By Paul Rincon
Science reporter, BBC News, Llano de Chajnantor, Chile
Away in the distance, I can see a volcano spewing sulphurous fumes. Under foot, the earth is so dry few plants can survive in it.
The Alma array of 64 antennas will offer an unprecedented view
And here, at an altitude of 5,300m, the atmospheric pressure is halfway between sea level and the vacuum of space. The scarcity of oxygen means that visitors need to inhale regularly from canisters of O2.
Llano de Chajnantor, a plateau high in the Chilean Andes, is the sort of extreme environment where humans really shouldn't venture. But to astronomers, it is prime real estate.
Chajnantor has been chosen as the site for the Atacama Large Millimeter Array (Alma), a major telescope array that aims to illuminate one half of the Universe that has hitherto been shrouded in darkness.
It will be able to observe some of the first galaxies to form after the Big Bang, and catch planets in the act of forming around young stars.
"We expect to produce pictures of a comparable resolution to those you are used to seeing from space telescopes," says Alma project director, Dr Massimo Tarenghi.
The £472m ($900m) project is overseen by the European Southern Observatory (Eso), which is made up of 12 member states including the UK. When it is completed in 2012, Alma is expected to become the largest antenna array anywhere in the world.
New world view
Objects in the night sky emit many different kinds of light in addition to that which is visible to the naked eye. These include radio waves, infrared waves, X-rays and gamma rays. But most of the energy in the Universe is contained in the form of sub-millimetre waves.
The array will study distant objects for sub-millimetre waves, sometimes referred to as terahertz waves, with a crisp resolution no other telescope has been able to provide.
"Alma will open a new view of the Universe. We're looking at wavelengths 1,000 times longer than optical light," explains Dr John Richer, Alma project scientist from the University of Cambridge.
Chile's Chajnantor plateau: only for the foolhardy and astronomers
"The importance of terahertz astronomy has been recognised since the 1970s. But Alma is going to be the first major telescope which allows us to make high quality images at these wavelengths."
Building work at the site is well underway; a "pathfinder" dish called Apex was installed on the plateau in 2005.
Chajnantor provides the ideal conditions for sub-millimetre stargazing. First and foremost, it is incredibly dry. In this part of the desert, you notice how quickly the moisture drains from your mouth and lips if you don't keep hydrated. Even after retiring to bed early, you can wake up in the morning feeling like you've spent a night on the tiles.
Water vapour absorbs sub-millimetre waves, interfering with observations using the telescope. But if you collected all the water vapour above Chajnantor, it would form a pool just 1mm deep.
However, the sheer altitude at which the site is located makes this an extremely challenging project; one that is, in many respects, more reminiscent of a space mission than a venture in ground astronomy. "It is the highest construction site of its type in the world - that I'm aware of," says Jorg Eschwey, site development manager for Alma.
Vehicles used to construct the array will have oxygenised cabins, so that drivers don't make any "silly decisions". The early stages of high altitude sickness are characterised by erratic behaviour. And luckily for the Alma project team, prevailing winds send the ash from the nearby Lascar volcano over to Argentina.
Alma's prelude: the Apex "pathfinder" telescope
On our drive to the Alma site, the rolling, rock-strewn desert temporarily becomes a green oasis grazed by llamas. Then it switches just as swiftly back to the dusty, Mars-like scenery the Atacama is known for.
Our first pit-stop is a base camp of temporary buildings on the slopes of Chajnantor. Inside, it already feels like a permanent workplace, with offices, a canteen, a recreational centre with pool tables and a gym, and living quarters installed with satellite TV.
The high altitude medical centre has an ambulance so well equipped, the nearby town of San Pedro has asked to borrow it occasionally.
This site will eventually house Alma's control centre, known as the Operations Support Facility (OSF), which will be manned around the clock once the array is operational. Staff will remotely control the 64 antennas on the plateau above, pointing them at numerous targets in the sky each night.
At an altitude of 2,900m the OSF is high by most standards, but at a much more comfortable altitude for workers than the plateau.
"Get all your questions in there," John Richer advises, "before we go up the mountain."
The array site lies at about half the cruising altitude of a 747. Atmospheric pressure there is 50% that at sea level. After a blood pressure check-up at the OSF, we're issued with aerosol canisters that dispense pure oxygen. Our guides advise us to take two shots in quick succession, as required.
"If the wind blows your hat away, don't run after it because you will collapse," warns Massimo Tarenghi.
We drive up to the plateau in a white pick-up along a wide, dusty dirt track that will soon carry construction lorries to the top of the mountain and back. After another stop at 3,200m in order to acclimatise, we press on to reach the array site.
It's a bumpy drive to the very top. As soon as I get out of the truck, light-headedness kicks in. My body adjusts to this fairly quickly. But my legs feel a bit like they're walking on a bouncy castle, rather than gravel and rock.
There's a lot of trial and error involved in getting used to being at this altitude. I overcompensate a bit at first; treading slowly and surely on the irregular terrain, I feel perfectly comfortable. Then, after walking at a fast pace for some distance, my brain starts to feel like candy floss - my cue to slow down.
We carry on to the array site, where the 64 antennas will eventually be installed, a construction project that will require 12,000 tonnes of concrete for 197 foundations. Contractors will begin building three prototype antennas at the beginning of next year. Each antenna will weigh about 120 tonnes, with a dish measuring 12m across and a surface engineered to be accurate to within 20 microns.
The dishes are designed to swing round very fast, taking just 5-10 seconds to switch between different targets in the sky. And individual antennas can be re-positioned, giving the array a "zoom-lens" capability. Their "superconducting" receivers have to operate at just four degrees above absolute zero (-269C).
Alma's base camp is at the more comfortable altitude of 2,900m
A custom-built transporter with 28 wheels - itself weighing 136 tonnes - will heave the antennas up the mountain.
Once operational, Alma will be able to see very cold objects obscured by clouds of cosmic dust and gas that are much stronger at sub-millimetre wavelengths.
"It turns out roughly half of the stars that form in the Universe are obscured by dust. If you want to see the other half of the Universe you need these complementary measurements using terahertz telescopes," says Dr Richer.
It should be able to see past the dust and gas into so-called proto-planetary disks - sites where planets are forming around young stars. "We'd like to understand what a typical Solar System looks like and therefore what the planetary conditions are going to be like. Then the habitability becomes of interest," adds John Richer.
The nearby Lascar volcano is very much active
In addition to detecting so-called proto-planets, Alma will also discover many whole galaxies that appear very faint in optical and infrared light, but are bright at terahertz wavelengths. In fact, project scientists predict that each time Alma observes a new part of the sky, it will find a new galaxy every three minutes.
A key target will be those sub-millimetre galaxies that were the first to appear after the Big Bang. Analysing their spectral "fingerprints" should let scientists know how far away they are, and, by extension, how old they are.
By studying lots of faint galaxies - which have formed at different stages in cosmic history - astronomers should be able to build a better picture of how the Universe got from the Big Bang to its present state.
Project managers hope to begin the first science with Alma in 2008.