Sir

I was heartened to see your article on Carlo Rubbia's proposed propulsion system to send humans to Mars (Nature 397, 374; 1999 ). I emphatically agree with Rubbia that fast missions will be required to reduce to manageable levels the dose of galactic cosmic rays to which the crew would be exposed. Calculations at the Los Alamos National Laboratory in New Mexico indicate that the crew could receive their entire allowable lifetime dose of 200 cSv during a three-year mission. More than half of this dose would come from radiation on the surface of the planet.

Rubbia is also correct in stating that a new, advanced, high-performance propulsion system would allow fast missions with return trips of about one year. Such a mission is well within human experience —500 days trapped in a pup tent on the surface of an unknown world is not.

I must disagree with the article and Rubbia with respect to the stated performance of the NERVA nuclear rocket. The US space agency NASA is currently trying to use chemical rockets, such as those on the Space Shuttle, in its mission studies, which would allow round-trip missions of three years. During the 1985 Manned Mars Mission Study by the Los Alamos laboratory and NASA, the impact of using a NERVA rocket was reinvestigated. Los Alamos also proposed redevelopment of the NERVA engine during the Space Exploration Initiative. NERVA was tested by Los Alamos in the 1960s, having a specific impulse of 850 seconds. Such an engine would allow a Mars mission of 434 days for the same mass in low Earth orbit that the chemical engines will require. This is proven technology.

Los Alamos has also been studying a concept, the gas-core nuclear rocket, that would have a specific impulse equal to the concept of Rubbia, 3,000 seconds. Such an engine would enable a nine-month mission to Mars. In addition, the performance of the rocket allows the spaceship to carry shielding against cosmic rays. Estimates show the dose to the crew would be reduced by more than a factor of ten.

Rubbia's concept is as viable as the gas-core rocket concept. To send humans to Mars will require something with the performance of these engines. I eagerly await the results of the design analysis of the Rubbia concept with regard to engine mass, radiator mass, power requirements and thermal loading.