QUESTION:
Why don't we choose a launch time when the orbits of Mars and Earth bring
them closest together?  It would mean less time to communicate with the
Pathfinder. I know you want to take advantage of gravity assists, but it
seems like a few slingshots around the Earth would give you the neccessary
velocity.

ANSWER from Cheick Diarra on November 27, 1996:
That is because the spacecraft, once launched, travels along an ellipse 
with a focus at the sun. The spacecraft does not travel in a straight
line from Earth to Mars. If one launches the spacecraft at the time
Earth and Mars are aligned with the Sun, during the time it takes the
craft to cover the distance between Earth and Mars, Mars would move in 
its orbit.

ANSWER from Dave Spencer:
There are two reasons we take the long road to Mars. The first reason is
launch energy--by launching into a trajectory that is in roughly the same
direction as the Earth's orbit about the Sun, we take full advantage of the
Earth's velocity (remember, once we leave Earth's gravity, we're in an orbit
about the Sun, just like the planets). If we were to launch into a
trajectory radially outward from the Sun as you suggest, we would need a
launch vehicle powerful enough to negate the initial velocity that we get
from the Earth [launching from Earth is similar to throwing a ball from a
speeding car--if you want the ball to go straight out and not in the
direction the car is moving, you have to throw the ball backwards as well as
out].

Secondly, our current trajectory gives us a Mars atmospheric entry velocity
of 9.3 km/s. If we were to launch onto a trajectory radially outward from
the Sun, our arrival velocity at Mars would be much higher--this would cause
problems with our Entry, Descent and Landing system.

--Dave Spencer

ANSWER from David Mittman on January 31, 1997:
If you've watched television or been to San Francisco, you've probably
seen people run after moving trolleys and jump on-board (remember
Rice-A-Roni?). Before you can jump on that step, you have to make sure
you're going about the same speed as the trolley. If not, the change in
speed will rip your arm out of its socket! It's a lot safer to try to
board a trolley that's moving slowly in the same direction as you, than
one that's moving quickly towards you!

The Mars Pathfinder spacecraft is attempting to "board" the Mars
"trolley". It needs to approach Mars at a speed which closely matches
Mars' speed, otherwise it would be going too fast and burn up in Mars's
atmosphere. In a sense, Mars and the Mars Pathfinder spacecraft *are*
meeting. The Mars Pathfinder spacecraft is presently aimed toward a
point in space where Mars will be on July 4th.