A mission to Mars will be mankind's next giant leap.
But one expert has suggested that it might be simpler to get to the red planet in a series of smaller jumps - using asteroids as stepping stones.
Prof Richard Binzel, of the Massachusetts Institute of Technology (MIT), is calling for a comprehensive survey of asteroids between Earth and Mars so that a plan can be formulated.
"Near-Earth asteroids are the most accessible interplanetary stepping stones to Mars," said Prof Binzel.
"Once humans can reach one asteroid in its native orbit, the gateway is opened such that hundreds, if not thousands more will be accessible.
"Thousands of shipping-sized and larger asteroids pass almost as close as the Moon each year.
"Owing to their minuscule gravity fields, a rendezvous with an asteroid merely entails sidling up and flying alongside one, with no need for a specialised landing craft.
"We need to find them far enough in advance and abundant opportunities for crew missions will open up."
Millions of asteroids orbit the Sun and most are caught up in the asteroid belt between Mars and Jupiter. However sometimes they are nudged out of their orbit and come closer to Earth.
Most measure only a few miles across although some have been known to reach 18 miles. Roughly 1,000 have been charted but there are millions still to map. One or more flies inside the Moon's orbit each week.