This Month's Article
by Doug Robertson
Ex-CAP Chap Tries to Land Spacecraft on Mars
Beagle 2 is the Lander part of the European Space Agency (ESA)'s mission to Mars in 2003. Beagle 2 is a micro (less than 30 kg) geo-chemical laboratory being placed onto the surface of Mars to search for signs of life, both past and present, by collecting and analysing samples of rocks, soil and the atmosphere. The relative positions of Earth and Mars later this year give rise to a launch window, lasting from May 23rd to June 28th, which gives the shortest journey from Earth to Mars in terms of distance (only 250 million miles!) and time (about six-and-a-half months). The chosen launch date is June 2nd, although technical hitches and/or adverse weather conditions could delay this slightly. This launch date equates to a predicted landing date in the second half of December. Whatever the exact date on which the unmanned Beagle 2 Lander attempts its landing on Mars, there will be an ex-CAP chap at the controls (so to speak!).
Beagle 2 will be carried to Mars by the Mars Express spacecraft which will itself be launched by a Soyuz/Fregat rocket from the Baikonur Cosmodrome in Kazakhstan. Beagle 2 is a British-led project and (non-American) Readers will be pleased to know that, in the race to put the next spacecraft onto the surface of Mars, the British Beagle 2 Lander will beat the American Athena Rover by a short period (or should that be a short head?).
A number of Readers will undoubtedly know that "On the Origin of Species" was written by Charles Darwin. Probably, far fewer of those Readers will know that when he set out in 1831 for Tierra del Fuego it was in a ship named HMS Beagle. Whilst not searching for life forms, he was searching for an explanation of way in which the multitude of species found on Earth could have arisen. How natural then, when a Lander was to be sent to search for other (non-terrestrial) life forms in our solar system, that it should be named Beagle 2 in recognition of his pioneering work all those years ago.
In the 1970's, NASA thought that it had answered the question as to whether there was life on Mars with a definitive 'No'. This was on the basis that only one out of a great number of experiments had given an (apparently) positive result and this was interpreted as being due to a chemical reaction and thus discounted. This brought space exploration of Mars to a halt but studies of Mars received a boost when it was realised that we already had rocks from Mars.
Meteorite EETA 79001, found in Antarctica, was proved to be of Martian origin by analysis of the gases trapped in glass inclusions which matched, like a fingerprint, the analysis of the Martian atmosphere performed by NASA's Viking spacecraft. In fact, the match is much better than a fingerprint test and when oxygen isotope data is included becomes more like a type of meteorite DNA test. Sub-tests for the amounts of carbon dioxide, nitrogen, neon, argon, krypton and xenon, plus sub-tests for the relative isotopic ratios of oxygen and nitrogen, all matched to one part in ten thousand million
Other Martian meteorites have given different evidence about the planet. One meteorite, found in Egypt, was the first to provide evidence that Martian carbonates (minerals deposited in water) existed. Meteorite ALH 84001, also found in Antarctica, also showed the presence of carbonates but in this case it was also possible to determine that they had been deposited at temperatures between 10 ºC and 90 ºC--the temperature range in which life on Earth thrives..
|When NASA scientists announced in August 1996 that they had found evidence of fossilised bacteria in ALH 84001, it caused unprecedented scientific, public and media interest in Mars. Overnight Mars shot to the top of the league in terms of importance in the search for life in our solar system. Martian meteorites have been found to contain carbonates and organic material. Now, all living things leave behind evidence of their existence in the form of organic compounds and/or chemical fossils, even long after visible fossil traces have disappeared. Like the fossil evidence, the evidence of the organic material is controversial as there is no way to conclusively prove that it was present in the meteorite before it reached Earth. However, the suggested mechanisms for how the organic matter could have entered the meteorite after reaching Earth seem equally improbable.|
The only way to resolve this is to go to Mars and look for 'in situ' evidence of organic material and minerals associated with flowing water. NASA has pictures which show features attributable to water activity so water must have been present on Mars at some point in the past.
As the Martian environment is a harsh, oxidising one, exposed organic material would be rapidly destroyed. Therefore, evidence of organic material is best sought in protected locations such as the interiors of rocks and buried in soil shielded by large boulders.
When, in 1997, ESA announced a mission to Mars, named Mars Express, to be launched in 2003, the Open University (OU)'s professor, Colin Pillinger, immediately put forward the idea of including a Mars Lander as part of the mission. This Lander would be dedicated to looking for signs of life by performing geo-chemical and atmospheric analyses.
For superstitious Readers there are a number of uncanny parallels between the voyages of HMS Beagle and the Beagle2 Lander.
When Charles Darwin first told his father about his proposed voyage, the response was, "It is a wild scheme and no good will come of it.". When professor Colin Pillinger first told ESA of his idea for a Lander as part of the Mars Express mission, the response was, "It is a wild scheme and quite impossible as there is no mass budget for Mars Express to carry anything.".
Darwin's destination of Tierra del Fuego was described as 'desolate, depressing, sterile and repulsive'. Beagle 2's destination of Mars has been described in similar terms and is equally, if not even more, inhospitable. After Darwin was accepted to go on the HMS Beagle voyage, it was found that there wasn't really enough room, with the result that the ship's padre had to be left behind. After Beagle 2 was accepted to go on Mars Express with an assigned mass budget of 108 kg, it was found that Mars Express could really carry only 60 kg of extra mass, with the result that 48 kg of instruments had to be left behind.
When Darwin, a tall man, was assigned to his small cabin, a drawer at the end of his bed had to be removed to make room for his feet because the bunk was too short. When the Beagle 2 project was informed of the overall dimensions into which the Lander had to fit, the robot arm, the longest instrument, would not fit and Beagle 2's interior layout had to be redesigned in order to fit the robot arm in.
HMS Beagle's voyage was successful in that the observational evidence that Darwin obtained allowed him to formulate his theory of how species originated in his book "On the Origin of Species", which revolutionised our idea of the evolution of life on Earth. Is this an omen for Beagle 2's voyage being equally successful in obtaining analysis results proving the existence of life on another planet, which will revolutionise our idea of the evolution of life in the solar system?
3. Science Mission
Beagle 2 is the smallest (less than 30 kg), most heavily instrumented, soft-landing spacecraft ever produced, being the most ambitious science payload to system mass ratio ever attempted. The science mission has several objectives but only the following two are related to the search for life:
(a) to look for evidence of past life on Mars;
(b) to look for trace atmospheric chemicals indicative of present life.
3.1 Gas Analysis Package
The most important instrument in this search for life is the Gas Analysis Package (GAP), which contains a mass spectrometer to analyse samples for evidence of the chemical signatures of biological processes. This is achieved by the temperature-stepped combustion of soil and rock samples, when all carbon compounds will be converted to carbon dioxide either by combustion or decomposition, leading to a determination of the total amount of carbon present. The ratio of the relative abundance of the two stable isotopes of carbon will also be determined and will give clear evidence of the presence or absence of biological processes. The GAP can also be used to detect traces of methane in atmospheric samples.
The ratio of the relative abundance of the two stable isotopes of carbon will distinguish between organic carbon (microbial remains) and inorganic carbon (carbonate minerals). Traces of methane in atmospheric samples will be strong evidence for present life forms, possibly deep below the surface. Methane is rapidly destroyed on the Martian surface by the harsh, oxidising environment. Methane produced in the past will not now be present so the detection of traces of methane would be evidence of a continuing, present-day source of supply for which biological activity would be far and away the most likely cause. Atmospheric circulation will enable the detection of microbes up to 1,000 km away.
On Earth, microbes produce methane in cows' stomachs, termites, anaerobic paddy fields, peat bogs and landfill sites (yes, detection of the little green men's local council's rubbish tip is eminently feasible!). Biological processes preferentially use the lighter 12C carbon isotope rather than the heavier 13C carbon isotope. Non-biological processes also preferentially use the lighter 12C carbon isotope but the difference is much smaller than for biological processes as processes such as precipitation are much simpler. Therefore, it is possible to distinguish between biological processes and non-biological processes and even between different biological processes such as photosynthesis and methanogenesis.
This characteristic signature remains long after the organisms have died and even after all visible signs of fossils have been obliterated. This technique has been used to show that life started on Earth four thousand million years ago.
Samples for analysis will be collected by the Mole (see below) from below the surface and, ideally, from below a boulder. Other protected samples will be collected by the Corer (also see below) by grinding / drilling into large rocks.
A crawling Mole is used to gather sub-surface soil samples and return them to the Lander's analytical laboratory. The Mole thus provides an element of mobility to the stationary Lander. The Mole can crawl across the Martian surface at about 10 cm per minute, collecting a sample in a cavity in its tip which opens when the Mole reaches a sampling location. In addition to its horizontal movement, the Mole can burrow into the ground and under a boulder, 1 mm at a time. In total, the Mole can crawl about 3 m away from the Lander, including the distance travelled during the burrowing phase.
To overcome the problems of a naturally occurring dust layer and an obscuring weathered rind on rocks, Beagle 2 is equipped with a Grinder and Corer. Those Readers who have tried D.I.Y. jobs using a hammer-head drill will know that when drilling into a hard surface the drill bit can tend to wander over the surface before it bites in. This effect is utilised by the Beagle 2 rock drill which is initially allowed to wander over small area of the selected surface to clean off the weathered rind.
When this translational motion is stopped, the drill bit bites into the rock, penetrating up to 1 cm. The bit is split and hollow, trapping a sample inside. Reversing the drilling action removes the 1 cm long, 1 mm radius, cylindrical core sample weighing about 60 mg.
Drilling to these dimensions may have reminded some Readers of having a tooth filled so, to those Readers, it will come as no surprise to learn that the design of this drill was, indeed, supplied by a dentist!
The sample size is important and 60 mg is enough to allow the detection of carbon at the sub-parts-per-thousand-million level.
3.4 Thermal Control
The temperature of the Lander needs to be maintained within the range compatible with the proper functioning of its instruments during all stages of the mission. During a Martian night, temperatures can plunge to below -70 ºC.
One operating procedure designed to conserve battery power is to run those experiments which generate heat during the night so that the energy released helps to keep the Lander warm. If the Reader thought that off-peak heating plans were only for houses on Earth, then this Martian equivalent might come as a surprise!
4.1 Landing Site
The landing site has been chosen with great care. The chosen landing site, and others that were considered, all show evidence of fluvial processing by large volumes of water at some time in the past.
Other important criteria were altitude and latitude. Too great an altitude and the parachutes wouldn't be able to slow Beagle 2 down sufficiently for a safe landing. Too far from the equator would use up too much battery power just keeping Beagle 2 warm enough to function, as the climate rapidly becomes colder the further away from the equator it lands. In this respect the northern hemisphere is better than the southern, as the northern Martian lowlands will be going from spring into summer during 2004 while Beagle 2 is carrying out its scientific experiments.
Yet other criteria were terrain slope and presence of boulders. Too great a slope would cause problems with stability and with the effectiveness of the solar panels. Some rocks are needed for the success of the scientific mission so too few rocks would put this in jeopardy but on the other hand too many (sharp) rocks would risk rupturing the gas-filled landing bags and thus preventing a safe landing from occurring.
It was finally decided that Beagle 2 will land in the Isidis Planitia basin, a 1,000 miles diameter bay at longitude 90 ºE and latitude 10.6 ºN.
Depending on the exact angle at which Beagle 2 enters the Martian atmosphere (between 15º and 20º), the target area, assuming a 3-sigma error, is 200 to 310 miles long by 60 miles wide--about the area of southern England below the M4 motorway!
4.2 Landing Sequence
Beagle 2 has no propulsion system of its own which is why it hitches a taxi ride to Mars on the Mars Express spacecraft, which will then go into orbit around Mars. Before that manoeuvre, Mars Express will power up the Beagle 2 batteries and eject Beagle 2 leaving it on an atmospheric entry trajectory. After this separation there is no comms capability until after landing so Beagle 2 is on its own as regards stability, power, thermal control and the entry, descent and landing sequence, which includes parachute braking and inflation of the gas-filled landing bags.
Five days before Mars Express reaches Mars, Beagle 2 will be separated from the mother craft by the Spin-Up and Eject Mechanism (SUEM). This is a critical point for both halves of the project. If separation does not occur, Beagle 2 cannot land still attached to Mars Express but also Mars Express cannot enter the correct orbit to map the surface of Mars carrying extra, unexpected weight.
The Entry, Descent and Landing System (EDLS) consists of three sub-systems. The Entry Sub-system consists of front shield / aero-shell, back cover / bio-shield and release mechanisms. This sub-system protects the Lander from contamination on Earth, in the space environment during the six-and-a-half month journey to Mars and during entry into the Martian atmosphere at Mach 31.5 (over 14,000 m.p.h.).
The Descent sub-system consists of a mortar, pilot 'chute, main 'chute and main 'chute release mechanism. The Landing sub-system consists of a gas generator and three gas bags which are filled to cushion the initial impact with the Martian surface.
Starting from over 14,000 m.p.h.; applying the emergency braking system too late to avoid a collision at over 60 m.p.h.; cart-wheeling, bouncing and rolling; and finally coming to rest, possibly upside down, is not a sequence likely to cause an M.o.T. examiner to give an L-test driver a 'pass' certificate but it is the sequence planned to give a successful Beagle 2 landing!
After Beagle 2 has separated from Mars Express and a few hours before reaching the edge of the Martian atmosphere, a hardware timer will activate the EDLS computer software program. This program starts monitoring the deceleration of the Lander to detect the point at which the pilot 'chute can safely be deployed.
During the first part of the descent phase, the heat shield slows the Lander from its inter-planetary velocity (Mach 31.5) to Mach 2.5, energy being lost by frictional heating.
At the correct point (Mach 2.5) the EDLS program will fire the mortar through a patch in the back cover thus deploying the pilot 'chute and will explode the pyrotechnic bolts thus severing the heat shield connections. Timing is critical for, if the mortar is fired a second or two too early, the main 'chute will be torn off the Lander which will then crash into the surface of Mars. On the other hand, if the mortar is fired a second or two too late, there will be insufficient time for the parachute braking affect to slow the Lander down sufficiently for a safe landing and the scientific instruments will not survive the initial impact with the Martian surface.
The pilot 'chute causes slight braking (thus dividing the Lander into two sections as the freed heat shield drops away) and pulls the main 'chute out from its stowed position to provide the main braking. The EDLS program then determines the correct point to cause the gas generator to inflate the three gas bags. These must be fully inflated before the initial impact with the Martian surface so that the scientific instruments have sufficient cushioning to withstand the impact.
The EDLS program then has to correctly determine the initial impact with the Martian surface and immediately release the main 'chute so that the Lander can bounce away free from underneath the canopy. The main 'chute cannot be released early as every fraction of a second's braking effect is needed to slow the Lander down sufficiently for the scientific instruments to survive the initial impact with the Martian surface. Neither can it be released too late as, because the Lander will hit the surface at an angle of between 15º and 20º, the Lander will cart-wheel after initial surface impact and, if the main 'chute is still attached, it will become wrapped round the Lander thus preventing the release of the gas bags and thus preventing the deployment of the solar panels so Beagle 2 will quickly freeze to death.
The EDLS program then has to correctly determine when the Lander has stopped cart-wheeling, bouncing and rolling, and cut the lace holding the three gas bags pressed onto the Lander, allowing them to reform into their spherical shapes and roll away thus dropping Beagle 2 onto the surface of Mars. Whichever way up it falls, the clam-shell lid opens through 180º. It is hoped that the lighter clam-shell lid will end up on top and can open up, over and off the heavier clam-shell base. However, if the clam-shell lid ends up underneath, the opening mechanism is powerful enough to open the heavier clam-shell base through 180º up, over and off the lighter clam-shell lid. Thus either way the Lander is now free to deploy the other solar panels, to begin communicating again and to commence the science mission.
5. Look Out For ...
(a) The Beagle 2 launch, scheduled for June 2nd 2003, although technical hitches and/or adverse weather conditions could delay this slightly. Should be covered on T.V. news and in the newspapers.
(b) The Beagle 2 landing, predicted to be in the second half of December 2003. The exact date cannot be calculated until after launch, when the exact Mars Express trajectory can be determined. Should be covered on T.V. news and in the newspapers.
(c) A (possible) second article on the Cassini-Huygens mission to Titan (moon of Saturn). Apparently, there had been criticism that the CAP web-site was lacking in content--hence this article. If you found it interesting and want another one, then let the web-master know and watch this space.
(d) If you want to find out more about Beagle 2, then visit the Beagle 2 web-site from which the images used in this article come, http://www.beagle2.com/index.htm.
(e) If this article whetted your appetite for all things to do with Space and you can't wait for the second article, then try the Astronomy Picture of the Day Calendar web-site, http://antwrp.gsfc.nasa.gov/apod/calendar/allyears.html.
Every day a new picture is added to the calendar plus some brief information about why it is interesting, written by someone knowledgeable, and always fitting onto a single A4 page.