Only a small part of the infrared regionof the spectrum can be observed fromthe ground. There are several windows

in Earth’s atmosphere where we can seeinfrared radiation from astronomical objects,from red visible light (0.8 µm) to around30 µm. From 30 µm to around 400 µm,astronomers must get above as much of Earth’satmosphere as possible, going to high moun-tains, flying balloons or aircraft, or going intospace. Some of the main scientific reasons forusing the infrared region of the spectrum are toprobe dense dust clouds, to study moleculesand to observe distant galaxies that emit most

strongly in the infrared through their powerfulbursts of star formation.

When we compare the view we get of the cen-tre of our own galaxy in the visible with thatfrom the infrared (even at 2 µm), the differenceis obvious since the infrared penetrates the dustclouds between us and the galactic centre.However, some clouds have such dense cores,that even at 15 µm they can block the radiationfrom background objects. Instruments anddetectors have improved enormously duringthe last four decades. In 1968, Gerry Neuge-bauer and Eric Becklin scanned across thegalactic centre using a single detector on the200 inch telescope at Palomar. In 1995 thesame amount of time was used by Ian Gatley tomap the whole galactic centre region in severalinfrared filters.

5.10 October 2000 Vol 41

A brief history of iIt is 200 years since William Herschel

discovered and began to explore

infrared radiation. An exhibition on the

many uses of the infrared part of the

spectrum for the RAS President’s

evening earlier this year drew together

the many discoveries and applications

of this versatile waveband. Early

observations focused on the

usefulness of infrared as a measure of

temperature, for example of the Moon

and the solar corona. Advances in

detector technology led to the first sky

Helen J Walker sums up the first200 years of infrared astronomy.

Infrared astronomy

1: The galactic centre as seen by 2MASS andMSX. The 1.25 µm 2MASS data are codedblue, the 2.17 µm 2MASS data are codedgreen, and the 6–11 µm MSX data are codedred. The picture is 5° long by 1° wide, and thegalactic centre appears to be bright yellow.Even at 11 µm there are some dust clouds toodense to see through.

Early pioneersAlthough Sir William Herschel discovered andexplored infrared radiation in 1800, throughhis series of 881 experiments, little astronomicaluse was made of infrared until the 20th century.Piazzi Smyth detected the full Moon in theinfrared in 1856, using a thermocouple. In1881, Samuel Pierpont Langley (who is com-memorated on a US stamp for his work towardsheavier-than-air flight) invented an instrumenthe called a bolometer to study solar radiation.One of his students penned these “immortal”lines in celebration of his achievement:

Prof. Langley devised a BolometerIt’s really a sort of ThermometerIt’ll detect the heatOf a Polar Bear’s feetAt a distance of Half-a Kilometre!

From 1868 to 1890, the Fourth Earl of Rossetackled the problem of measuring the heat ofthe Moon, the brightest object in the sky afterthe Sun. His final result for the maximum lunartemperature at the equator during the lunarday differed from Langley’s results, causingconsiderable controversy, but posterity hasshown that Rosse was nearer the truth.Thomas Alva Edison also investigated theinfrared, using an instrument he inventedcalled the Tasimeter. He intended to measurethe temperature of the solar corona by com-paring it to Arcturus and attempted to measurethe corona when the Sun’s disk was obscured,during a total eclipse of the Sun in Wyoming.Anecdotally, he is said to have observed theeclipse from the door of a chicken run, all thebest sites and best accommodation having been

5.11October 2000 Vol 41

nfrared astronomysurveys in the 1960s and 1970s. Now

infrared astronomy is part of the

armoury of the modern astronomer

interested in anything from the origins

of the universe to the disposition of

water and other molecules in

interstellar space. Notable successes

have come from instruments such as

UKIRT, IRAS and ISO, and much is

expected in the future from Gemini,

the Next Generation Space Telescope,

FIRST and Planck, among other

planned instruments.

Infrared astronomy

taken by “proper” astronomers. In the early1900s, infrared radiation was detected fromseveral astronomical sources such as Jupiter,Saturn and Vega, as well as the first systematicobservations, made by Seth Nicholson and Edi-son Petit, and others, in America.

Modern infrared astronomy perhaps began inthe 1950s, when lead sulphide detectors becameavailable to astronomers. The PbS cell wascooled to 77 K in liquid nitrogen to increase thesensitivity. Around 1960, Harold L Johnsondefined the first infrared magnitude system. Hisinfrared photometer covered the wavebands R,I, J, K, L (out to 4 µm), and Johnson and histeam measured thousands of stars.

Getting above the cloudsIn the 1960s, infrared astronomy took off liter-ally as well as metaphorically. Neugebauer andLeighton in California (at Mount Wilson) start-ed work on a survey of the northern sky (givingus the NML, CIT and IRC objects), andStephan Price made a similar survey of thesouthern sky. Frank Low invented the first ger-maniun bolometer (cooled using liquid heliumto 4 K) which worked beyond 10 µm, and hemade the first observations at 35 and 350 µm,as well as observing at 100 µm using a Lear jetto get above most of the atmosphere. Low andJohnson were using the 60 inch telescope sitedon Mount Lemmon (in the Catalina Mountainsof Arizona) in the 70s. Pioneering work in theUK in infrared astronomy was led by Jim Ring’sgroup from Imperial College London, with the

Infrared Flux Collector (IRFC) on Tenerife. Thetelescope was similar in size to the telescopeused at Mount Lemmon, but as David Allenpoints out in his book INFRARED The NewAstronomy, astronomers had to bring their ownequipment (a big disadvantage).

Stars and Stellar Systems (Volume II) report-ed that, in 1959, a balloon experiment waslaunched by the group at Johns Hopkins Uni-versity to look for water vapour in the atmo-spheres of Venus and Mars. Balloons could goas high as 25 miles above the Earth’s surface.The Goddard Institute of Space Sciences usedballoons in 1966 to survey the sky at 100 µmand they detected around 120 bright infraredobjects near the galactic plane. In the 1970sand 1980s, many balloon experiments flewfrom Palestine, Texas. A typical experimentinvolved an ESA spectrometer on a UCL bal-loon platform. It operated between 40 µm and100 µm, looking at fine structure lines of oxy-gen in the Orion and M17 nebulae. The UCLastronomy group, led by Dick Jennings, col-laborated with the astronomy division of theSpace Science Department at ESTEC (includingRoger Emery and Brian Fitton). The balloonwas filled with helium and its release with theexperiment from the truck, Tiny Tim, was verycarefully timed. Retrieval of the experimentafter the flight often provided some excitingmoments, from whether the experiment (andconsequently the data) survived the landing towhether it landed at all (there is a photographof a balloon experiment stuck 25 m above

ground in “the only tree in Texas”).In 1967, the Mauna Kea Observatories site

for astronomy was founded 4200 m above sea-level on an extinct volcano in Hawaii. TheUnited Kingdom Infrared Telescope (UKIRT)was built there, using an exceptionally thin3.8 m mirror, making the telescope a third asmassive as a normal telescope of that size(6.5 tonnes). The site at Mauna Kea is home tomany telescopes now, including the NASAInfrared Telescope Facility (IRTF) and the newGemini North telescope, which has infraredcapability out to 25 µm.

Another way in which astronomers beat theatmosphere was by flying to high altitudes. TheC-141A jet transport plane, the Kuiper Air-borne Observatory (KAO), based at the NASAAmes Research Center in California, startedflights in 1974 (figure 4). The KAO could reachan altitude of 41 000 feet and it had a 0.9 mtelescope on board (the hole through which itlooked is just visible near the top of the plane).Astronomers brought their own experiments toput on to the telescope, or collaborated withgroups which had suitable equipment. Planningthe night’s observing was more interesting onthe KAO than on other telescopes, because thelegs of the flight path (corresponding to theobjects observed) had to return the aircraft tothe airfield from which it took off at the start ofthe night. The rings around Uranus were foundusing the KAO in 1977.

SOFIA (Stratospheric Observatory For Infra-red Astronomy) is a project funded by NASA

Infrared astronomy

5.12 October 2000 Vol 41

2: A picture of thegalactic centreregion, taken withUKIRT. Data at J arecoded blue, data atH are coded green,and data at K arecoded red. Theregion covered is7.5′ by 4.5′. (Image:Ant Chrysostomau.)

3: A map of the entire pointsource sky as seen by IRAS(in Aitoff projection), with thegalactic centre in the middleof the picture. The data arecolour coded, and generallystars appear blue, galaxiesand young (dust-shrouded)stars appear yellow/green.(Infrared Processing andAnalysis Center, Caltech/JPL.IPAC is NASA’s InfraredAstrophysics Data Center.)

4: The Kuiper Airborne Observatorypreparing for take-off at NASAAmes Research Center.

and the German Space Agency DLR, to put a2.5 m telescope into a Boeing 747-SP. SOFIAwill have a set of observatory instruments forall astronomers to use, and is due to startflights in 2002.

Infrared in spaceThe Aerobee Infrared Telescope rocket waslaunched in 1967, to measure the infrared skybackground. In 1976, a landmark cataloguewas published from observations made withsounding rockets, the Air Force GeophysicsLaboratory catalogue (AFGL) by Stephan Priceand Russell Walker, with 2363 objects (wave-length coverage 4–30 µm). The data, from afull sky survey (taken during several rocketflights), took a total of 30 minutes to collect.IRAS (the Infrared Astronomical Satellite), ajoint Dutch, American, and British project, waslaunched in 1983 and made an all-sky surveyfrom 10 µm to 100 µm. The 0.6 m telescopeand detectors were cooled with liquid helium.The mission lasted for nine months, but in itsfirst 12 hours of operation IRAS detected moreobjects than were in the AFGL catalogue. TheIRAS Point Source Catalogue contains 245 839sources and in addition to the Point Sources,IRAS produced maps showing the extent of thewarm and cool dust. COBE (the Cosmic Back-ground Explorer satellite) was launched in1989 to survey the infrared background. It con-firmed the smoothness of the 2.7 K spectrumand detected ripples in the spatially smoothcosmic background (extremely small tempera-

ture variations) which may have led to the for-mation of galaxies.

ISO (the Infrared Space Observatory) waslaunched in 1995 (figure 5). It was an ESA mis-sion, with a 0.6 m telescope and four instru-ments on board, operating between 2 and240 µm. The satellite weighed around 2500 kgand was launched with over 2000 l of liquidhelium coolant on board, which lasted for29 months. ISO made around 30000 observa-tions of astronomical objects, from planets andstars to distant galaxies. The Japanese alsolaunched an infrared satellite (IRTS) in 1995,although this had only a 28-day mission. TheAmerican MSX (the Midcourse Space Experi-ment) satellite was launched in 1996. It weighed2700 kg, and was cooled by a solid hydrogenblock, which lasted for 10 months. MSX oper-ated between 4 and 26 µm, with much higherresolution than IRAS.

The future for infrared astronomyMany ground-based telescopes now haveinfrared capabilities, including Gemini andKeck. Ground-based surveys at high sensitivityhave been made, most notably DENIS (a deepnear-infrared survey of the southern sky) and2MASS (the 2 µm all-sky survey). NICMOS(the near-infrared camera and multi-objectspectrometer) was fitted to the Hubble SpaceTelescope in 1997. The replacement spaceobservatory, NGST (Next Generation SpaceTelescope), will also have infrared capabilitiescovering 0.6–20 µm using an 8 m telescope.

The next infrared satellites planned for launchare SIRTF by the Americans (in 2002) and IRISby the Japanese (in 2003). SIRTF will belaunched into an Earth-trailing orbit ratherthan the conventional Earth orbit, and employpassive-cooling techniques in addition to liquidhelium coolant.

In 2007, the ESA satellites FIRST (Far Infraredand Submillimetre Telescope) and Planck willshare a launch. FIRST will be an observatorysatellite making observations of specified tar-gets, but Planck will follow COBE in making anall-sky survey of the cosmic background at high-er sensitivity and higher spatial resolution. ESAis studying the Infrared Space InterferometerDarwin (IRSI Darwin), which involves betweenfour and six 3.5 m telescopes (each mounted ona separate spacecraft) acting as an interferome-ter with baselines between 50 m and 500 m.The interferometer would be used for astro-physics and planetary systems research andwould work between 5 µm and 28 µm. It is acandidate for an ESA Horizons 2000 corner-stone mission, for launch around 2012. NASAhas a similar mission called TPF (TerrestrialPlanet Finder). �

Helen J Walker FRAS, Rutherford AppletonLaboratory, UK. Acknowledgements: My thanks to the IPAC time-line, Peter Hingley and David A Allen’s bookINFRARED The New Astronomy. My apologiesfor all the landmark events (and people) I havefailed to mention.

5.13October 2000 Vol 41

Infrared astronomy

5: The Infrared SpaceObservatory (ISO) in the

clean room at ESTEC.