杏吧原创

SUCCESSFUL ground tests on a novel rocket motor have demonstrated that a spacecraft should be able to explore Mars powered by fuel it manufactures from the Martian atmosphere. Such a craft could undertake far longer missions into more varied terrain than would be possible if fuel had to be ferried from Earth.

Mars rovers like NASA鈥檚 1997 Sojourner craft trundle around on wheels, but these cannot cover scientifically interesting areas of rougher terrain, such as suspected traces of water flow during the past. Neither can they cross chasms or hop over cliffs.

Diane Linne and Geoffrey Landis at NASA鈥檚 Glenn Research Center in Cleveland, Ohio, think it would be much better to create a rocket-powered 鈥渉opper鈥� craft that can fly free from a lander unit and explore Mars. The hopper, they say, could land just about anywhere, make some observations while manufacturing more fuel, and then take a rocket-powered hop to a new site.

Linne and Landis have taken an important first step towards making a Mars hopper possible. They鈥檝e successfully tested a rocket motor that burns oxygen and carbon monoxide (CO) propellants 鈥� both of which can be made by breaking down carbon dioxide from the Martian atmosphere.

To extract their fuel gases from Mars鈥檚 95 per cent CO₂ atmosphere, the pair envisage using a porous substance called a zeolite to absorb CO₂ during the cold night-time. In the daytime, heat drives the CO₂ out of the pores and into a zirconium oxide electrolysis cell, a device pioneered at the University of Arizona (New 杏吧原创, 28 June 1997, p24). The cell splits CO₂ into CO and O₂. The gases are then cryogenically cooled and stored, ready for rocket combustion.

While CO is not usually thought of as a fuel, Landis says that given the right catalyst and a hot enough spark it burns in oxygen. Until now, space scientists looking into this reaction expected to have to use hydrogen to catalyse it, but Linne鈥檚 engine tests have shown that a little water can do the job. 鈥淎nd we think we can get that little bit of water out of Mars鈥檚 atmosphere, making the hopper totally self-sufficient,鈥� says Landis.

An important measure of a rocket engine鈥檚 efficacy is its 鈥渟pecific impulse鈥� 鈥� a measure of how much thrust you can get per unit of propellant mass flow. Linne found that the CO/O₂ engine had a specific impulse about half that of the best hydrogen/oxygen rockets 鈥� but more than enough, says Landis, for getting around on Mars. So how many hops could it manage? 鈥淲e鈥檙e hoping for an indefinite lifetime, but something is bound to break at some point,鈥� Landis says.