The lowdown on Herschel

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OK – so now you’re up to date with the current launch status. That’s all fine and well – but I’m sure you’re asking, what *is* Herschel, and what is it designed to do? Very good questions indeed – and we’ll try to answer them as best we can.

What is the Herschel Space Observatory?

The Herschel Space Observatory (formerly known as FIRST) is the fourth cornerstone mission in the European Space Agency (ESA) science programme. It will perform imaging photometry and spectroscopy in the far infrared and submillimetre part of the spectrum, covering approximately the 55-672 µm range.

Tell me more about the satellite itself.

The Herschel satellite is composed of three sections.

Herschel satellite overview

Herschel satellite overview

First is the telescope, which has a 3.5 m-diameter primary mirror protected by a sunshade.

The telescope focuses light onto three scientific instruments housed in a giant thermos flask, known as a cryostat. The cryostat contains liquid superfluid helium colder than –271°C to make the instruments as sensitive as possible. The instruments and the cryostat make up the second section – the payload module. The third element of the satellite is the service module below the payload module. It houses the instrument electronics and the components for making the satellite function, such as the hardware for communicating with Earth. The service module hosts the data processing and spacecraft control electronics, operating at ambient temperature.

Harvesting infrared light with the largest mirror Herschel’s primary mirror is the telescope’s light collector. It captures the light from astronomical objects and directs it towards a second, smaller mirror. This completes the focusing and sends the light to the instruments, where the light is detected and recorded by computer. The size of the primary mirror is the key to a telescope’s sensitivity: the bigger it is, the more light it collects and so the fainter the objects it sees. It also determines the telescope’s ability to distinguish fine details. The surface of the mirror is very important, too. It has to be precisely shaped and perfectly smooth, since the slightest roughness distorts the final image. Herschel’s mirror is the largest ever built for a space telescope. The mirror has to be very light (as do all satellite components); it has to withstand the extreme conditions of launch and the low temperatures of outer space; and any bump on its surface must be less than a thousandth of a millimetre high.

Why must the telescope be so cold?

An infrared detector must be cooled to an extremely low temperature so that it is not warmer than the radiation it is trying to measure. Because the temperature of some astronomical objects is close to absolute zero (–273.15°C or 0K), trying to observe them with a warmer instrument would be like trying to see a star against the glare of the midday Sun.

All three Herschel instruments will be cooled by the cryostat filled at launch with more than 2000 litres of superfluid helium kept colder than –271°C. Further cooling – down to 0.3K – is required for the SPIRE and PACS ‘bolometer’ detectors. The role of the cryostat is fundamental because it determines the lifetime of the observatory. The superfluid helium evaporates at a constant rate, gradually emptying the tank. When it has all gone, the temperature of the instruments will start to rise and Herschel will no longer be able to perform observations. However, the data that Herschel will have supplied will keep astronomers busy for decades.

What instruments will be on board Herschel?

A telescope’s instruments turn it from a mere light collector into a set of technological eyes. The instrument detectors form the retina, where the light from astronomical objects is really seen. The instruments detect and analyse the light in many different ways. Herschel carries three scientific instruments:

  • HIFI (Heterodyne Instrument for the Far Infrared), a high resolution spectrometer;
  • PACS (Photoconductor Array Camera and Spectrometer);
  • SPIRE (Spectral and Photometric Imaging REceiver), a camera/spectrometer.

These instruments were developed by nearly 40 institutes, mainly European but with American and Canadian participation.

The Heterodyne Instrument for the Far Infrared (HIFI)

The Heterodyne Instrument for the Far Infrared (HIFI)

The Heterodyne Instrument for the Far Infrared (HIFI) produces high-resolution spectra with thousands of frequencies simultaneously. It covers two bands, 480–1250 GHz and 1410–1910 GHz, and uses superconducting mixers as detectors. It was developed by a consortium led by SRON (Groningen, The Netherlands).

PACS (Photoconductor Array Camera and Spectrometer)

PACS (Photoconductor Array Camera and Spectrometer)

The Photoconductor Array Camera and Spectrometer (PACS) is an infrared camera and a spectrometer. It operates simultaneously in two wavelength bands, 60–130 and 130–210 micron, with bolometer and photoconductor array detectors. It was developed by a consortium led by MPE (Garching, Germany).

SPIRE (Spectral and Photometric Imaging REceiver)

SPIRE (Spectral and Photometric Imaging REceiver)

And finally, the Spectral and Photometric Imaging REceiver (SPIRE) is a camera and spectrometer. It provides broadband photometry simultaneously in three bands centred on 250, 350 and 500 micron. It was developed by a consortium led by the University of Wales (Cardiff, UK).

What makes Herschel unique? And why is Herschel’s mission so important?

Herschel is the only space facility dedicated to the submillimetre and far infrared part of the spectrum. Its vantage point in space provides several decisive advantages, including a low and stable background and full access to this part of the spectrum.

Herschel has the potential of discovering the earliest epoch proto-galaxies, revealing the cosmologically evolving AGN-starburst symbiosis, and unraveling the mechanisms involved in the formation of stars and planetary system bodies. The key science objectives emphasise specifically the formation of stars and galaxies, and the interrelation between the two, but also includes the physics of the interstellar medium, astrochemistry, and solar system studies.

Where will Herschel be launched with, and where will it be launched from? How long will Herschel observe for? Who can apply for time to observe and how can you do this?

Herschel will be placed in a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 ECA (shared with Planck) in April 2009, launched from Kourou, French Guiana. Once operational Herschel will offer a minimum of 3 years of routine observations; roughly 2/3 of the available observing time is open to the general astronomical community through a standard competitive proposal procedure.

A rather nice brochure on Herschel, and its prime science goals can be found here.

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So – we’re in for quite a mission, all in all, I think you’ll agree. If you have any questions – please, ask away! The Herschel team would be more than happy to answer!

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