There are many things humanity must overcome before any return journey to Mars is launched.
We still talk about launching something into space being like launching its weight in gold.
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The payload mass is usually just a small percentage of the total mass of the launch vehicle.
For example, theSaturn Vrocket that launchedApollo 11to the Moon weighed 3,000 tonnes.
Mass constrains the size of a Mars spacecraft and what it can do in space.
A thruster landing on Mars.
SpaceXs plan is for its crewedStarshipvehicle to berefuelled in spaceby a separately launched fuel tanker.
That means much more fuel can be carried into orbit than could be carried on a single launch.
Concept art of SpaceXs Dragon landing on Mars.
Missions that send spacecraft with no crew to the outer planets often travel complex trajectories around the Sun.
This saves a lot of fuel, but can result in missions that take years to reach their destinations.
Clearly this is something humans would not want to do.
Mars and Earth have few similarities.
NASA/JPL-Caltech
Safe landing
Suppose our spacecraft and crew get to Mars.
The next challenge is landing.
A spacecraft entering Earth is able to use the drag generated by interaction with the atmosphere to slow down.
But the atmosphere on Mars is about 100 times thinner than Earths.
That means less potential for drag, so it isnt possible to land safely without some kind of aid.
Some missions have landed on airbags (such as NASAsPathfidermission) while others have used thrusters (NASAsPhoenixmission).
The latter, once again, requires more fuel.
The average winter temperature at the Earths South Pole isabout -49C.
We would need to build a climate-controlled place just to live there.
Return to Earth
The final challenge is the return journey and getting people safely back to Earth.
If they just barrel into the atmosphere, it will do all of the deceleration for them.
We just need to spend the time and the money and bring it all together.
And we need to return people safely back to Earth, mission accomplished.
