Although the energy of radiant heat glows all around us, it was not until a simple, intuitive experiment by the astronomer Sir William Herschel in 1800 that it was recognized as a natural part of the continuous spectrum of electromagnetic energy. While experimenting with the heating properties of different colors, Herschel noted that his thermometer measured the highest temperatures when he placed it beyond the red region of a prism spectrum, in an area where apparently there was no light. He had discovered "calorific rays"‹what we know today as infrared.

In Herschel's own century astronomers used thermocouples, devices that convert heat to electric current, to detect this invisible infrared radiation from space. The Moon was first observed in this way in 1856, and by the early twentieth century most of the bright visible objects in the sky had also been observed in parts of the infrared. By the 1960s, the same decade that saw a boom in radio, x-ray, and ultraviolet astronomies, infrared observers began to benefit from new techniques, particularly the use of supercooled (cryogenic) detectors. Infrared telescopes were moved to higher and drier locations, and observers lofted their sensors by balloon, rocket, and airplane above the infrared-absorbing water of atmosphere.

A few preliminary surveys of the infrared sky, beginning in 1968 with the California Institute of Technology short-wavelength survey at 2 microns for northern latitudes, catalogued many new sources. A similar, though less complete, survey from a New Zealand observatory in the same year revealed some of the brightest infrared objects in the southem sky. These were to be followed in the 1970s by the U.S. Air Force survey with rockets at longer wavelengths, up to about 30 microns, and with the Naval Research Laboratory, the far Infrared Space Experiment which observed at the still longer wavelength of 100 microns.

Until 1983, however, there was no attempt to take a complete inventory of the major infrared emitters in the universe. This is the task of the Infrared Astronomical Satellite (IRAS), launched on January 25, 1983. An international effort involving the United States, United Kingdom, and The Netherlands, IRAS is performing the first all-sky survey in a wide range of infrared wavelengths with a sensitivity 100 to 1000 times greater than any previous work. At the end of nearly a year of observation from Earth orbit, IRAS data will be used to produce a comprehensive catalog and maps of significant infrared sources in the universe.

The Infrared Spectrum

The electromagnetic spectrum is divided into several categories of radiation, each with different wavelengths. At one end are the low-energy radio waves with wavelengths up to tens of thousands of meters. At the other end are the gamma rays whose wavelengths are smaller than the diameter of an atom. The smaller the wavelength, the greater the energy of the radiation.

Between these two extremes lies the infrared region, with wavelengths from one millimeter (the shortest radio waves) to approximately 0.8 microns (.0008 millimeters), the longest waves of visible red light. The familiar infrared heat photographs are made by films sensitive to only the shortest waves closest to visible light.

Virtually everything radiates in the infrared. Astronomical objects generally emit their energy over a wide range of wavelengths, and the hotter an object is, the more its energy output is concentrated at the short end of the spectrum. Hot stars therefore appear blue (short waves) while cooler stars are red. When an object is not quite hot enough to shine in visible light it emits the bulk of its energy in the infrared, like a stove burner before it begins to glow red hot. Infrared astronomy is thus the study of relatively cool objects below about 6000° Kelvin (10,000° F) that astronomers believe account for a significant amount of the universe's total energy output.

Aside from the ability to detect cool objects, there are other advantages to observing the universe in the infrared. Between the stars of our galaxy there is a large amount of cosmic dust that effectively blocks out light at visible wavelengths, because these particular waves are about the same size as the dust particles. The interstellar dust is especially thick in the plane of our galaxy, so that visible light coming from the Milky Way's center is reduced by a factor of ten billion by the time it reaches Earth. Not so with the longer waves of infrared, which are reduced by only one tenth. Because of this relative transparency, infrared astronomy is ideal for studying the bright and dense core of the Milky Way.

In addition, certain changes in energy state within hot gases and interstellar clouds of molecules have their signature in the infrared spectrum. By studying emissions from these regions it is possible to reconstruct the type of chemistry taking place there.

With all of this scientific information contained in photons of infrared light it is annoying, at least for astronomy, that after billions of miles of generally unimpeded travel from the far parts of the universe most of them are blocked just as they reach Earth. Water and carbon dioxide in the atmosphere absorb the bulk of infrared radiation from space. Only a few wavelengths make it through to the ground in narrow observational windows centered at 1.25,1.65, 2.2, 3.5, 4.75,10.5,19.5, 35, 350, and 800 microns. Even at these transparent windows there are problems‹the air above us glows brightly in the infrared, so even on the darkest nights infrared astronomers must resort to techniques like nodding their telescopes back and forth to sort out sky "noise" from astronomical sources.

The solution is to orbit an infrared telescope above the atmosphere where it is exposed to pure, unfiltered radiation and can survey the whole sky, north and south, even at wavelengths between the ground-based windows. This is the purpose of the Infrared Astronomical Satellite.

The IRAS Project

Like many of the ventures into space planned for the 1980s and 1990s, IRAS features international cooperation. The Netherlands Aerospace Agency (NIVR) supervised the design and manufacture of the spacecraft bus that supports and powers the main telescope, and the University of Groningen provided a Dutch Additional Experiment package.

The Ames Research Center and the Jet Propulsion Laboratory (JPL), both in California, developed the infrared telescope for NASA. JPL is also processing IRAS data into final infrared catalogs and maps. The United Kingdom contributes to the project through its Science and Engineering Research Council by tracking the satellite and receiving its radioed data.

As an international project, the satellite is managed by a Joint IRAS Project Executive Group made up of representatives from the participating agencies, institutes, and industries. Similarly, the IRAS science team draws its 18 members from the three cooperating nations.

IRAS was launched by a Delta rocket from the Western Missile and Space Center in California on January 25, 1983 and placed in a near-polar circular orbit (Florida launch sites are used for east-west orbits) at 900 kilometers (560 miles) altitude, well above the atmosphere but below the Van Allen radiation belts. The 1,076-kilogram (2,365-pound) cylindrical satellite is about the size of a passenger van, measuring 3.6 meters (12 feet) in length and 2.16 meters (7 feet) in diameter. Its main component, a 57-centimeter (22.4-inch) Ritchey Chretien-type reflecting telescope, has an array of infrared detectors mounted at the focal plane. At its base the telescope is attached to a Dutch-built spacecraft bus that contains all the electronic "housekeeping" equipment for computing, power distribution, tape recording, communications, and telescope pointing control. A large visor-like sun shade cuts down the amount of stray light reaching the telescope, and two solar panels catch sunlight for converting to electricity.

As with infrared telescopes on Earth, the IRAS instrument's heat-sensitive detectors are kept at very low temperatures. But for the purposes of this highly sensitive survey, the telescope itself must also be super-cooled to prevent its infrared heat from interfering with the detection of very faint astronomical sources. For this reason, the optical assembly and detectors are surrounded by a dewar, or cooling vessel, shaped like a doughnut stretched in height. The vessel contains 475 liters (125 gallons) of extremely cold liquid helium, spread out in the near zero-gravity of space into a film on the walls of its tank. The tank is in turn mounted inside a main shell, but is separated from the outer shell walls by a vacuum layer to further reduce heat flow to the telescope, like a giant thermos bottle. Thick insulation at the telescope's base shields it from the warmth of the spacecraft electronics.

Because of this cryogenic system, the first ever flown on an orbiting instrument, the temperature at the IRAS focal plane, where the infrared detectors are located, stays at a low 2° K (-455° F), or only two degrees above absolute zero‹the lowest temperature theoretically possible and the point at which all molecular motion comes to a halt. This 2° focal plane temperature is required for IRAS' great sensitivity‹the survey can detect objects in the universe as cold as 15°K (-432°F), sources whose energy is as faint as one million-trillionth of a watt per square centimeter by the time it reaches Earth.

The same cryogenics that allow IRAS to perform its mission, however, limit its lifetime. The liquid helium boils off even in cold space at a slow, steady rate, escaping as vapor through pores in a stainless steel plug in the tank. This leak rate dictates the useful life of the satellite, which was estimated to be approximately 11 months at the time the IRAS survey began.

The IRAS orbit was chosen with several factors in mind. First, the heat-sensitive telescope must always point more than 60° away from the Sun and more than 88° from the brightness of the Earth's limb, or edge. The power-producing solar panels must also receive sunlight at least part of the time. The satellite therefore follows a nearly polar orbit closely aligned with the Earth's terminator, or sunrise-sunset line. The orbit is Sun-synchronous‹it shifts approximately one degree each day to keep the same attitude relative to the Sun as the Earth travels its seasonal journey. Horizon sensors on the satellite warn it away from bright objects and keep it looking at dark space. They also help in the computer-guided, gyro-assisted attitude control system. Star sensors are used to achieve a telescope pointing accuracy down to only a few seconds of arc.

The purpose of all this caution and engineering is to maximize the amount of radiation striking the IRAS telescope's sensitive infrared detectors on each survey scan. There are 62 detectors in all, sensitive in four wavelength bands. Rectangular in shape, they average about the size of a medium-length printed word on this page. They work on the principle that exposure to infrared radiation reduces the electrical resistance of their crystals by a known amount, so that the amount of radiation reaching the telescope's focal plane can be read directly as an increase in current. The Band 1 detectors, observing at shorter wavelengths, typically reveal the emissions of hotter point sources such as stars. Band 4, on the other hand, observes cooler or more extended objects, such as dust clouds, with its longer wavelength sensitivity.

Mounted with the array of detectors in the telescope's focal plane are three more instruments, known collectively as the Dutch Additional Experiment. They include two photometers and a low-resolution spectrometer. The spectrometer is used with the main telescope to obtain spectra for strong point sources emitting in the 7.4-23 micron range. This helps in their classification. For statistically measuring the distribution of infrared sources in areas of high stellar density, there is a short wavelength (4.1-8 microns) channel photometer, also used with the main survey telescope.

In addition, another long-wavelength photometer maps the areas of high and low infrared radiation in large, extended sources by giving data on relative intensities. This instrument is not used while the survey is in progress, but requires a specific "pointed" observation mode.

Surveying the Infrared Sky

Following a two-week check-out of its telescope and operational systems, IRAS began history's first infrared survey of the entire sky on February 9, 1983. The first scan recorded infrared sources in the constellations Taurus and Hercules in the northern sky and Scorpius and Eridanus in the southern, among others, as the satellite traced a circle around the Earth.

For the purpose of the survey, mission planners have divided the celestial sphere into overlapping banana peel-shaped segments called "lunes." Lunes are the areas sliced out between two great circles of ecliptic (as opposed to Earth) longitude, and they each cover 30° of the celestial sphere. The telescope scans a thin (1/2° field of view) segment in each of two lunes on every north-south orbit‹first rising through one, then descending through the corresponding lune on the other side of the Earth.

As IRAS scans the sky, the faint radiation from astronomical sources is focused onto the array of detectors at its focal plane. These detectors are arranged so that the energy from any source strikes two of each kind of detector at a time. The same segment of the sky is then rescanned on the next orbit to give a total of four data readings for each source in each of the four infrared channels. Then, later in the survey, the telescope returns to do another double scan of that portion of the sky, so that every area is covered at least four times (some even more), and every astronomical source has a possible eight detections overall in each IRAS waveband. This repetition is needed to make sure that the survey is thorough, and also to prevent transitory objects‹asteroids, comets, even glints of moonlight‹from being wrongly interpreted as infrared sources in deep space.

Orbit Geometry

The IRAS orbital path is closely aligned with Earth's sunrise/ sunset line, and shifts about one degree each day to maintain that position throughout the changing seasons. Solar panels acquire sunlight while the telescope (protected by a sunshade) points outward to dark space.

The IRAS focal plane assembly includes 62 small rectangular infrared detectors, sensitive in four wavebands.

IRAS circles Earth 14 times a day, once every 103 minutes, and its telescope is always pointed away from Earth and out at the dark sky. Only about 60% of its time will be spent on the survey, however, due in part to "noise" created by Earth's Van Allen radiation belts, which dip below the altitude of the IRAS orbit in a region centered over the south Atlantic ocean. Trying to conduct a sensitive survey in this region would be pointless‹protons in the radiation belts would create too many false detector readings.

So during these periods, the IRAS telescope is focused on specific astronomical objects that mission planners have targeted as the most interesting for observing in the infrared. Hundreds of these additional observations‹of unusual galaxies, bright infrared stars, and nebulae‹are done each week, not only when the turning Earth brings the south Atlantic under IRAS, but also when the satellite passes over the poles, where overlapping survey coverage is not needed as often as in other areas.

Twice each day IRAS radios scientific data down to Earth as it passes over a receiving station at Rutherford Appleton Laboratories in Chilton, England. Two on-board tape recorders with a combined capacity of 900 million bits of information store the data while IRAS is surveying, then "dump" the previous half day's data at a rate of one million bits per second at Chilton. Engineering information on satellite operations is also sent down with the scientific data.

Infrared Band Type of Detector Number Spectral Range

IR Band 1 Silicon Arsenide 15 8.5 - 15 microns

IR Band 2 Silicon Antimonide 16 19 - 30 microns

IR Band 3 Germanium Gallium 16 4 - 80 microns

IR Band 4 Germanium Gallium 15 83 - 119 microns

62 Total

At the same time, instructions for carrying out the next half day of the survey are radioed up to the IRAS computers, after a team in England has determined that all spacecraft operations and data are normal. Only a preliminary check-out of the data is performed in England, whereas the full set of IRAS data is sent by communications satellite from there to the Jet Propulsion Laboratory for extensive computer processing.

IRAS and Infrared Astronomy

According to modern theory, a star is formed from a cool cloud of interstellar dust and gas that is somehow triggered to begin condensing. The mutual gravitational pull of its particles causes the cloud to contract further and heat up. Eventually it collapses into a hot, dense sphere, and internal pressures and temperatures reach a point where nuclear fusion processes begin. A star is born.

The unique contribution of infrared astronomy is that it can detect these proto-stars long before they "turn on" in visible light by sensing the heat they emit as they contract. The prototype for this kind of object, the Becklin-Neugebauer Kleinmann-Low source in Orion, was discovered by infrared astronomers in the mid-1960s. The nebula is about ten times the mass of the Sun, but has a temperature of only 600°K (620°F). IRAS should be able to locate proto-stars that are much smaller‹down to the size of the Sun‹over much of the galaxy.

Stellar "hatcheries" leave other infrared clues as well. Very young, hot stars, for example, emit ultraviolet light that breaks up the atoms in nearby clouds of hydrogen and leaves the gas ionized. Such clouds of ionized hydrogen are called H 11 regions, and are believed to mark the sites of ongoing star formation. They also appear at all IRAS survey wavelengths.

IRAS will be particularly good at picking out these H 11 regions throughout the galaxy. In addition, the IRAS spectrometer should detect certain interstellar molecules, among them water and ammonia that appear widely throughout the galaxy as very cool clouds of dust and gas. These clouds of molecules also are thought to be associated with the birth of stars.

By detecting, identifying, and showing the distribution of these dusty proto-stars, H 11 regions, and clouds of molecules, the IRAS survey will allow astronomers to estimate the rate at which stars are forming in our galaxy, and by extension, other galaxies in the universe.

At the waning end of a star's life cycle it runs low on its own nuclear fuel and begins to redden and die. As it does so it may cough out clouds of material from the interior‹heavy elements formed from fusion processes deep inside the star. Dust can envelop the fading star so thickly as to block its visible light completely. Here infrared astronomy can contribute in two ways. First, since the dust is much more transparent at long infrared wavelengths, astronomers are still able to see infrared light from the fading star. Secondly, the surrounding dust is itself heated up by the star to reradiate its own infrared light. Because of this, IRAS can take inventory of the stars dying in our galaxy.

The material ejected from these fading stars in turn resupplies the large clouds that will eventually become new stars. The dust is composed mainly of smoke-sized silicate grains, and these have characteristic signatures at specific infrared wavelengths. By mapping the galaxy-wide distribution of this silicate dust, IRAS will be an important key to understanding the ongoing natural processes of stellar birth, death, and the recycling of matter.

Infrared also opens up an important region of the sky nearly invisible to optical astronomers‹the center of our Milky Way galaxy, located in the direction of the constellation Sagittarius. With its ability to see through the thick dust in the plane of the galaxy, IRAS can map a previously unseen structure at its core, including further detail in the giant H 11 regions and molecular clouds that have already been observed there at radio and infrared wavelengths. Infrared sources and structure in nearby galaxies like the Magellanic Clouds will also be revealed in the IRAS survey.

Perhaps most intriguing will be the information IRAS returns on certain unusual objects outside the Milky Way that astronomers know already emit large amounts of infrared energy.

Quasars and Seyfert galaxies, for example, pour out unexpectedly large amounts of energy for their size at all wavelengths. Some galaxies are known to have particularly large excesses in the infrared, and the reasons why are not clear. The IRAS survey is able to catalog such unusual sources so that their energy output in the infrared can be compared with that at other wavelengths.

Closer to home, the IRAS survey can observe many objects within our own solar system. Asteroids, especially, should appear in the survey by the thousands, and infrared data can help to determine their reflectivity, their surface composition, and even their diameters. Also visible to the sensitive detectors is the zodiacal light caused by sunlight reflecting from dust within the plane of the solar system. To map the distribution of this dust is to gain insight into how our planetary system was formed. The IRAS telescope avoids the bright planet Jupiter, however, because brightness is heat, and heat boils off the telescope's cryogenic helium faster than the desired rate.

Approximately one year after IRAS ends its survey in space, the Jet Propulsion Laboratory plans to release the mission's final products‹maps of the infrared sky and a catalog of all major infrared sources, along with critical information on each one. This information will be available to the entire scientific community worldwide.

The IRAS maps, when completed, will show a much different sky than our familiar array of constellations, a different sky even than the one astronomers know from previous infrared study. We can only speculate on the total number of new sources that will be discovered. It is even quite possible that IRAS will discover whole new classes of astronomical objects whose only signature is in the faint infrared radiation they emit.

As we learn to open our electronic eyes to the many "colors" of the electromagnetic spectrum, we come closer to putting together all the clues, to understanding the great and deep complications of the universe.

Classroom Activities

1. Construct a three-dimensional model of the Earth, Moon, and Sun in space. Calculate the angular size of the Earth as seen from the 900-kilometer-high IRAS orbit. Then plan a strategy for surveying the entire sky in the fewest possible orbits, keeping in mind that the telescope must point away from the Sun, Moon, and Earth, and remembering the restrictions against observing when the satellite is over the south Atlantic.

2. Using the sources listed in the bibliography below as well as other articles published in scientific journals, make a map comparing the fifty brightest known infrared sources with the fifty brightest visible objects in the sky.

3. Perform the experiment by which Herschel discovered infrared radiation.

4. Write a report based on preliminary IRAS findings published in such magazines as Science, Astronomy and Astrophysics, and Nature.

Suggested Reading

G. Neugebauer and R.B. Leighton. "The Infrared Sky," Scientific American, August, 1968, pp. 50-60.

G. Neugebauer and E.E. Becklin. "The Brightest Infrared Sources," Scientific American, April, 1973, pp. 28-40.

C.H. Annett. "The Infrared Universe," Astronomy, October, 1981, pp. 74-79.

David A. Allen. "Infrared, the New Astronomy," New York, K. Reid, 1975.