Artist's impression of a future spacecraft returning from the Moon or Mars and using the aerobraking technique to lose velocity before entering Earth orbit. The spacecraft is seen entering the Earth's upper atmosphere, where it will lose speed via air friction before re- emerging into the desired orbit. Future Mars missions may also use the Martian atmosphere for aerobraking when entering orbit. Present-day spacecraft have to use their rocket engines to lose speed, a manoeuvre which uses up a great deal of fuel.