From: alain245@videotron.ca   
      
   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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)