With the Herschel Space Observatory believed to have entered its final year of operation, we take a look back at the telescope’s short life.

The European Space Agency’s Herschel Space Observatory was launched in May 2009 and is currently expected to remain operational for around another year.

Compared to the Hubble Space Telescope, which launched in 1990 and is still producing useful science, it seems like a short lifespan – especially when compared to the 17 years it took to plan and build. But thanks the particular challenges of the infrared astronomy that Herschel is designed to do, it was always destined to be short-lived.

Any object with a temperature will emit radiation according to the blackbody law. At the temperatures commonly encountered on Earth, much of this radiation is at infrared wavelengths – this is picked up by the Observatory’s imaging equipment and interferes with the picture being taken, rather like trying to take a photograph in too-bright light.

Although there is less of this background infrared radiation in space, the telescope’s images can still be compromised by the heat given off from its own instruments. They therefore have to be cooled.

Herschel therefore carries more than 2000 litres of liquid helium on board to cool the telescope’s instruments down to a temperature only fractionally above that of the cosmic microwave background. In addition, it is in orbit around the Sun at the Sun-Earth system’s second Lagrangian point, where it maintains a constant position with respect to both bodies and where, sitting in the Earth’s shadow, is shaded from most of the Sun’s radiation. As carrying the coolant out to this position, 1.5m km from Earth, adds to the fuel cost, there is a necessary trade-off between the cost of the mission and its duration.

The Observatory carries three types of instrument on board. There are spectrometers to determine the chemical composition of celestial phenomena from their spectral lines, and two kinds of imaging sensor – bolometers, which determine the intensity of incoming radiation from the change in their material’s electrical resistance as they heat up; and photometers, which detect incoming photons directly.

By the spring of 2013, Herschel will have been in operation for four years, six months longer than originally conceived.

It has spent that time studying various phenomena that are interesting at far-infrared and submillimetre wavelengths: principally the formation of galaxies and stars. Both of these involve large amounts of (initially) cold gas and dust, which cannot be seen at visible wavelengths but which are a lot brighter further down the electromagnetic spectrum.

Its spectrometers are also being used to research the chemical composition of stars and planets, and of the dust in interstellar space. Discoveries made include the first detection of molecular oxygen in space, and the presence of water vapour around a nearby star.

But aside from its own discoveries, it is expected that Herschel will pave the way for future missions by helping to identify celestial bodies to focus particular attention on. For one of these future missions, ESA, along with the Canadian Space Agency, is contributing to NASA’s James Webb Space Telescope, also designed to make observations at infrared wavelengths, which is expected to launch in 2018. It will partly pick up where Herschel left off, ensuring a valuable legacy – despite its short lifespan.

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