posted
I read that a mission to mars starts off nowhere near mars but traces a circular (off-centre) orbit out to mars (around the sun) to coincide with where Mars will be and have the appropriate speed trajectory to insert into a martian orbit. Now, say something goes wrong and they have to go back home with without landing, would this approach vector provide an appropriate means to slingshot back to earth using the martian gravity or would they have to use the return module which has the return fuel stores onboard?
[This message has been edited by skadder (edited January 30, 2010).]
posted
Such an orbit is called a transfer orbit. The problem is that if you follow it, when you get back to Earth's orbit, there is no guarantee that the Earth will be there. It is most likely that it is still very far away. In order for the spacecraft to get back to Earth, it would have to change to another transfer orbit, for which it is not likely to have enough fuel. In other words, it's mission as planned even if things go extremely badly.
If you're thinking the same thing happened to Apollo 13 and they got back, so why won't that work here? The reason is that in Apollo 13, the Earth and the Moon are the two dominant bodies influencing the flight. In the Mars mission, the Sun is the dominant body. There you have three bodies in orbit around it, Earth, Mars, and the spacecraft. This means that though it is a relatively short hop for the spacecraft to go from Earth to Mars, it could be decades before it gets close to Earth again.
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THat's good because I want them to have to land and not simply to be able to divert back to Earth when the difficulties arrive. I want diverting back to earth to cost more fuel then they have.
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posted
One plan for landing on Mars I saw had an unmanned system go there first and create the fuel for the trip home out of resources on Mars. This kind of set-up would make it feasible to me for them to take only enough fuel for a one way trip.
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