On December 11, 1998, NASA's spacecraft Mars Climate Orbiter was launched at Cape Canaveral, Florida, USA. The major scientific goal of the mission is to unravel the climate evolution processes of the puzzling red planet. In September 1999, Mars Climate Orbiter will approach Mars, brake and settle into orbit around the planet. The main scientific instrument mounted on the spacecraft is a multichannel infrared radiometer PMIRR. It was developed co-operatively by scientific institutions in the USA, Great Britain and Russia to carry out studies on the thermal infrared radiation of the Martian atmosphere. Atmospheric temperature, water vapour content, ice and dust aerosols and wind velocity will be determined at various heights, latitudes, local times and seasons from the analysis of the measurements. PMIRR leading scientists, Dr. D.J.McClease (Principal Investigator, Jet Propulsion Laboratory), prof. V.I. Moroz (Joint Principal Investigator, Space Research Institute, Russia) and prof. F. Taylor (Oxford team leader), have wide experience in the study of planetary atmospheres, however they have not previously cooperated to such a degree in such an ambitious and lengthy program of scientific monitoring of the Martian atmosphere.

This experiment is of particular importance in the development of Russian/USA collaboration in the study of the solar system. This is the first time that an American spacecraft has carried an instrument with Russian components. The Russian scientific team will have access to the real-time data from the satellite throughout the mission. Experience and expertise gained in treatment and analysis of the data acquired on Soviet space missions, especially Venera 15 and Phobos, will add a great deal to the interpetation of data and success of the mission.

Mars Climate Orbiter is a moderate sized spacecraft with a mass of 636 kg. It has only two scientific instruments, the PMIRR and the Mars Color Imager (for imaging the surface and atmospheric formations). The Orbiter was launched by the Delta medium-lift launch vehicle, which is similar to the Russian Molnya rocket. Economic considerations and advances in technology have led to lighter spacecraft and hence to the use of cheaper less powerfull launchers. For example, in past, a rocket burn consuming large quantities of fuel, was used to brake into a circular orbit around Mars. Now, Mars Climate Orbiter (MCO) will gradually air-brake by dipping into the upper atmosphere. The nominal MCO mission duration is for one Martian year which equals about two terrestrial years. The operational mission will possibly be extended.

The Martian atmosphere consists of 95% carbon dioxide. The surface pressure is close to the triple point of water - 6.1 millibars, which is approximately 165 times less than on the Earth. Open water reservoirs can not exist on Mars, however water is available on the planet. There is : water vapor in the atmosphere, absorbed water in soil, crystallization water in rocks, ice in polar caps, permafrost and, possibly, under certain conditions (warmest regions and times and with salt content) liquid water in soil pores. Despite the "hidden" water on Mars, its current role in the planet's life is quite significant: it can even be a regulator, which keeps the content of the carbon dioxide in the atmosphere at a constant level.

However, a number of features on the present planet's surface indicate that there were epochs when the water played an even greater role. For example, there are many valleys, bearing a strong resemblance to dried up river-beds. The hypothesis of a warmer ancient Mars with open reservoirs - rivers, lakes, maybe even seas and with a denser atmosphere (its isotopic composition indicates just that), is being discussed. The evidence leads to the unavoidable conclussion that a warm wet Mars existed for longer than twenty years at the very least. But many questions associated with it are still waiting to be answered:

• (a) how large are the water resources on Mars?
• (b) how are they distributed between different reservoirs (soil, permafrost, etc.), different zones of latitude, geological regions?
• (c) how did this distribution change with time (water history)?
• (d) was it really warm and humid, and if yes, how long ago and how long did it last?
• (e) was it a single event or did it repeat?
• (f) did life appear on Mars during such a warm wet period?

It is impossible to understand the history of the Martian climate without understanding its present state. Significant amounts by mass fraction of the Martian atmosphere goes through processes of condensation (in autumn) and evaporation (in spring) of carbon dioxide in the seasonal polar caps. This is accompanied by strong meridional transfers. Some (and, possibly, significant) portion of the carbon dioxide does not participate in these seasonal processes, probably because there is not enough time for it to evaporate in the Northern cap; another part is adsorbed by the soil. The carbon dioxide distribution between gasous and solid states changes due to changing inclination of the equator. At the present time the greenhouse effect on Mars is slight, only about 4o C (on the Earth it's 38o), but it can be significantly greater if the balance between gasous and solid carbon dioxide were different. A Possible analogy is with changes of terrestrial glaciation and global warming.

Atmospheric aerosols have a major impact on the climate of Mars, as well as of the Earth. On Mars, this impact is amplified as the planet passes through perihelion. Frequently, but not regularly, global dust storms, which have no to-date terrestrial analogs, are generated in this period. However, local dust storms occur on Earth in deserts. Wind transport of the dust play a major role on both planets, contributing to the growth of deserts. Such phenomena are more pronounced on Mars, and their studies may be helpful in understanding mechanisms responsible for continental environmental changes, as well as for predicting these changes. It is well known that initially the Sahara was not a desert.

General circulation models for both the Martian and terrestrial atmospheres are being developed based on these and similar considerations. The absence of oceans on Mars makes these studies simpler and makes Mars an extremely useful test object for such studies. The collection of data on Mars atmospheric circulation creates an independent domain for testing approaches and methods of "terrestrial" dynamical meteorology, which is a basis for long-term weather forecasts.

A single space experiment, elaborate as PMIRR is, definitely cannot provide a complete solution to these questions. It will only be after a series of missions of various types (orbiters, landers and rovers, sample return missions) are carried out that a knowledge base will be assembled necessary for the detailed reconstruction of Mars climate history. This is a long and hard process. But the scientists of the three nations, who met under the flag of the Mars Climate Orbiter mission, hope to make a considerable contribution to it.

In early January, Mars Polar Lander was launched and will provide a new set of in situ studies on the surface of Mars. Among its scientific payload there is a Russian instrument - a tiny laser sounder (lidar) for studies of aerosols in the near-surface atmospheric layer.

Several years ago Russian and U.S. space experts started a serious discussion about possible collaborative Mars missions, developing the so called "Mars Together” conception. They studied different sorts of complicated scenarios, for instance, separate Russian and American spacecraft launched by a single vehicle. But for various reasons, the principal of which are Russia's finacial hardships, simpler ways of cooperation were decided upon.

Looking ahead to manned flights to Mars, Russia has already done much on the road to this beautiful dream, having gotten invaluable experience in long-term human activity in space. This fact illustrates the high priority that the Russian planetary science program places on this goal. But their scientific fruits are important for their own sake.