Page last updated at 01:17 GMT, Monday, 9 February 2009

'Silver sensation' seeks cold cosmos

By Jonathan Amos
Science reporter, BBC News


The Herschel space observatory is ready for its great voyage

Stare into the curve of Herschel's mirror too long and you get a slightly giddy feeling that comes from not being able to judge where its surface really starts.

It is enchanting, spectacular and - at 3.5m in diameter - it will soon become the biggest telescope mirror in space, surpassing that of Hubble.

The great 18th Century astronomer William Herschel would have been astonished by the silver sensation that now bears his name.

The design keeps Herschel's critical detectors in an ultra-cold state

The European Space Agency (Esa) is certainly very proud of its new observatory. It has been working on the venture for more than 20 years.

"The mirror is an enormous piece of hardware," enthused Thomas Passvogel, Esa's programme manager on the Herschel space observatory.

"It's a ceramic mirror; it's the biggest piece ever made from silicon carbide. It's very hard but much, much lighter than glass and the performance is excellent."

This week, the finished observatory will be flown to Europe's Kourou spaceport in South America. There, it will be bolted to an Ariane rocket and hurled into orbit.

It will take up a vantage point a million-and-a-half kilometres from Earth, to open up what scientists expect to be an utterly fascinating new vista on the Universe.

"Very simply, the science pillars of Herschel are to understand better how stars and galaxies form and how they evolve," Göran Pilbratt, Esa's project scientist on Herschel, told BBC News.

Herschel (BBC)
Hubble has viewed some near-infrared wavelengths. Its "successor", James Webb (2013), will seek infrared light also but with an even bigger mirror

Unlike Hubble, which is tuned to see the cosmos in the same light that is visible to our eyes, Herschel will go after much longer wavelength radiation - in the far-infrared and sub-millimetre range.

It will permit Herschel to see past the dust that scatters Hubble's visible wavelengths, and to gaze at really cold places and objects in the Universe - from the birthing clouds of new stars to the icy comets that live far out in the Solar System.

Some of these targets, though, are frigid in the extreme (between five and 50K; or -268 to -223C); and for Herschel to register them requires an even colder state be achieved on the observatory itself.

This involves the use of a cryostat. It is akin to a giant "thermos" bottle. Filled with more than 2,000 litres of liquid helium, its systems will plunge Herschel's science instruments into the deepest of chills.

Critical detectors will be taken to just fractions of a degree above absolute zero (0K; -273C), from where they can make the most of their remarkable design performance.

"Imagine one million, million, millionth of the brightness of a 60W lightbulb - that's what we can detect with one of our detectors," explained Professor Matt Griffin, who leads the international consortium behind SPIRE (Spectral and Photometric Imaging Receiver), one of Herschel's three instruments.

"Turning that around - imagine observing one of our very faint sources; let's say a very distant galaxy. If we were to observe it with SPIRE for a billion years, we would collect enough energy to light that 60W lightbulb for just one-twentieth of a second," the Cardiff University, UK, researcher said.

Herschel's other instruments are HIFI (the Heterodyne Instrument for the Far Infrared) and PACS (Photodetector Array Camera and Spectrometer).

With the entire package, the observatory can investigate a broad range of wavelengths (55-672 microns), including a swathe that has hitherto been missed by orbiting telescopes.

Eagle Nebula at different wavelengths (Nasa/Esa)
The classic "Pillars of Creation", great columns of gas and dust. Viewing the star-forming region at progressively longer wavelengths opens up new features
(A) Visible light: Reflected light from the nebula is seen (0.5µm)
(B) Near-infrared: Nebula suddenly becomes transparent (1-2µm)
(C) Even longer: Possible to see emission from the nebula itself (7µm)
(D) Longer still: Different structures start to become apparent (50µm)

Herschel's interest will be piqued near and far.

Close to home, it will study the mountainous balls of ice, dust and rock (some of them comets) that orbit our Sun beyond Neptune. The nature of these "primitive" objects has an important bearing on the story of how our Solar System came into being.

And beyond our little corner of space, Herschel's vision will allow it to see inside the clouds of gas and dust that give rise to stars in the Milky Way galaxy today, to see the conditions "in the womb". Studying these embryonic events will give astronomers further insights into the Solar System's beginnings 4.5 billion years ago.

Herschel instruments (Esa)
Once the liquid helium boils off, Herschel's instruments will go blind

Another key target for Herschel's investigations will be those galaxies that thrived when the Universe was roughly a half to a fifth of its present age. It is a period in cosmic history when it is thought star formation was at its most prolific.

Herschel will need to look deep into space to make these observations. The data will be used by scientists to test their models of how and when the galaxies formed their stars and how successive generations of those stars produced the abundance of heavy elements (everything heavier than hydrogen and helium) that now exist in the Universe.

Professor Griffin summed up the Esa mission in this way: "Herschel is not about studying mature stars or galaxies; it is really about studying the processes by which they are created.

"We know very little about that and we need to understand it in order to put together a picture of how the Universe we live in today grew from the earliest stages after the Big Bang."

A double deal

Herschel's launch will be doubly significant because it sees Esa loft two major science missions on a single rocket. The other passenger on the Ariane will be the Planck telescope, which will look at even longer wavelength (microwave) radiation.

Planck (Esa)
Herschel will share its ride with the Planck telescope

One reason for the dual launch, says Esa's head of science projects, Jacques Louet, is logistics. Both telescopes have been designed to operate at the so-called Lagrange Point 2, a gravitational "sweet spot" in space where the observatories can stay fixed in the same location relative to the Earth and the Sun.

"The other reason is that we have coupled them industrially," he told BBC News.

"Both spacecraft share the same service module, so there is an economy in building them together. And because you build them together, you have basically the same timing on each mission. So, overall, I think it is a good strategy, but a risky strategy."

At a combined value for Hershel and Planck of approximately 1.7 billion euros, you get an idea of just how risky this strategy is. If the rocket fails, both missions are lost.

One is tempted to say "good luck"; but as Göran Pilbratt points out, when you have put as much effort into these missions as Esa has over the past 20 years, "luck doesn't come into it".

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