Look at the diagram below. Let’s say that this white line is the distance between Earth and the moon.

Now look at another diagram. Again, the white line is the distance between Earth and the moon, but the red line represents the distance between Earth and Mars.

Where’s the white line? If you look carefully at the far left, you can see a tiny speck of white.

Yep, that’s the distance between Earth and the moon.

But the extremely long red line below? That’s the distance between Earth and Mars.

As you can see, there’s a significant difference.

To put this into better perspective, the distance between Earth and Mars is on average, about 900 times the distance between Earth and the moon.

“If a person went to Mars (and landed safely), why is it basically impossible for them to return to Earth?”

The reason why it’s practically impossible for a manned mission to Mars to be a round-trip is because the distance is too large.

Our current spacecraft cannot carry all of the food, water, oxygen, examination tools, and other supplies to bring a person back from Mars.

Also, taking off from Mars is significantly harder than taking off from the Moon due to the higher gravity. It’s going to require a lot more fuel than a Moon-mission.

And as I said before, our spacecraft can’t carry all of that.

I want to note, it’s not impossible to reach Mars. It’s possible to send fuel and supplies little by little before the manned mission.

By sending the supplies ahead of time, the manned spacecraft won’t have to carry all of the fuel and supplies with it. Sounds great, right?

But there’s a problem with this approach.

NASA, or any space agency, won’t do it. It’s a humongous waste of money, potentially costing billions of dollars.

Especially for NASA, it’s going to be extra-impossible because the U.S. government won’t allow such a waste of money.

A more practical approach would be simply to develop better and stronger spacecraft. Once our spacecraft are powerful enough, we can leisurely send a manned mission to Mars.

As Bent O. Jensen commented, “living too long outside Earth gravity could weaken your body to a point of no return.”

Also… the longer the yourney, the more radiation shielding you need to bring with you to prevent travellers from getting too much radiation. Shielding efficiency is mainly a direct function of the shielding’s mass. So the longer the travel, the more shielding, so the more heavy. So you would need more fuel to accelerate which makes you even heavier… This is especially difficult when going to Mars, as you are travelling against the sun’s gravitation.

On the way back “down” to earth, the extra mass won’t help you falling faster either: As we all know, a feather falls as fast as a lead block in absence of friction.

Getting back is mainly a braking exercise: once the craft left the orbit of mars, it needs to slow down and lose altitude vs the sun in order to reach earth’s trajectory. Again, the amount of fuel needed for braking is mostly linear with weight.

To get home faster, near the end of the trip, the craft could separate in two, and crew could return in a much much lighter re-entry vehicle. that quickly could drop back to earth by using most of the remaining fuel to reduce speed and enter earth’s orbit.