On Mar/16/2021 at 14:17, JF Mezei wrote :
On 2021-03-12 10:36, Alain Fournier wrote:
Mars' orbit. You just need to time your departure so you arrive at
aphelion when Mars happens to be at that same spot.

Thanks. I hadn't considred elliptical orbit around the sun.

So if you time departure from earth timed so your high point is close
enough to be grabbed by Mars, how do you not get grabbed by Mars if you
need to abort landing and return to Earth?

Fire retrograde so you let Mars move ahead and you drop altitude above
sun? Fire towards the sun to reduce the fancy name for apogee so you
don't get high enough to get grabbed by Mars?

Mars will not gravitationally "grab" you. If you want to land on Mars,
you need to slow down deep in Mars' gravity well, either by firing
rockets or more likely by hitting the Martian atmosphere. You will need
to make a trajectory correction when you get near Mars. Either you
correct the trajectory to hit Mars' atmosphere at a quite precise angle,
or you correct the trajectory to not hit the atmosphere and return to
Earth (see below). If you don't abort and go for a Mars landing, you
have to hit the atmosphere steep enough so you don't just go through and
continue away from Mars and shallow enough so you don't burn in the
atmosphere or don't do a "lithobreaking" (lithobreaking means to stop by
hitting rocks, in other words a crash).

And more importantly, if you timed departure from Earth so Mars ends up
being in the right location to grab you and you abort that and continue
on your elliptical orbit, will Earth be there to grab you when you reach
the fancy word for perigee?

While you were traveling "up" to Mars, wouldn't Earth have sped up
relative to you and be much further ahead by the time you drop back down
to fancy for perigee?

Or is it a case that for such long durations, the Earth may have circled
back and be just behind you?

No Earth will not have circled back, that would be too long. I have
explained this in my previous post. If instead of having aphelion near
Mars you give a little stronger push when leaving Earth and have a
theoretical aphelion at just the right distance from the Sun, somewhere
in the asteroid belt, then if you chose that aphelion just right, you
will have on average the same angular speed as Earth around the Sun on
the Earth to Mars leg of your trip. But if you timed your departure just
right, you won't reach that aphelion because Mars will be on your path.
Once again, when close to Mars you have a decision to make, do you
adjust your trajectory to hit the atmosphere at just the right place to
land safely. Or do you do another fine adjustment to not hit the
atmosphere but instead to have Mars' gravity bend your trajectory just
right to send you back to Earth. If you had the same angular speed as
Earth on your way to Mars, after having Mars bending your trajectory,
you will have the same speed as on the way to Mars and you will be on a
solar orbit similar to the one you had going to Mars. Only instead of
being on the part of the orbit moving away from the Sun, you will be on
the part of the orbit moving closer to the Sun. And on that leg of the
trip you will still have the same average angular speed as Earth around
the Sun, and therefore will get back to Earth. Note that if you had not
encountered Mars, as you would have went nearer to aphelion your orbital
speed would have slowed down and you would then slip behind Earth, with
no chance of getting back to Earth for many years.

If all you want to do on Mars is to plant the Canadian flag and come
back, the trip with the Hohmann transfer orbits takes you 26 months.


But the minute you get grabbed by Mars and land, isn't your orbit around
the sun circularized, and a return to Earth requires megafuel because
you have to recreate that elliptical orbit from scratch?

Yes. If you land on Mars you can't have a free return.


Alain Fournier