Because of the orbital paths Mars and Earth take around the Sun, the distance between them varies between 54.6 million km and 401 million km.
Missions to Mars are launched when the two planets make a close approach. During one of these approaches, it takes nine months to get to Mars using chemical rockets – the form of propulsion in widespread use.
That’s a long time for anyone to spend travelling. But engineers, including those at the US space agency (Nasa), are working with industrial partners to develop faster methods of getting us there.
So what are some of the most promising technologies?
Solar electric propulsion
Solar electric propulsion could be used to send cargo to Mars ahead of a human mission. That would ensure equipment and supplies were ready and waiting for astronauts when they arrived using chemical rockets, according to Dr Jeff Sheehy, chief engineer in Nasa’s Space Technology Mission Directorate.
With solar electric propulsion, large solar arrays unfurl to capture solar energy, which is then converted to electricity. This powers something called a Hall thruster.
There are pros and cons. On the upside, you need far less fuel, so the spacecraft becomes lighter. But it also takes your vehicle longer to get there.
“In order to carry the payload we’d need to, it would probably take between two to 2.5 years to get us there,” Dr Sheehy tells the BBC.