The story so far has been of stars shrouded in dust, clouds of gas and dust where new stars are forming, and nearby star-forming galaxies. Now we shift the stage to the whole universe with the discovery of background radiation at microwave wavelengths. The cosmic microwave background (CMB) radiation is the dying whisper of the initial fireball phase of the hot Big Bang universe, and its discovery transformed our understanding of the universe. This background is the dominant form of astronomical radiation at submillimetre wavelengths outside the Milky Way. When astronomers were trying to detect submillimetre sources in the 1970s, nodding the telescope between the source position and a nearby position on the sky to subtract out the emission from the Earth’s atmosphere, each telescope beam would detect an amount of CMB radiation far brighter than the source they were trying to detect. However, because they were only interested in the difference between the two measurements, the CMB radiation exactly cancelled and did not affect the observations. In this chapter, I describe the discovery of the radiation and its impact on cosmology.

The story of the discovery of the cosmic microwave background by Arno Penzias and Bob Wilson in 1965 has been told many times. Initially this was a story of short-wavelength radio astronomy and microwaves (centimetre wavelength radiation), but gradually it became clear that the wavelength of peak energy is around 1 millimetre, so while about half the CMB is microwave, half is submillimetre. To understand the significance of this radiation, we have to understand the origin and evolution of the universe itself.

The enormous success of IRAS stimulated both the European Space Agency (ESA) and NASA to develop new space infrared observatories that would follow up the wealth of discoveries about the infrared universe made with IRAS.

Early in February 1983, the European Space Agency met to select a new medium-sized astronomy space mission. Peter Clegg was able to place on the table at the meeting the first scan around the sky from IRAS, and its quality was sufficient to convince the European Space Agency to select the Infrared Space Observatory (ISO). The idea for a European infrared space observatory had been first proposed in 1979. ISO was finally launched in November 1995 with a planned life of 18 months (Figure 10.1). In fact, its helium coolant lasted until April 1998, almost a year longer than expected.

ISO had a camera, ISOCAM, led by Catherine Cesarsky and a spectrometer, SWS, led by Thijs de Graauw, working at the near- and mid-infrared wavelengths (3–20 microns); and a camera, ISOPHOT, led by Dietrich Lemke and a spectrometer, LWS, led by Peter Clegg, working at far-infrared (40–160 micron) wavelengths. The two cameras also had smaller low-resolution spectrometers as part of their capability. The spectrometers of ISO were especially powerful in unravelling the nature of the dust around stars and in interstellar space, and in probing young stars in the process of formation.

The launch of the Infrared Astronomical Satellite (IRAS) on 25 January 1983 was a very exciting moment in the history of infrared astronomy. It had been designed to survey the whole sky at far-infrared wavelengths between 10 and 100 microns. When the helium coolant ran out ten months later on 22 November, IRAS had surveyed 96% of the sky. Over the next few years there were a string of discoveries that still comprise the core of our knowledge of the dusty universe: the zodiacal dust bands, the link between Apollo asteroids and comets, the infrared ‘cirrus’, debris disks and protoplanetary systems, ultraluminous and hyperluminous infrared galaxies, dust tori around active galactic nuclei, young stellar objects and ‘protostars’, and the origin of our Galaxy’s motion through the cosmic frame.

The IRAS Story

During the 1970s, Dutch astronomers had been exploring the concept of a dedicated infrared satellite and managed to get U.S. astronomers and NASA interested in the idea, with the latter proposing that the wavelength range be extended to 100 microns. In 1975, Nancy Boggess, head of infrared astronomy at NASA, assembled a group of infrared astronomers at Snowmass, a ski resort in Colorado. To their surprise, she announced that NASA intended to build a space facility for infrared astronomy and to operate it from the instrument bay of the space shuttle. They had even given it a name, the Shuttle Infrared Telescope Facility, or SIRTF. The astronomers argued instead for a smaller survey satellite in Earth orbit which would map the whole sky in a few weeks, a proposal that Neugebauer and others had originally made in 1971. The survey satellite concept did not make much progress until the Dutch began to propose a joint U.S.-Dutch mission. In 1976, on the initiative of the Dutch astronomers, the United Kingdom was also invited to join what became the Infrared Astronomical Satellite, or IRAS.

Finally, we look ahead to the next decade of infrared and submillimetre astronomy. This will be an era in which the infrared and submillimetre wavebands continue to have a dominant role, with spectacular space missions and giant, new ground-based facilities. I describe the three most recent infrared and submillimetre missions to be launched, Herschel, Planck, and WISE; the future planned missions, the James Webb Space Telescope and SPICA; and the future ground-based facilities, the Atacama Millimetre/Submillimetre Array and the very large 30–40-metre ground-based telescopes. We will see that the next decade will be just as exciting as the 25 years since the dramatic days of the IRAS mission have been.

Herschel and Planck: probing the cold universe

On 14 May 2009, the European Space Agency (ESA) launched together on top of a single Ariane 5 rocket two major space astronomy missions, the Herschel Space Observatory and Planck. The dual launch of these complex missions, at ESA’s space port at Kourou in French Guiana, was a great moment for European space science. It was an extremely moving moment for those of us who had worked on these missions since their inception. They were finally approved by ESA in 1993, Herschel (then known as FIRST, for Far InfraRed Space Telescope) as the fourth of ESA’s Horizon 2000 ‘cornerstone’ missions providing a multi-instrument observatory working at far-infrared and submillimetre wavelengths, and Planck (then known as Cobras-Samba) as a ‘medium’ mission to map the cosmic microwave background radiation. Cobras and Samba had been submitted as separate proposals, but ESA decided they should be merged into a single mission with two instruments, the Low Frequency Instrument (LFI), led by Reno Mandolesi and the High Frequency Instrument, led by Jean-Loup Puget. Both missions had been studied for many years before 1993.

Figures 1.1, 1.4, 6.2, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 8.1, 8.3, 9.1, 10.8, 10.10, 11.4, 12.7, Plates I, III, IV, V, VI, VII, VIII, IX, XI, XII, XIII, XIV, XV, XVI, XVII, XVIII, XIX, XX, XXI, XXII: NASA; Figure 1.3: NRAO; Figure 2.1: John Herschel-Shorland; Figures 2.2, 2.3: Science Photo Library; Figure 3.1: Royal Observatory Edinburgh; Figure 4.1: D. Milon; Figure 4.3: Edith Low; Figures 4.4, 4.7: California Institute of Technology Archives; Figure 4.6: Bob Gehrz; Figure 4.8: George Rieke; Figure 4.9: Astrophysical Journal; Figure 4.10: Louis Allamandola; Figure 5.1: adapted from Naylor et al. (2000). Figure 5.2: Pat Daly; Figure 5.3: Ian Robson; Figure 5.4: Martin Harwit; Figure 5.5: Bill Hoffmann; Figure 6.1: Alcatel-Lucent; Figure 9.2: Gemini Observatory; Figures 10.1, 10.3, 10.4, 11.7, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, Plates XXIII, XXIV, XXV, XXVI, XXVII, XXVIII, XXIX, XXX, XXXI: ESA; Figure 10.2: Ewine van Dishoeck; Figures 10.6, 10.7: Mike Werner; Figure 10.9: Eiichi Egami; Figure 11.1: Brian May; Figure 12.8, Plate XIII: ALMA Observatory; Figures 11.3, 12.9: European Southern Observatory; Plate X: STFC; Plate XIV: Hervé Aussel.

We have seen that at near- and mid-infrared wavelengths there are a few wavelength windows accessible from mountaintop observatories, at 1.25, 1.65, 2.2, 3.5, 5, 10 and 20 microns. In the previous chapter, I described the advances that came with pushing out into these near- and mid-infrared windows with ground-based telescopes and with telescopes on rockets. Beyond 20 microns, carbon dioxide and especially water vapour in the Earth’s atmosphere make astronomy from the ground almost impossible until we reach a few submillimetre windows at 350, 450, 850 and 1250 microns (Figure 5.1). In this chapter, I describe how ground-based submillimetre astronomy in these windows developed and how far-infrared and submillimetre astronomy was opened up using aircraft and balloons. The cosmic landscape these wavelengths opened up was one of dense clouds of dust and molecular gas, and of new stars being formed in these clouds.

Submillimetre Astronomy

The father of far-infrared and submillimetre astronomy was undoubtedly Frank Low, who in 1961 had invented the gallium-doped germanium bolometer and the Low dewar. In the previous chapter, I discussed how Low and his collaborators applied this detector to 10- and 20-micron observations of stars and of clouds of cool gas where new stars are forming. In the late 1960s, Frank Low began to work at genuinely far-infrared wavelengths using a 12-inch telescope aboard a Lear executive jet (Figure 5.2). His first attempts at airborne astronomy had begun soon after his move to Arizona in the 1960s, when with Carl Gillespie he observed the Sun at a wavelength of 1 millimetre (1000 microns) from a twin-jet Navy bomber. The Lear jet was an early executive jet, and the telescope was installed in the emergency door opening. The jet flew at 50,000 feet, and the cabin air pressure was so low that the pilots and astronomer operating the telescope needed full breathing equipment. To subtract out the foreground infrared emission from the residual atmosphere, Low installed a tilting secondary mirror operated by a magnetic drive.