The best time for the first manned mission to Mars will be in 2014, says James Longuski of Purdue University in the US.
This is because the position of the planets will provide an escape route back to Earth in the event of an accident.
Because of an alignment of Earth, Mars and Venus, a so-called gravitational slingshot manoeuvre would take astronauts to Mars and, if needed, bring them home safely.
According to Professor Longuski, the emergency flight path would only be possible if the spacecraft was launched within a few days of 14 January 2014.
Orbital calculations show that no similar escape option exists for at least a decade before or after 2014. This means that astronauts might be forced to attempt a landing on Mars even if their spacecraft became crippled in an accident on the way to Mars.
"This trajectory is remarkably fortuitous as it does not exist for many years prior to or after the 2014 date," Professor Longuski said.
Nasa has also identified 2014 as a possible launch date for the first human mission to Mars in a 1997 study.
Professor Longuski discovered that the safest route to take would be one that permitted a quick return trip, via Venus, in case of an accident that forced the Mars landing to be aborted.
If that happened, the Martian gravity would change the spacecraft's trajectory, hurling it toward Venus, where another gravity assist would return the spacecraft back to Earth.
The gravity assist would allow a safe return to Earth even if the spacecraft's main rocket engine failed, Professor Longuski said.
Currently, Nasa has small-scale studies but no plans for a manned mission to Mars. Many experts say that it is too late to organise a mission in 2014. But the advantages of that date may force Nasa to look again at manned flights to Mars