Source of alien life forms intent on destroying the Earth or a future home for man - Mars has captured the imagination of astronomers and science fiction writers for centuries.
Since the 1960s, probes, orbiters and landers have been sent to discover more about the most Earth-like of planets in the solar system, specifically whether there is, or ever was, life on Mars.
The latest search for water on Mars ended in high-profile failure at the end of 1999. Nasa's Polar Lander was sent to look for ice in the south polar region but 10 minutes before it was expec ted to land it lost contac t w ith Ear th . Now it is the turn of the British.
The Beagle 2 lander, named after the ship that carried Charles Darwin on the voyage that lead to his Origin of Species theory, is being developed by a consortium of British universities and industrial partners. The lander is a key part of the European Space Agency's ECU150M (£94M) Mars Express project.This is due to launch in early June 2003 from the historic Baikonur Cosmodrome in Kazakhstan - the first man in space, Yuri Gagarin, was launched from here in 1961 - arriving in December that year.
The scientific aspects of Beagle 2's mission, which gained ESA approval in November last year, are being led by the Open University, with the University of Leicester as project manager.
Matra Marconi Space is the lead engineering contractor while Martin-Baker Aircraft is managing the entry, descent and landing system.
Mars Express will carry out comprehensive surveys of the Martian geology and atmosphere. It aims to discover evidence of liquid water (and therefore any indications of past life) on the red planet.
It has been known for some time that while the surface of Mars is barren, there is ice in the polar regions. Previous missions to the planet found water vapour in the atmosphere.
'The idea that there is no liquid water on Mars is wrong, 'says Colin Pillinger, professor of planetary sciences at the Open University and one of the driving forces behind Beagle 2.
Martian exploration has had a chequered history, Pillinger explains. Speculation that Mars was home to intelligent life was first sparked when what were believed to be canals were spotted on the planet's surface.
'Although astronomers knew as early as 1910 that this was not the case, the public were not convinced, 'he says.
The Mariner missions in the 1960s provided photographic proof that the features were in fact natural and evidence that there was once water on Mars.
Pillinger says: 'This was instrumental in turning off looking at Mars for signs of life', as dating by crater frequency showed that water could not have existed on the surface for some 3.5bn years.Either water was lost to space along w i th th e a tmo sph e re o r i t i s t r app ed a t d ep th .
However, the perceived benefits of going to Mars were revived with the Viking missions in 1976, which set out to search for signs of life and to give a detailed picture of the atmosphere.
Data gathered by the landers and orbiters confirmed that meteorites on Earth thought to be from Mars were indeed from the planet, says Pillinger. In the mid 1990s, scientists discovered 'microfossils' in one of the meteorites, and proposed that these may be evidence for past organic life on Mars. Carbonate deposits showed evidence of water percolating through the rock.
Debate still rages over whether this organic evidence is Martian in origin or if it was caused by terrestrial contamination after the meteorite's impact on Earth.
Pillinger believes isotopic differences between Martian and terrestrial water is evidence that the water percolation occurred on Mars.
The most recent meteorites showing evidence of water have been dated as forming between 180Ma and 360Ma and left the planet about 600,000 years ago. Water could have percolated through any time up to the moment they left, Pillinger says.
Mars Express will try to detect the presence ofwater below the planet's surface, using Beagle 2 and instruments mounted on the orbiter. The changing orbit of the spacecraft will allow detailed global photogeology of the surface at 10m resolution, mineralogical mapping of the surface at 100m resolution and mapping of subsurface structures using radar.
Instruments will also assess atmospheric composition and circulation to build up an accurate picture of the Martian climate.
Five days before Mars Express reaches Mars, Beagle 2 will be powered up and jettisoned from the orbiter. The lander's entry, descent and landing system is similar to that used for the Pathfinder mission in 1997.A heat shield will protect the lander as it slows in the Martian atmosphere. Parachutes will then be deployed and three gas-filled landing bags will inflate.These will cushion the impact and allow the lander to bounce to a stop. The bags will then detach and the 'clam-shell' lander will spring open and begin work, taking samples of soil and rock and analysing them using its onboard laboratory.
Two sampling methods will be used: a robot arm (dubbed Beagle 2's 'paw') and a mole, or 'pluto' - the planetary undersurface tool. The arm is mounted with a variety of sensors and has a grinder to remove dust and weathered rind and a corer that hammers into the ground and is capable of taking 10mm long, 2mm diameter cores, if there is rock close by.
It is hoped that understanding of Martian weathering processes will be increased by using the grinder, as samples can be analysed before and after grinding.
The arm also deploys the mole. This ingenious little tool was originally designed to take cores at up to 1m depth by hammering vertically into the Martian surface using a compressed spring mechanism but, as Pillinger explains, it was soon discovered that it could be used in a different way.'We suggested that the mole could crawl along the ground and take samples.'
This will be particularly useful, he says, as one of the landing sites, Chryse, was chosen because it shows evidence of large scale flooding. If there are any large boulders present, they were moved by the flood waters and not by the wind, giving the best chance of sampling unweathered and unoxidised material, as they will not have moved again since the flooding.
Limited only by the length of the cable attached to the arm (probably about 5m, including burrowing), the mole will crawl along at about 6m an hour.When it reaches a boulder it will begin to burrow beneath it, taking a sample in its jaw.Samples are retrieved by winding the cable back.
At the heart of Beagle 2 is the mass spectrometer that will analyse samples for the chemical signatures of biological processes.
Solid samples (soils or rock) will be heated at a range of temperatures in oxygen - any carbon present will burn to give carbon dioxide, the amount of gas generated being measured by the mass spectrometer. It can distinguish between organic and inorganic carbon and hence between biological remains and carbonate minerals.
Samples will also be analysed by an X-ray detector which will measure the amount of potassium and this will be compared with the amount of argon in the atmosphere (measured by the mass spectrometer) to calculate the amount of potassium that has decayed to argon to date the rock.
The X-ray detector will also detect major elements (magnesium, aluminium, silicon, sulfur, calcium, titanium, chromium, manganese and iron) to help identify rock types. A Mossbauer spectrometer will analyse the iron content and the degree of oxidisation.
As with any site investigation, a paramount objective is to obtain a representative sample of the area. Pillinger believes that Beagle 2 will obtain a representative sample.
'In hindsight all the lunar samples were, 'he says, although he admits that Mars'weathering processes are the main problem - and one not shared by the Moon. Evidence of organic matter and features and minerals related to flowing water must be looked at insitu, in areas protected from the harsh oxidising environment inside rocks and underneath boulders, before a full picture can be assembled.
Another problem is terrestrial contamination. Pillinger says exploration for life on Mars must be done before any manned missions land, as humans will bring a variety of micro organisms with them.
It will also be very difficult to keep samples clean if they are brought back to Earth, he says, because they would have to go through quarantine which would lead inevitably to terrestrial contamination. 'All the lunar samples were contaminated, 'he comments.
Even robotic explorers must be kept clean to ensure there is no forward contamination of the planet. Beagle 2 can only be launched if it has no more than 300 spores/m 2and a total of less than 3x10 5spores.
Beagle 2 will have up to four camera systems.A pair of cameras will be mounted on the robotic arm to give panoramic views of the landing site and to monitor sampling. A wide angle camera will be fixed on the lander to monitor the robot arm and a microscope will be fitted to the arm to examine fresh rock surfaces cleaned of weathered debris by the rock grinder. A miniature camera may also be used to take pictures of the surface during landing.
The lander will be bristling with environmental sensors, including a UV sensor, a hydrogen peroxide sensor, a radiation sensor, an accelerometer to measure atmospheric density and pressure, thermocouples, a pressure sensor, a wind gauge and dust impact detectors.
Beagle 2 is powered by a combination of batteries and solar panels and is expected to work on its primary mission for about 180 days, with an extended mission lasting up to two years (one Martian year) after landing.The Mars Express Orbiter is designed to operate for two years, beaming back the results to Earth.
Pillinger is confident that the mission will be a success and he believes that it can avoid problems experienced by earlier expeditions.
While development continues, the main problem is securing further funding for the lander. He says corporate sponsorship is needed, and he expects that this will start to come in as the launch date gets closer, in the way the Millennium Dome obtained a lot of its revenue in the last year - 'like a mini-dome', he says. Hopefully this is one British endeavour that the public will take to its heart.
www.beagle2.com - the official Beagle 2 website sci.esa.int/marsexpress - the European Space Agency's Mars Express website