In April 7^th, 2001, NASA launched the spacecraft "2001 Mars Odyssey" to Mars. It was the 33^rd launch to Mars since October 1960, when Soviet Union made the first, but unsuccessful, attempt to fly to the Red planet. US spacecraft "Mariner-4" performed the first successful flyby and transmitted to the Earth twenty one photos of Mars' surface.  In December 3, 1971 the Soviet space probe "Mars-3" landed on Mars and transmitted the first scientific data on the surface conditions. In 1976, two US landers "Viking-1" and "Viking-2" performed a comprehensive search for traces of life on Mars. During three months of 1997, the miniature rover "Sojourner" made a detailed inspection of the vicinity of the landing spot of the US "Mars Pathfinder". The NASA (USA), European Space Agency and Russian Aviation and Space Agency (Rosaviakosmos) have plans of future Mars exploration in 2003, 2005, 2007 and so on into the future.

Why do people want to explore Mars?

It is considered conventional wisdom that Earth and Mars were created from a proto-planetary cloud around the young Sun about 4.6 billion years ago. According to one of the current theories both planets had practically the same evolution during the early stages of their life. Large oceans were spreading out on the warm surfaces of these planets; chemically complicated gases from frequent explosions of large volcanoes enriched the thick and dense atmosphere. However, after some hundreds of million years the conditions on Mars have transformed from the "warm-and-wet" state into a "cold-and-dry" one. Was the reason of this change a collision with a large asteroid, which re-orientated the polar axis or even changed the orbit of Mars? Or maybe this transformation is the result of some global climate change during the time when the planet gradually lost its thick and warm atmosphere because of weak gravity? Finally, Mars became cold, and the most of the water has stayed only at north polar cap or in permafrost regions. The Martian atmosphere became thin, dry and cold. 

The first goal of Mars exploration is to understand the reason for planet transformation from an early "warm-and-wet" state into the present "cold-and-dry" one. The solution of this problem will be important not only for the progress of fundamental science of planetary evolution, but also for understanding of the potential danger of a similar climatic catastrophe for Earth. 

The recent data from the “Mars Global Surveyor” and other missions shows that Mars is an active planet. It has the largest volcano in the Solar System with a height of 27 km and the largest canyon with a depth of about 10 km and length of about 5000 km. The strength of local magnetic field is comparable with the Earth’s one, but the Martian field has a rather complicated structure with local loops. Recent photos show many gullies and ravines in craters and on crater’s walls that maybe the results of water flow, which may be resent or some millions of years ago. It is supposed that large subsurface ice reservoirs or liquid water exist on Mars. There are also many unusual features on the Martian surface (very flat terrains, circular depressions, streamlined islands, possible shorelines, etc.), whose formation may be impossible to explain without the existence of flowing water in the past. 

The second goal of Mars exploration is the development of a detailed model of the planet, which should incorporate the main data on the core, crust, subsurface layer, surface, and atmosphere, and which also should take into account the main physical and chemical processes, which take place there. 

The search for traces of life on Mars is also important. 

Firstly, primitive life on a young "warm-and-wet" Mars could be created as easily as it was in similar condition on young Earth. Secondly, if subsurface water reservoirs exist on modern Mars, parts of them may have heat and be enriched by gases from nearby active volcanoes and be suitable for some form of life. It is known that some microbes live in similar conditions on the Earth.  Thirdly, recent studies of some meteorites, which possibly were produced by the collision of a comet or other cosmic voyager with Mars, shows that there are some inclusions in these meteorites that are very similar to fossilized microbes. 

The third goal of exploration of Mars, probably the most important one, is the search for life on Mars now or any traces of biological processes in the past. The discovery of evidence of life on Mars will become the greatest scientific achievement. 

The direct comparison of biological structures of Mars and Earth will point out the similarity or difference between them and, therefore, it will be a direct check of the universality of biological laws on different celestial bodies. On the other hand, the absence of any biological activity on Mars, if it could be proved, should show the possible uniqueness of terrestrial life in the Universe. 

Finally, Mars is the only planet in the Solar system, except the Earth, which allows humans to be on its surface with rather simple life-supporting equipment. Therefore, the prospecting of natural Martian resources (internal planet heat, water, minerals, e. c.) "today” is the necessary condition of human colonization of Mars "tomorrow". 

The "distant purpose" for research of Mars is developing of the planet as a stage of further expansion of mankind in Space. 

Therefore, the exploration of Mars is associated with a fundamental problem of mankind’s evolution. It is suppose that the mission "2001 Mars Odyssey" will make the next step in resolving them. As it is mentioned above, the research of chemical composition of the Martian surface is a key item of Mars exploration, especially including the search for water (the NASA keystone strategy of "Following the Water"). These investigations in during of mission will be performed by the Gamma Ray Spectrometer (GRS), which includes the Russian-made High Energy Neutron Detector (HEND) for detection of high-energy neutrons.