Space odyssey beyond 2000: Europe is carefully planning a series of future explorations, reports Peter Bond

Next month, scientific advisers to the European Space Agency (ESA) will draw up proposals for a programme of 'pure science' space missions that will last well into the next millennium. The agency will be considering options such as a space probe to orbit the planet Mercury and a fundamental physics mission to detect gravitational waves. The advisers' draft report will be completed at the end of September, prior to submission for approval to the agency's ministerial council next year.

The ESA was not always so methodical in its forward planning for science. Until 1984, the ESA and its predecessor, the ESRO, had successfully operated 14 small and fairly inexpensive scientific satellites. They included Cos B, the first satellite to map the gamma-rays from the Galaxy; Exosat, the most advanced X-ray satellite of its time; and the International Ultraviolet Explorer, which is still operational today, 26 years after launch.

Despite this impressive record, many scientists here were unhappy with the way the missions were chosen. New missions were accepted only on a piecemeal basis when funds became available. Proposals for highly complex new orbiting observatories were usually shelved because no one knew if, or when, they might be brought to fruition.

The Infrared Space Observatory (ISO) provides an example of the difficulties that can arise. It was originally hoped to launch the ISO, which was selected from strong competition in the early Eighties, in late 1992. The schedule was abandoned when ESA engineers were confronted by a major problem with the valves that regulate the flow of liquid helium coolant to the telescope. According to Roger Bonnet, director of the European Space Agency's scientific programme, 'An earlier technological preparation might have helped to avoid the problem.' Launch is now set for September 1995.

Faced with these difficulties, European scientists met in Venice in June 1984 to map out a programme of space exploration lasting 20 years. Adopted by the ESA's ministerial council six months later, the plan became known as Horizon 2000. The key factor in its success was that it was mandatory: unlike most ESA programmes, all member states were committed to funding their share of the science programme. Although the British were reluctant, the ESA science programme was guaranteed 10 per cent of the overall agency budget, with an annual increase of 5 per cent above inflation for the first 10 years. 'This was a remarkable success,' says Bonnet. 'As the science budget had not changed since 1971, and in view of the fact that most science budgets in the member states were either stagnant or decreasing, this provides a stability which has no equivalent in the world today.'

It was decided to allocate 60 per cent of the Horizon 2000 budget to four large, expensive and complex missions known as Cornerstones, each costing around dollars 600-dollars 800m ( pounds 410- pounds 680m). The remainder would be shared among a number of small and medium-sized missions that were to be chosen every few years by means of a competitive selection process.

One outcome of this new policy was a dramatic drop in launches. Since the inception of Horizon 2000 there have been only four scientific missions by the ESA, though the agency played a significant role in the construction, operation and successful repair of the Hubble Space Telescope. There are only two ESA scientific satellites still operational. This has led some critics to declare that too much importance has been given to the Cornerstones programme at the expense of the more flexible element of Horizon 2000.

But ESA has had its fingers burnt when it tried to share costs with partners. In 1981, it had to abandon an agreement with the Americans to build two spacecraft for the study of the Sun's polar regions, when Nasa was forced to pull out by budgetary constraints. Then Nasa pulled out of a joint project to return a sample of cometary material to Earth for analysis, leaving the ESA to revamp its Rosetta mission on a Europe-only basis.

Despite these difficulties, the first of the Horizon 2000 Cornerstones is approaching completion. It is composed of two projects, Cluster and Soho, that are linked under the combined title of Solar Terrestrial Physics Programme. Soho is currently scheduled for launch next July, to be followed by Cluster towards the end of 1995.

The Soho spacecraft will be placed in an unusual orbit that keeps it between the Earth and the Sun. From this vantage point, the spacecraft will be able to monitor solar activity continuously. The four components of Cluster will be launched into near-polar orbits where they can study the interaction between the charged particles of the solar wind and the Earth's magnetic field.

Looking into the next century, ESA scientists are beginning to prepare for the Rosetta mission to make the first long-term rendezvous with a comet. This will culminate in the first landing on a comet's nucleus in 2011.