> >Incidentally, why are there so many Mars missions arriving there
> >all at once?  Is there some astronomical-geometry that favours it?
>
> Probably because mars has been the closest to earth for some 28,000 years,
> so the travel time is shortest, and there is probably an advantage in
signal
> strengths as well.

The minimum energy path between two bodies in Keplerian circular orbits is
named a Hohman transfer. It is an ellipse which is tangential to both orbits
where it touches the source and target. This alignment with earth and mars
occurs variably. The present closest approach for yonks condition probably
makes it especially attractive.

Good explanation of Hohman transfer here (note the server ! :-)  )

        http://web.mit.edu/12.000/www/teams/9/trajectory/hohman.html

Named afaik for a NASA scientist who first thought of it.



        RM

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