Video: See animations of the telescope and launch
FROM an astronomer鈥檚 perspective, the universe is a pretty cool place. In fact it鈥檚 positively chilly, with more than half the energy emitted by normal matter coming from clouds of gas and dust too cold to radiate visible light 鈥� and which therefore cannot be seen with traditional telescopes. The same problem plagues observations of the chilled-out photons of the . Closer to home, cool, dark objects litter our immediate cosmic neighbourhood.
To obtain a complete picture of our surroundings, astronomers must resort to the equivalent of night-vision goggles or a thermal-imaging camera. These are specialised space telescopes that scan the sky at infrared and microwave wavelengths much longer than those of visible light, allowing them to spy out the faint traces of heat that dark bodies imprint on the sky.
On 29 April [Note: this date has been postponed to mid May since the time of writing], the (ESA) will launch two such instruments from its . Piggy-backing in the nose of a single Ariane V rocket will be the , the . Once in space, the two will go their separate ways: Planck to study in detail the cosmic microwave background, and Herschel to spy on the cool gas and dust clouds that are the nurseries of stars and galaxies.
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This second satellite will incorporate the largest space telescope ever launched, named after William Herschel, the German musician turned British astronomer who discovered infrared radiation in 1800. The size is needed to produce razor-sharp images: the precision of a telescope鈥檚 images decreases as the wavelength of the light it focuses gets longer, but can be boosted again by using a bigger mirror to scoop up more photons. Herschel鈥檚 huge, 3.5-metre-wide mirror will allow it to provide images of high quality even at the longest infrared wavelengths, and so bridge the gap between previous infrared space telescopes (see diagram) and ground-based telescopes that probe longer radio wavelengths.
Space telescopes are indispensable for scanning the infrared sky. Water vapour in our atmosphere absorbs virtually the entire infrared spectrum at wavelengths from 20 micrometres to a millimetre, cutting it off before it reaches the ground. What鈥檚 more, water emits infrared radiation of its own, completely fogging up our view. Herschel鈥檚 wavelength range 鈥� from 60 to 670 micrometres 鈥� spans the only radiation band yet to be surveyed. Apart from two narrow windows in the water-vapour fog around 350 and 450 micrometres, this light never reaches the ground.
Versatile observatory
The launch will position Herschel in a remote orbit about 1.5 million kilometres from Earth 鈥� about four times as far away as the moon, and far removed from the disruptive infrared radiation emanating from Earth. A sun shield will not only protect the telescope from solar radiation but also carry photovoltaic arrays to provide power for the spacecraft.
Operating an infrared telescope in space has certain collateral benefits: in this particular orbit, the telescope is naturally cooled to a temperature of just 80 kelvin, reducing its own infrared heat emissions which would otherwise swamp the sensitive instrumentation. The detectors themselves have to be cooled even further, though, to within just a couple of degrees of absolute zero using liquid helium. Herschel鈥檚 working lifetime will be limited to about three years by how long its liquid helium supply 鈥� a whopping 2200 litres 鈥� will last.
The infrared photons collected by Herschel鈥檚 mirror will be focused onto three instruments designed to respond to the 60 to 670-micrometre range. First, a high-resolution instrument called HIFI will use superconducting detectors to identify the atoms and molecules present in a particular region of the sky by the precise spectrum of light they emit. The Photodetector Array Camera and Spectrometer (PACS), meanwhile, will provide sharp, but less spectrally precise, images at the short-wavelength end of Herschel鈥檚 range, between 60 and 200 micrometres. A third instrument, the Spectral and Photometric Imaging Receiver (SPIRE), will do something similar for the band between 195 and 670 micrometres.
Together, these detectors should provide an enormously versatile space observatory 鈥� so what do we hope to learn from it?
Most generally, it should allow us to study the physics and chemistry of cool celestial objects at all scales: from the comets, asteroids and planets of our solar system to very distant galaxies that we see as they were forming, when the universe was just a tenth to a half of its present age.
In addition, Herschel will beam back unprecedentedly detailed maps of gas and dust in galaxies closer to us, as well as of interstellar clouds in our own galaxy. When stars and planets form within these clouds, they are too cool to produce much visible light, but at infrared wavelengths we should see the process in action (see illustration): from its beginnings in the gravitational collapse of regions of the clouds, past the ignition of young stars, through to the accumulation around forming stars of rings of debris from which planets are later built. Herschel鈥檚 high-resolution imaging capabilities should allow it to zoom in on regions of interest, in both our own galaxy and its neighbours, to study systematically what is going on and inform our ideas of how solar systems form.
Mature galaxies like our own Milky Way are giant collections of hundreds of billions of stars, but they started life in the early universe as more modest objects which grew through dramatic collisions that triggered enormous bouts of star formation. Peering beyond our neighbourhood, Herschel will be able to look back in time to see that happening, and perform the first census of star-forming galaxies throughout the universe at the epoch of peak star formation, about 3 billion years after the big bang. That will allow us to chart how stars formed and galaxies evolved throughout the history of the universe.
There will no doubt be an element of surprise in what emerges. After all, this is an entirely new wavelength band that we are exploring. For that reason and more, astronomers鈥� eyes will be fixed on the launch of Herschel and its microwave partner, Planck. Expectation is high that they will shed some exciting light on the universe鈥檚 dark places.
