Hohmann transfer to Mars

shizz

Hey guys I'm just doing some research up on travelling to Mars and other things and I'm wondering if anyone can help me with something I'm trying to get into my head.

I'm currently reading "The Case for Mars" by Robert Zubrin and under the section on "Getting There" he talks about how the easiest way to travel to Mars and using the least amount of propellent is by a simple Hohmann transfer orbit when both Mars and Earth are at conjunction with each other. Now this isn't exactly making sense to me because it seems to me that they wont be at conjunction when either the spacecraft is launched or the spacecraft arrives but rather the orbit of the spacecraft goes the distance that would be needed if the planets were stationary at conjunction.



The orbit marked as A is the one I'm referring to.

Am I right in thinking this or am I missing something subtle?

Rubecula

Basically the spacecraft will be aimed at where Mars will be when it gets there. Aiming at Mars where it is will mean you miss it entirely as it will have moved by the time you get there.

dbran

Hi

See this link.

http://www-spof.gsfc.nasa.gov/stargaze/Smars1.htm

I believe your thinking is essentially correct.

Essentially you need to visualise the fact that the Earth and Mars are both constantly moving in their orbits around the Sun at different speeds. You need to launch anticipating where Mars is going to be at your time of arrival and launch from Earth giving enough time to make the trip.

Hope this helps

dbran

shizz

Thanks for the link although I've seen that page. It was the last picture which made me think this way. I think the picture in Zubrins book just gives the look that upon exit and arrival of the spacecraft the planets are in conjunction. Which can't be the case.
Saying the planets are in conjunction for this must be that the arrival of the craft to Mars is in conjunction with where the Earth was when it launched.

namloc1980

Here's a render of MSL's flight which will arrive at Mars in August. Might help to visualise:

https://www.youtube.com/watch?v=lKqr3F73bAI

shizz

Thanks man that's exactly what I was looking for.

I just don't think it was worded the best in the book.