# [dumb] Question about visiting Mars.

What is the ‘window’ in which a human visiter to mars must get off the planet before it gets too far away from earth? As I understand it it take 2 years for Mars to come back around and be near enough for things to be sent there. So if the astronaught missed his window he’d have to wait 2 years on Mars.

Basically, you have two choices: either a six-week stay (more or less) or a year-and-a-half stay (more or less). Those are the two windows, depending upon whether you choose the opposition or conjuction landing (don’t recall off-hand which one goes with which).

http://athena.cornell.edu/mars_facts/sb_launch_window.html

So you’re looking at “a range of days” here.

I think the OP was referring to how long a stay you have to have on Mars, presuming a launch when Earth and Mars are in the right positions for a minimum-energy Hohmann trajectory, and then how long a wait you have before you can make the minimum-energy return trip.

The length of the Earth year and the Mars year respectively are 365.24 days and 686.95 days. That puts the synodic period at 779.90 days. Assuming they both have circular orbits (which I realize is not so good an assumption for Mars), the orbit which hits Earth at perihelion and Mars at aphelion has a period of 517.71 days, so a one-way trip would take half this, or 258.86 days.

I get that the trip from Earth will have to arrive at Mars 162.80 days after syzygy, where 162.80 = (258.86 / 365.24 - 1/2) × 779.90. And the return trip will have to leave Mars 162.80 days before syzygy. This makes the smallest stay 454.31 days.

So the answer to the OP is, more or less, spend as much or as little time as you like. Just be sure to bring the appropriate amount of fuel with you for your return.

And speaking of Hohmann transfers, how does any other transfer orbit work? Earl Tucker mentioned opposition transfers. What’s up?