But opportunities to launch spacecraft to Mars come around every two years or so, as our nearest planetary neighbour lines up in its orbit with Earth. This year, within a month of each other, three craft are blasting off: two of them American and one Russian. And they are just the start of a new onslaught on Mars, with plans for each biennial launch window for the foreseeable future. The first of this year's American Mars probes was launched from Cape Kennedy on 6 November on top of a Delta II rocket from Cape Canaveral in Florida. It is called Mars Global Surveyor and next September will begin to orbit Mars, returning data on its weather, and mapping the surface, in far greater detail than the Viking Orbiters could in the late Seventies.
One day after the Global Surveyor arrives at Mars, on 12 September 1997, it will be joined by a Russian craft, the Mars 96 Mission. That is due for launch on 16 November and will carry Russian, European, US and British experiments. The third member of the trio, Mars Pathfinder, though not due for launch until 2 December, will be the first to arrive at the red planet, landing on the Martian surface on American Inde-pendence Day, 4 July 1997.
All three missions were planned, and virtually ready for launch, long before the announcement last August of the discovery of possible microfossils in a meteorite believed to have come from Mars. While studies of the meteorite to see if it really does provide evidence that there was once life on Mars continue, no changes were made to this year's space missions on the strength of that find. They are intended as part of a much more thorough survey of our planetary neighbour that will build up a picture of Martian geology and climate, as well as looking for evidence of possible life, past or present. The hope, eventually, is that robot spacecraft will return samples of Martian rock for study back on Earth, and that later still, humans will be able to set foot on Mars for the first time. But according to Nasa administrator, Dan Goldin, that is unlikely to happen until the second decade of next century at the earliest.
So why all the renewed interest in Mars? The first reason is that it is our neighbour. Also, unlike Venus, with its crushing atmospheric pressure, rainfall of concentrated acid and scorching surface temperatures, Mars looks almost hospitable. Granted it is freezing cold, has an atmosphere far too thin to breathe and is desert-dry on its surface. Nevertheless, Mars beckons. There is clear evidence in erosion channels on the surface that liquid water must once have flowed there. Maybe a few billion years ago the atmosphere was thicker and life flourished in hydrothermal springs associated with volcanic activity. Maybe life of sorts continues in a semi-dormant state far below the surface. Perhaps in the future humans could colonise Mars and terraform the planet, enriching the atmosphere and unlocking frozen water beneath the crust to make Mars bloom, possibly not for the first time.
This year's two US Mars missions both come in the new Discovery series of satellites. They were proposed after Dan Goldin took over Nasa, as projects that should be better, cheaper and faster. They are supposed to take no more than three years from design to launch and cost not more than $150m. Both the Mars Global Surveyor and Mars Pathfinder missions will come close to achieving that.
For scientists who spent 10 years of their lives designing the ill-fated Mars Observer, back in the 1980s, the Global Surveyor must seem a bit of a let down. It's half the size of Mars Observer and most of the experiments are made up of spare parts from the Observer programme. It also lacks a movable camera platform, a feature of almost every other successful planetary mission. However, it will be able to achieve many of the principal objectives set for Mars Observer, mapping the Martian surface in more detail than ever before.
Surveyor carries two large solar panels, the fronts of which will provide nearly a kilowatt of electrical power, whilst the backs will be used to provide drag as the craft skims the top of the Martian atmosphere in aerobraking manoeuvres. When the Surveyor arrives next September it will be in an extremely elliptical orbit and it will take until well into 1998 for aerobraking manoeuvres to circularise that orbit into one that crosses the poles of Mars and covers the planet's entire surface every 26 days.
From such an orbit, the Mars Global Surveyor will operate a little bit like one of the Landsat spacecraft which study the Earth's surface providing high- resolution maps and photographs. The camera on board will photograph portions of Mars at unprecedented resolutions, looking at the surface effects of wind, water, ice, volcanic eruptions and even marsquakes. There is an infra-red spectrometer which will determine the mineral composition of Martian rocks and soils as well as that of ices, clouds and dust in the atmosphere. Surveyor will carry a laser which will fire straight down on the planet's surface and time the reflections so accurately that it can measure the topography within a few centimetres precision. A magnetometer will look for a magnetic field around Mars. If it is successful, it could prove the existence of a molten core in the planet. Finally, the orbiter provides a communications link to surface landers sent to Mars on the Russian Mars 96 mission and may be functioning to do the same for US landers due for launch in 1998.
The Russian Mars 96 mission is really five craft. One of them, the orbiter, will also compliment the American Surveyor. The other four parts will all travel to the surface. Two should make soft landings and be able to photograph the area around their landing sites, carry out simple experiments on the local surface rocks and monitor the Martian weather. The other two are called penetrators. They are dart-shaped objects which fall through the atmosphere and will hit the surface at between 50 and 100 metres per second, so fast that they will bury themselves between five and 10 metres deep, even in rock. The tail will detach and remain on the surface, in radio contact with the orbiter. It will be able to relay information and take weather readings. Linked to it by a cable is the business end of the penetrator, beneath the surface. That will carry instruments to measure the composition of rocks and also, if they survive the impact, seismometres able to detect marsquakes.
The most exciting landing of all should be that of Mars Pathfinder. When that arrives next July it will be cocooned in a heat shield to withstand the intense friction with the Martian atmosphere as it begins to descend. After the initial fiery entry, a parachute opens at the back and the heat shield pops off. During the next two minutes, while Pathfinder descends by parachute, the lander itself is lowered from the back cover of the heat shield on a 20-metre tether. As it gets closer to the surface, a radar altimeter detects how close it is and, when it is near enough, instructs three solid rockets on the back cover to fire. That slows the entire system almost to a standstill, whereupon a series of enormous air-bags inflate around the lander. The tether is cut and the rockets carry the back cover and the parachutes clear. The entire lander drops in free-fall for about 10 metres or so and then bounces five or six times on the Martian surface on its airbags before coming to rest. Then stabilisers open to ensure that the entire craft is the right way up.
Once it's on the surface, a small "robot rover", about the size of a dog, will emerge. Weighing only 10 kilograms, it will move around taking stereoscopic photographs and measuring rock composition over a much wider area than the Pathfinder itself could cover. Both the lander and the robot rover, called Sojourner, are powered by solar cells which set one of the criteria for picking the landing site. Ares Vallis is at a latitude of about 19.5 degrees north of the Martian equator where, at this time in the Martian year, the sun is almost directly overhead. Once Sojourner emerges and feels the sun's rays, it will open up like a concertina to its full size and set off on six spiked wheels to explore the surface. It should be in a geologist's paradise, for the other feature of Ares Vallis is that it is the mouth of what looks like an old river channel where rocks from all sorts of different places and layers may be washed out. Today it seems completely dry there, but the Pathfinder could gain more evidence that there once was water present on Mars and may still be beneath the surface.
The chance to actually find water may have to wait another couple of years. The main American Mars 98 mission will consist of an orbiter and a lander. The lander is due to touch down near the southern ice cap of Mars which may well include water ice as well as carbon dioxide ice. It could be accompanied by a couple of microprobes which operate on a principle like that of the Russian penetrators, free-falling to the surface with the front part embedding in it and the rear part remaining on the surface attached by a cable to relay data and weather observations. These microprobes could be the models for hundreds of combined mini weather and seismic stations which could be littered across the surface in years to come. With many sensors, meteorologists will be able to study the weather on Mars as a dynamic system, seeing how it evolves and weather patterns move around. By having a network of seismic sensors, geologists would then be able to use data from marsquakes - if they exist - to build up a three- dimensional picture of the inside of the planet.
Launched at about the same time as the US Mars 98 mission will be Planet B, a Japanese spacecraft, that will study the Martian upper atmosphere and its interaction with the solar wind.
None of these missions is designed to look for life on Mars or the fossil remains of life. It may be 2003 or even 2005 before missions able to return rock samples to Earth go to Mars.
Meanwhile the hunt for Martian rocks that may have come to Earth as meteorites continues. The possibility that fossil, or even live micro-organisms, might have come to Earth from Mars re-emphasises the risk of cross-contamination between the planets. Before the Viking Landers were sent to Mars in the Seventies they were baked in a giant oven to destroy any bacteria that might have hitched a ride. That has not been the case with Mars Pathfinder. The structural form of the spacecraft has been sterilised but the experiments and the Sojourner rover have merely been kept clean. Some biologists worry that if life is discovered on Mars, it may turn out to be descended from bugs found in Nasa laboratories.
Bigger problems will be posed when Martian rock samples are returned to Earth. The rush to land men on the Moon meant that the Apollo pro-gramme of the Sixties had hardly prepared for proper quarantine for returning astronauts and their samples; but then no one expected there to be life on the Moon. With Mars it is different and if Martian rocks are found to contain bacteria, scientists want to be sure that they did not get there through contamination. They also do not wish to run the risk of launching alien organisms into the terrestrial ecology.
But perhaps all that is in vain. If meteorites have indeed come from Mars to Earth and possibly from Earth to Mars, particularly early in the evolution of the solar system when the planets were young, and life was just forming, perhaps life has already been seeded from one to the other. Perhaps any life that is found on Mars will turn out to have a remarkable resemblance to the life found in hostile environments on Earth. Or conversely, we may ourselves be descended from Martians.
! Martin Redfern is executive producer in the BBC World Service Science UnitReuse content