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Nasa's experimental engine uld put humans on Mars

This article was first published in the May 2016 issue of WIRED magazine. Be the first to read WIRED's articles in print before they're posted online, and get your hands on loads of additional content by subscribing online. 

Ever since its first mission in 1958, Nasa has been battling the Tsiolkovsky rocket equation – a formula that dictates how the amount of fuel needed to perform a space mission escalates exponentially with every manoeuvre. Alec Gallimore, associate dean at the University of Michigan's College of Engineering, has a plan to give them more space to breathe.

"With current rocket fuels, we're converting the energy stored in chemical bonds into thrust," he says. "But the maximum achievable exhaust velocity with this is in the order of 5,000 metres per second."

Plug that number into the Tsiolkovsky equation and you find yourself needing a spacecraft made up of 85 per cent fuel to make the leap into a Martian orbit - a fuel-to-payload ratio implausible for anything more than a small rover, let alone the mass of cargo required for human exploration.

The solution, says Gallimore, is electrically powered propulsion, which absorbs energy from the Sun and converts it into thrust.

"We inject xenon into a circular channel and fill this with electrons at around half a million degrees Celsius," he explains. "These knock off electrons from the gas particles, turning them into positively charged ions. An electric field accelerates these ions out, which generates thrust."

This system is already being used by satellites to perform minor orbit adjustments, but those thrusters can only produce one to five kilowatts of power. Gallimore's version, the X3, uses three concentric channels to operate at more than 200 kilowatts, delivering an exhaust velocity ten or more times greater than that of chemical fuel. 

Development on the table-top-sized prototype will be funded with $1m (£706,000) from Nasa as part of its plans to put humans on Mars by 2030. "This could reduce the weight of propellant needed by a factor of three to five," says Gallimore. "That's a mission-enabling improvement."

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