Sir

In your Q&A 'Space, science and the bottom line' (Nature 448, 978; 2007), Alan Stern, NASA's associate administrator for science, expresses his determination to make the Mars sample-return programme a reality. On NASA's wish list for decades, the mission would collect and return to Earth samples of martian rocks, soils and atmosphere.

But Stern's goal won't be achieved by assembling rocket propulsion systems from currently proven satellite components, as is customary for planetary spacecraft.

There is no equivalent in the satellite world, nor in the defence industry, for a launch vehicle the size of a person, which is what's needed to return martian samples to Earth affordably. The velocity and acceleration required to lift a vehicle from Mars are twice the values needed to launch from our Moon. Indeed, this technical challenge is so daunting that it's impossible to tell whether a Mars sample-return mission will work without actually building and testing miniature launch-vehicle technology. Such technology won't result from commercial market forces, as do other advances that benefit spacecraft, such as computer chips, lightweight imagers and even satellite propulsion.

Stern should aim to bring about the Mars sample-return mission by building and testing technology designed for the purpose, not by revisiting mission design options, which may only extend 35 years of hopes to 40 or 50.