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200 Years of infrared discoveries
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The German-born British astronomer and musician, William Herschel
(Courtesy of Royal Astronomical Observatory). Background: Hubble / WFPC2
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As recently as 200 years ago the Earth was widely thought to be only about six thousand years old - in 1650
Bishop Ussher had famously calculated the date of creation as 4004BC. The first to recognise the true age of the Earth was a Scottish physician
called James Hutton, an amateur geologist, who, in 1790, realised from his study of rock formations that the Earth had to be much older. It was
so many millions of years older than previously imagined that it made Hutton's head spin to be "looking so far into the abyss of time". Geology
was not the only science at that time to radically revise and expand the view of the world around us. An unexpected discovery made by an
astronomical contemporary of Hutton's would later lead astronomers to revise their view of the Universe just as dramatically.
In 1800, the German-born British astronomer and musician, William Herschel - famous for his discovery of the planet Uranus a few years
earlier - described that the differently coloured filters through which he observed the Sun allowed different levels of heat to pass. He performed
a simple experiment to study the "heating powers of coloured rays": he split the sunlight with a glass prism into its different constituent rainbow
colours and measured the temperature of each colour. He observed an increase in temperature as he moved a thermometer from the violet to the
red part of the 'rainbow'. Out of curiosity Herschel also measured temperatures in the region just beyond the red colour, where no light was visible,
and to his surprise, he recorded the highest temperature there. He deduced the presence of invisible "calorific rays".
Today these invisible rays are called infrared radiation. All objects emit infrared radiation and much can be learned about an object from this emission.
However, the true significance of infrared radiation for astronomy was not immediately apparent. Although the first instruments to detect the invisible light
were built soon after Herschel's experiment, not even the Moon was observed in the infrared until 1856. Astronomers had to wait a further one and a half
centuries to experience the full startling impact of the sky revealed in the infrared.
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The planetary nabula called the Egg Nebula seen in near-infrared
light. Blue corresponds to starlight reflected by dust particles, and red corresponds to heat radiation emitted by warm hydrogen. By comparing
it with the optical image below, it can be seen with stunning clarity how the infrared perspective can reveal entirely new progresses. (Hubble / NICMOS)
A view in visible light of the Egg Nebula, shown on first image in infrared. A dust
disc blocking the view to the 'waist' of the star is seen in the second image. The infrared (first) image penetrates through most of the dust, and reveals
a thick disc of warm gas there.(Hubble / WFPC2)
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The first infrared technology was not developed for scientific purposes, but for military applications. Infrared detectors
can 'see' heat-emitting bodies at night to give a kind of 'night-vision'. This has great military importance and was the main impetus behind the
development of infrared detectors during much of the twentieth century.
Only in the sixties were these new detectors pointed at the sky, and even then infrared astronomy was just expected to be an adjunct to
optical astronomy. "Stars, after all, were known to be visible objects; and the Universe appeared to be an aggregate of stars", writes US infrared pioneer
Martin Harwit in the book "A Century of Space Science".
The first infrared survey of the sky, performed by Gerry Neugebauer and Robert Leighton - who built their own telescope for the purpose - changed
this view completely. The results were published in 1965 and Harwit describes them as "electrifying": they revealed ten objects that were completely
invisible to other existing telescopes, which merely detected visible (optical) light.
This raised an unsettling question: if a first look at the infrared sky had yielded about ten new, odd, infrared-bright objects, what would more detailed
observations reveal? In 1969 the first catalogue of infrared-bright objects was published, including thousands of intriguing objects never seen before.
A few years later more observations revealed the existence of distant galaxies whose radiation is stronger in the infrared than in the visible. There was
no immediate explanation for this.
Astronomers were seeing a qualitatively different sky, a sky with different rules from the familiar night sky. The brightest objects in the night sky, visible to
the human eye and optical telescopes, are typically those hot enough to emit light in the visible range. Furthermore, only those objects whose light is
not absorbed by intervening dust can be seen. In contrast, in the infrared sky, both cold and dusty objects may outshine the optically bright ones. The first infrared observations surprised everyone by revealing many more infrared-bright objects in the sky than expected.
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The star-forming region NGC 3606 imaged with the ESO ISAAC instrument on VLT Antu in Chile. The image demonstrates with all clarity that certain types of near-infrared imaging are possible from the ground. (ESO VLT / ISAAC)
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There were still serious hurdles for infrared astronomy to overcome. The Earth's atmosphere blocks most
of the infrared light from the sky and the atmosphere itself emits strongly in the infrared. Ground-based infrared astronomers have to try to
observe objects that can be a million times fainter than the emission from the sky - as difficult as trying to observe faint stars during the
day with an optical telescope.
A deep exploration of the infrared sky required observations from above the atmosphere. How could this be done? There were several
possibilities: detectors carried high by balloons, telescopes onboard aircraft and rockets... or a free-flying, space telescope.
The first balloons, launched in the sixties, reached altitudes of more than 40 kilometres. Infrared telescopes, carried by aircraft, followed a
decade later. Both strategies provided the first infrared views of the centre of our Galaxy, but the detectors were still not completely lifted
above the atmosphere. To see the full infrared sky astronomers needed to reach even higher, using rockets.
This was far from simple. The first launch of an infrared telescope onboard an Atlas rocket as part of a US Airforce military test mission was
unsuccessful. An infrared detector working in space has to be cooled to very low temperatures so that its own infrared emission does not
outshine the astronomical objects. This cooling requirement is not so strong for ground-based observations because the emission from the
atmosphere is already stronger than that of the detectors themselves. In space, cooling is a key requirement that can be achieved by
using liquid nitrogen (-196°C) or liquid helium (-269°C) to cool the instruments. Unfortunately these fluids evaporate very quickly.
The liquid nitrogen for the first rocket-borne infrared telescope was meant to last only six hours, so the nitrogen tanks were filled shortly before
the scheduled launch time. Then the launch was delayed... by precisely six hours! When the rocket finally lifted off the temperature of the
detectors was already too high for them to function. Harwit was leading the project and recalls the moment as "heartbreaking".
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The Infrared Astronomical Satellite (IRAS)
Infrared Space Observatory (ISO)
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Although "pioneering rocket astronomy was not a happy venture" as Harwit says, subsequent flights went much better.
He remembers the thrills of the first succesful rocket flights when the first liquid helium cooled telescope revealed five distant sources near the centre of
our Milky Way, all shining more brightly in the infrared than had ever been imagined. Rockets could reach an altitude of about 250 kilometres, providing
at least five valuable minutes of observing time above the atmosphere. But, obviously, infrared pioneers dreamt about the long periods of observing
time - months, or even years - that a true space telescope could provide. They started to push for such a facility.
The Infrared Astronomical Satellite (IRAS) was undertaken as a joint project of the US, the UK and the Netherlands at about this time. Its main
goal was to perform a complete, sensitive survey of the whole sky at several infrared wavelengths.
Many astronomers doubted whether the technology was ready for such a challenge and building IRAS proved a complex undertaking. IRAS was
launched in 1983 and was a great success from the very beginning. The telescope itself was 57 cm in diameter, cooled to -269°C and operated for 11 months.
As a result of IRAS about 250,000 infrared objects were catalogued, many of them opening up whole new areas of research, and posing many
unexpected questions. What was the nature of the completely unknown population of galaxies, much brighter in the infrared than in the optical,
discovered by IRAS? How many other stars were encircled by discs of dust, such as the one IRAS detected around the star Vega?
It was clear that the new science of infrared astronomy was here to stay. Even before IRAS had been launched, the European Space Agency
(ESA) had started preliminary studies for a possible 'successor', which was approved shortly after IRAS' launch and was called ISO (Infrared Space Observatory).
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The Rho Ophiuchi Cloud.
ISO colour image taken at 6.75 and 15 microns. The scattered bright dots are newly-born stars. (ISO / ISOCAM)
The Horsehead Nebula.
ISO image at 7 and 15 microns. Much of the bright infrared radiation comes from carbon compounds (PAHs). (ISO / ISOCAM)
Click on the images
to view larger-sized ones
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In February 1983, more than one hundred astronomers and space scientists, mostly European,
gathered at a workshop in Scheveningen, in the Netherlands, to introduce the scientific community to the five different proposals
for ESA's next space mission. Only one of the five, which included ISO, was to be selected by ESA's Science Programme
Committee a few months later. The lobbying for each mission was intense.
Then, "chance played in ISO's favour", recalls Martin Kessler, ISO Project Scientist. "The launch of IRAS had to be postponed to January
1983, so that the meeting at Scheveningen coincided with the early orbit phase of IRAS. We, the ISO people, were sitting there, hoping
desperately that IRAS could prove that the infrared technology worked well. At one stage, one of the main scientists involved with IRAS
came in holding the results of the first IRAS scans across the galactic plane. He exclaimed 'The damn thing works!' or some such
thing. IRAS was clearly working exceptionally well! ISO would probably have been selected in any case, but it was a helpful coincidence".
Even during the development phase of IRAS a large number of European astronomers were thinking about a follow-up for the results
from this satellite. If the IRAS surveys were successful "it was clear that we had to go back and look in detail with a much higher resolution",
Kessler says. This was ISO's goal: to focus on and take a much 'closer' look at the most interesting regions seen by IRAS. The decision was
taken unanimously by ESA's experts, and gave Europe a major lead in the discipline of infrared astronomy.
However, ISO's selection as an ESA mission was controversial. Many thought that the technological challenges were too great for such an
expensive mission. Parts of ISO had to be cooled to temperatures close to absolute zero, not a trivial technological task. Especially considering
that the detectors and cooling system - the so-called 'cryogenics' - used on IRAS had been developed by the US part of the team, so there was
a lack of obvious European expertise in this area. The four instruments onboard ISO were to be developed by multinational teams with leaders
in France, Germany, the Netherlands and the United Kingdom. The satellite itself was built by an industrial consortium of 32, mostly European, companies.
Fortunately ISO exceeded expectations. When ISO was launched in November 1995 the mission life expectancy was only 20 months - dictated
by the need to keep the telescope and instruments cold. The cryostat, a huge thermos flask surrounding the telescope and its instruments,
was filled with 2300 litres of superfluid helium as a coolant. The rate at which this helium evaporated would determine the lifetime of the mission.
The original estimates turned out to be pessimistic and the satellite's working life stretched to more than 28 months.
ISO's 'eyes' did indeed see much deeper than IRAS. Some 600 teams from all over the world made observations with ISO during its operational
lifetime and many astronomers continue to use the ISO data made public through the ISO Archive. "There's general agreement that ISO worked
exceptionally well in all areas - operational, technical and, above all, scientific. It solved questions triggered by IRAS, and of course also left a
legacy of new mysteries for future infrared space missions to solve", summarises Kessler.
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IRAS mapped nearly all the sky in four broad infrared bands or "colours", revealing the existence of
several completely new classes of infrared objects and giving astronomers a detailed catalogue of interesting positions on the sky. ISO
looked at individual sources with much higher sensitivity and greater spatial and spectral resolution than IRAS to detail the individual
characteristics of the various targets. However, with a mirror size of 60 cm, ISO gave less sharp images and discerned less of the detailed
structure than would be possible with a larger telescope. Thus, astronomers at the University of Arizona proposed using the Hubble Space
Telescope, the telescope with the sharpest vision, to take near-infrared images of these newly discovered objects.
Hubble was not originally designed to make infrared observations, so Rodger Thompson (University of Arizona) and his team designed
a new instrument for Hubble, the "Near Infrared Camera and Multi-Object Spectrograph" (NICMOS). This instrument is a combined
camera and spectrograph and was attached to Hubble by astronauts during the Second Servicing Mission in 1997. To cool an infrared instrument
attached to Hubble to temperatures as low as -213°C is not an easy task. Originally, NICMOS was cooled by a big block of nitrogen ice,
which kept the instrument cold for almost two years before the ice evaporated. A new mechanical cooling device that does not need nitrogen ice -
a so-called cryo-cooler - will be attached to NICMOS during the next Servicing Mission.
The combination of Hubble's very high resolution and the ability of infrared light to penetrate dust clouds, opaque to visible light, has
allowed astronomers to study the nuclei of galaxies and dust-enshrouded star-forming regions and has led to the discovery of objects that
are most probably nascent planetary systems.
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Last update: 20 December 2000 |
