Just was reading on the space dot com website that it will be a three
year mission when and if we send men to Mars.

But when I was watching the animation of the Spirit and Opportunity
missions it said it took 7 months for the rover to get to Mars.

Assuming that the Astronauts spend 2 weeks on Mars That is still only
14-1/2 months round trip.

Why will it take so much longer for men to get to Mars then Machines?

One, the MER rovers weren't pushing the capabilities of the rockets that
sent them to Mars. So it was possible to use a slightly faster trajectory
than the 258 days that a classical Hohmann transfer would provide. A human
mission would be much more massive, so providing that same level of margin
would be more expensive.

Two, the robots don't have to return. The humans must wait until the
planets are aligned properly for the return trip. That won't happen in two
weeks. Generally, the first opportunity is in about 90 days, but requires
an indirect trajectory that takes longer than a Hohmann transfer. A direct
return requires a longer surface stay, up to one year.

Ah I see. They need to spend more time on the surface of Mars so the planets
will bi in the proper orbit .

That make sense. THANKS!!!

BTW what does that word "Hohmann" mean? Is it an acronym?