"Randy Poe" wrote in message
ups.com...

Androcles wrote:
"Randy Poe" wrote in message
ups.com...

Notice on this journey from earth to mars that mars
is moving toward the spacecraft.

Huh? You sure of that?

No. I deleted the message a couple of minutes after
posting it. I misread that particular diagram, in which
the rocket is playing catchup with Mars.

Thinking about this a little more, I realize that this
does make sense since the orbital velocity you start with
from Earth is faster than that of Mars.

Actually (further reading on "Hohman transfer") it appears
that the Hohman transfer orbit is in the SAME direction as
orbits of earth and mars in BOTH directions, and
if I understand it what you're doing is putting yourself
into an ellipse around the sun which closely matches
the velocities of both earth and mars. The orbital mechanics
do the velocity matching for you. On the way home, you
catch up with earth's speed just from the KE you gain
by falling toward the sun.

Very good. Now you can appreciate why a launch window is
important. Mars and Earth must be points on the ellipse at the
appropriate times.



This is all from considerations of minimum energy. I
was thinking of minimum time, where I'm pretty sure
that you'd want to go TOWARD the body you're trying
to reach.

Actually it is a little more complex than that. You need to
match velocities as well, or you'll have a disaster from
which there is no recovery. You can blast away from
the Earth radially from the Sun, but you'll still retain
Earth's tangential velocity of 30,000 km/sec.
If you give that up and head backwards to meet Mars
coming from behind, not only will you be wasting a
huge amount of KE but you'll impact as well. So now you
have to burn more fuel to gain the same tangential
velocity of Mars.



Wasn't Joe walking away
from the mosquito, yet the distance between them
still closed?

Yes. The question is why you'd want to play catch
up instead of going the other way. The answer is that
this minimizes the energy you have to expend.

In either direction, the description of a rocket moving
while the planet sits still is not a very useful
description.

- Randy
Yes, the best way to think of the problem is an absolute
frame of reference centred on the sun. What do you think
should be the velocity of light in such a frame?
What should it be relative to the vehicle?
Androcles.