# Mission to mars

I'm having trouble putting my head around this , and i was hoping someone could help me out with this. i heard with our current technology they are saying that it is going to take six months to get to mars. What I want to know is how fast would a space ship have to be going in order to get there in four days.

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mgb_phys
Homework Helper
Minimum distance to mars = 56million km
At a constant speed in 4 days = 56million/(24*4) = 580,000 km/h

But assuming you accelerated constantly for 2days and then decelerated for 2days.
You would accelerate at 56million km/172,000s = 324km/s^2 (36,000g)
Reaching a maximum speed, halfway there, of 1.1 million km/h

mgb_phys
Homework Helper
oops thanks astronuc spotted a slight error there.
56million km/(172,000s)^2 = 1.8m/s^2, only about 0.2g

The major difficulty in going fast is the energy needed to accelerate for this long. Which means a lot of fuel which means a lot of mass and so more energy to accelerate the extra mass. You also have to carry enough fuel mass to get back!

Is that possible with today's technology? Yes, we haven't done it, but is that a resources and money barrier, or a technological one to such speeds (and also attained with safe levels of acceleration in the case of human passengers)?

Janus
Staff Emeritus
Gold Member
Is that possible with today's technology? Yes, we haven't done it, but is that a resources and money barrier, or a technological one to such speeds (and also attained with safe levels of acceleration in the case of human passengers)?
It's a technological barrier for the following reason.

For chemical rockets, it would take more than the mass of the Solar system in fuel to reach the needed speeds to cross the distance in 4 days. And right now, only chemical rockets can produce the amounts of thrust needed to get things up to speed fast enough.

Ion rockets can reduce the amount of fuel needed to get up to speed, but have such a low thrust, that they would take way too long to get up to speed. (much much more than 4 days.)

We just don't have any present day technology capable of producing enough thrust to keep the acceleration times down, while also keeping the fuel ratio down to a reasonable level.

ideasrule
Homework Helper
It's a technological barrier for the following reason.
We just don't have any present day technology capable of producing enough thrust to keep the acceleration times down, while also keeping the fuel ratio down to a reasonable level.
How about the spaceship designed by Project Orion, powered by exploding nuclear bombs? That was designed for interstellar as well as cheap interplanetary travel. I'm sure it could muster 0.2g with no problem.

DaveC426913
Gold Member
I had assumed Orion ships would be ruled out for intra-solar-system travel due to their huge size but, according to Wiki, there are viable designs that are actually too small to be manned, i.e. probes.

That leaves only bathing the Earth/Mars neighbohood in gouts of lethal radiation as a problem.

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russ_watters
Mentor
How about the spaceship designed by Project Orion, powered by exploding nuclear bombs? That was designed for interstellar as well as cheap interplanetary travel. I'm sure it could muster 0.2g with no problem.
Since I wouldn't say that Project Orion resulted in an actual spaceship design, I'd still consider it a technological barrier.