With a landed mass of less than 30 kilograms (66 lbs), Beagle 2 represents the most ambitious mass ratio for a science payload-to-system combination ever attempted.

Almost a third of the payload will carry out various types of analysis or be used to manipulate and collect samples for study on the surface of Mars. The probe's primary mission life is 180 Martian days (Sols; 1 Martian year equals 687 Earth Days). An extended mission could go on for three times that initial duration.

The lander destined for Mars will carry the instruments necessary to search for the signatures of life; it's name Beagle 2 commemorates Charles Darwin's ship and the voyage which he called "the most important event in my life". The project seeks in situ evidence of organic matter, associated with features and minerals related to flowing water.

In choosing the Beagle 2 landing site, special attention was given to finding locations where the harsh oxidizing environment has been prevented from destroying the evidence. Such a place will be within the interiors of rocks or in the soil shielded by large boulders.

Springtime on Mars
Beagle 2's target for landing on Mars will be a region which shows evidence of having been inundated by large volumes of water. The landing site is centered near 11oN, 270oW, in eastern Isidis Planitia. The name, Isidis Planitia, refers to the broad, relatively flat plain that covers the floor of an extremely ancient, large basin formed by an asteroid or comet impact perhaps more than 4 billion years ago.

As the third largest impact basin, its floor exhibits chains of pitted ridges, numerous smaller meteor impact craters, and a variety of light-toned ripples and small dunes. The number of rocks on the surface seems to be about right - not too many to threaten a safe landing, but enough to provide an interesting landscape for the experiments.

The region seems to be a sedimentary basin where traces of life could have been preserved. Other criteria than the likely presence of ancient water are the altitude and latitude on Mars.

Too high and there is insufficient atmospheric density to allow the Beagle 2 parachutes to slow the lander for a safe touch down. Both height and also too far north or south away from the equator will increase the need for additional thermal protection as the climate worsens.

The northern hemisphere was a favored destination because in the martian lowlands the planet will be coming from spring into summer during the time Beagle is working in 2004.

"This is the best site given the landing constraints and scientific aims of Beagle 2," said John Bridges from the Natural History Museum, London, who has been assessing several sites on behalf of the project.

Controlled collision
Like the mission plan for the 1997 Mars Pathfinder, the critical landing of Beagle 2 features a set of steps compared to a controlled collision. When the lander first encounters the Martian atmosphere, it will be travelling at more than 20,000 km/hr (12,000 mph).

Fortunately the probe has a clamshell protective armor, and also parachutes and airbags to cushion its rapid impact. The clam-like structure of Beagle 2 not only has to support all the elements required to carry out the scientific mission on the surface of Mars but must also ensure they survive the final impact of landing.

When Beagle 2 drops onto Mars out of its cocoon of gas-filled bags, the impact could be in any orientation with a contact area varying from a small, sharp rock to a large, flat surface. The design case is equivalent to pushing your PC off a chair (on Earth) onto a concrete floor and expecting it still to work.

Unfortunately Beagle 2 does not have the full choice of materials available to your PC manufacturer because it has to be as light as possible to maximize the payload inside the lander and to keep the systems warm through the martian night when temperatures down to -70oC will be reached.

One energy saving protocol was to carry out those experiments which generate heat within the lander during the night so that the energy dissipated helps to keep the lander warm - a martian off-peak heating strategy.

The landing solution chosen was to use an outer shell to absorb impact energy and also thermal insulation within a casing that must spread the impact loads and resist tearing. An inner shell comprising carbon-fiber skins on an aluminum honeycomb core makes up the primary structure.

All materials have to be compatible with more than the usual space environment (vacuum and radiation exposure, low outgassing, low mass overall, low cost) in that they have to face the sterilization process necessary for planetary protection requirements.

The two halves of the lander, lid and base are joined by a spring loaded hinge which irrespective of the orientation which Beagle 2 finds itself on Mars will open the clam in its correct position.

When Beagle 2 has landed, its precise position will need to be measured. One way of doing this will be to time the movement of the shadow of Mars's tiny moon, Phobos, as it passes over the lander during a partial eclipse of the Sun.

"In February 2004, the Sun will have a partial eclipse by Phobos. By timing the eclipse, we will be able to pinpoint the lander's position quite accurately," said Tom Duxbury from the Jet Propulsion Laboratory, Pasadena, California, who is helping to characterize the landing site using Mars Global Surveyor data.

Science on the surface
The lander's science targets center on the search for past life, including:

• presence of water
• existence of carbonate minerals
• occurrence of organic residues
• complexity and structure of organic material
• isotopic fractionation between organic and inorganic phases.

The hoped-for results are based on a plan to:

• Seek trace atmospheric species indicative of extant life.
• Measure the detailed atmospheric composition to establish the geological history of the planet and to document the processes involved in seasonal climatic changes or diurnal cycling.
• Investigate the oxidative state of the martian surface, rock interiors and beneath boulders.
• Examine the geological nature of the rocks, their chemistry, mineralogy, petrology and age.
• Characterize the geomorphology of the landing site.
• Appraise the environmental conditions including temperature, pressure, wind speed, UV flux, oxidation potential, dust environment etc.

Seeking all carbon-based lifeforms
In looking for past life, the element carbon is one starting point. So Beagle 2 will heat samples of soil or rock in oxygen. This is equivalent to burning, since all forms of carbon will convert to carbon dioxide via combustion or decompose to a rocky residue (carbonates). The organic matter, if any, will combust.

Landing ellipses centered on Mars' third largest impact basis Credit: NASA JPL/MSSS/MOC Most important for Beagle 2 is a stepped combustion method--a ramp to higher, then lower temperatures in stages-- to resolve between organic and inorganic types of carbon.

This should distinguish between possible microbial remains and carbonate minerals. The residue from fossils is enriched in the light carbon, 12C, than what is expected from mineral or ancient sediments alone.

Biological processes preferentially use the lighter of two stable isotopes of carbon 12C and 13C. The ratio of the two stable isotopes (isotope scientists use the term delta 13C to describe differences in the ratio) can give a clue to the type of life processes (e.g. photosynthesis to convert light to energy, methanogenesis to convert carbon dioxide to methane) which have resulted in the fractionation.

Since strong chemical reactions quickly destroy (oxidize) methane at the Martian surface, if methane is found today, there must be some replenishment that gives a clue to active biology. Biosynthesis leaves a ubiquitous signature of life even in specimens where there are no fossils visible.

If evidence for life is below the exposed surface of Mars, then Beagle 2 will try to dig using a mole-like sample retriever. It is expected that the mole will be able to crawl up to three meters (around 9 feet) away from the lander, including the burrowing phase.

Samples will be collected using the mole from below the harsh oxidizing surface, the best location being a region additionally protected by a boulder large enough not to have been disturbed since being emplaced.

What's Next Mars Express will be launched by a Soyuz-Fregat launcher from the Baikonur Cosmodrome in Kazakhstan in June 2003. Customs formalities were recently cleared in Moscow and the lander arrived at the airport in Baikonur on March 20th.

The Beagle 2 Probe was then transported by rail to the spacecraft assembly complex to be mated once more with the Mars Express spacecraft and prepared for launch. Once launched, the spacecraft will take six months to reach the Red Planet.

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