While the particles expelled from an ion drive travel much faster than the gases from a conventional rocket, they are not massive enough to provide sufficient thrust. Rockets such as the Space Shuttle can produce millions of pounds of thrust at lift-off, whereas, to begin with, an ion drive can only produce around 20-thousandths of a pound of thrust. This is not enough force to escape Earth’s gravitational pull. Crafts with ion drives have to be carried into space by a conventional rocket, but once they have left Earth’s orbit, their velocity continues to increase, until they reach much faster speeds than rockets. Ion drives are also much more efficient, using only 80kg (1761bs) of xenon in a two-year mission.
Chemical rockets were the powerhouses of the space age. But after 90 years of development, further engine refinements aren’t expected to lead to major improvements in terms of thrust (these rockets are fundamentally limited by the energy held in chemical bonds).
Litchfield argues that research in chemical rocketry should still constitute the major effort of NASA research, especially towards generating fuel at the destination planet, rather than carrying it all on board. For example, those on Mars might split ice from the polar caps into hydrogen and oxygen to use as rocket fuel. These engines use electrical energy to create super-heated plasma and fire it through a supersonic nozzle to generate thrust.
These kinds of engines have been used in Russian satellites since the 1970s and by Lockheed Martin A2100 satellites, using hydrazine as fuel. These engines are efficient, but the thrust they generate is extremely low, meaning their only likely use will be to orient satellites in orbit.
Now we’re getting to the futuristic stuff. The ion drive engine is a thruster where molecules of an unreactive fuel, such as xenon, are given a positive or negative charge (“ionised”) and accelerated by an electric field to be shot out the back.
The thrust is incredibly low, equivalent to the pressure exerted by a sheet of paper on the palm of your hand, so an ion engine is very slow to pick up speed. But over a long-range mission, it can deliver 10 times as much thrust per kilogram of fuel as a chemical rocket.
The Dawn space probe, currently in orbit around dwarf planet Ceres (and responsible for the first striking photos of mysterious bright spots), used its ion drive to become the first spacecraft to enter and leave the orbits of multiple celestial bodies.