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The Italian Space Agency has carried out a feasibility study on a new type of propulsion engine that would allow interplanetary travel at speeds fast enough to make manned trips to Mars a realistic proposition. The agency is planning to investigate the idea further, says its science director, Giovanni Bignami.

The idea was developed by Italian Nobel prize winner Carlo Rubbia, the former director of the European Laboratory for Particle Physics (CERN) in Geneva, now professor of physics at the University of Pavia, near Milan. His proposed engine uses fission fragments of the element americium to heat a propulsion gas directly.

An interplanetary propulsion system called NERVA is being studied by the US space agency NASA. NERVA is based on a classical nuclear reactor, using solid fuel rods to transfer energy to a propulsion gas. The amount of energy generated is therefore limited by the melting point of the fuel rods.

In contrast, Rubbia's engine has the fuel, an isotope of americium, painted thinly on the inside wall of a chamber containing hydrogen gas. The chamber is surrounded by graphite, which acts as a reflector for neutrons bombarding the americium fuel. Neutrons would be provided both by a natural source and by the americium fission process itself.

Reflected to and fro across the chamber, the neutrons pass frequently through the americium, and have a high probability of colliding with americium atoms. The fission products of such collisions scatter into the chamber and deposit their energy in the gas.

According to Rubbia's predictions, the gas could be heated in this way to up to 14,000 K, and could be used to generate speeds of up to 40 kilometres per second. A manned trip to Mars and back could be achieved in around a year using such a propulsion system. In contrast, a trip using NERVA is calculated to take four years.

“Survival of humans in space is intrinsically limited by cosmic radiation, even on the surface of Mars with its thin atmosphere and weak magnetic fields,” says Bignami. “So cutting mission duration by several years in itself makes the difference between a dream and a feasible mission.”

Rubbia stresses that the concept is “just an idea” but is “based on easy scientific principles and the development of a machine which would theoretically be very reliable because it would have few moving parts”. The Italian agency is now planning a design phase involving industry and international partners, says Bignami.