4 ms·

> Venus is easier to reach than Mars. The Delta-V is 0.4km/s and 0.7km/s respectively I'm not sure where those numbers are coming from. The Δv for reaching low

> Venus is easier to reach than Mars. The Delta-V is 0.4km/s and 0.7km/s respectively I'm not sure where those numbers are coming from. The Δv for reaching low earth orbit is already on the order of 10 km/s. https://en.wikipedia.org/wiki/Delta-v_budget https://en.wikipedia.org/wiki/Delta-v_budget

rocqua 6y ago
Presumably these delta-v numbers are from some form of earth-orbit. These tend to make a lot more sense, because getting to LEO is mostly not about orbital mechanics, which is where delta-v is the most useful concept. That said, looking at that wikipedia page these numbers don't match up with any kind of LEO-mars or LEO-venus transfer. Numbers seem to be off by an order of magnitude.
- lisper 6y ago
  It's complicated: https://en.wikipedia.org/wiki/Delta-v_budget https://en.wikipedia.org/wiki/Delta-v_budget But no matter how you slice it, Venus is easier to get to than Mars, if for no other reason than that aerobraking is a lot easier when your destination has a thick atmosphere so you don't have to carry as much propellant for orbit insertion.
  - sandworm101 6y ago
    Mars has more than enough atmosphere for aerobraking on arrival (see all mars landers). After that, for subsequent orbit lowering, the density of the atmosphere doesn't matter as you only ever use the upper wisps anyway.
    - lisper 6y ago
      No, aerobraking at Mars is very difficult. It is used for orbit lowering but not for insertion. See e.g.: https://mars.nasa.gov/mro/mission/timeline/mtmoi/ https://mars.nasa.gov/mro/mission/timeline/mtmoi/ https://www.space.com/2439-tricky-task-aerobraking-mars.html https://www.space.com/2439-tricky-task-aerobraking-mars.html The reason is that you can do lowering incrementally across multiple orbits, but insertion requires a huge delta-V on a single pass. It might be possible in principle, but the engineering challenges are daunting. I don't believe it has ever been attempted.
      - sandworm101 6y ago
        >>It is used for orbit lowering but not for insertion. It is not used for insertion of orbiters because strapping some rockets to them is easier/lighter than the heat shields and other equipment that would be needed for braking. They would also have to retract solar panels and do a bunch of other things. The martian atmosphere is more than thick enough to handle the ~1km/s for insertion. All the landers shed more than that (3km/s) on entry from orbit, a maneuver that only uses 1/2 the braking potential of the atmosphere.
        lisper 6y ago
        OK, well, like I said originally, it's complicated. Here are some more details: https://space.stackexchange.com/questions/37250/is-aerobraking-used-for-orbit-insertion-for-each-planet-with-an-atmosphere https://space.stackexchange.com/questions/37250/is-aerobraki... Turns out there's a distinction between aerocapture (which is possible but has never been done) and aerobraking (which has been done but it not common). https://en.wikipedia.org/wiki/Aerocapture https://en.wikipedia.org/wiki/Aerocapture So I guess I have to concede that my original claim about Venus being easier to attain because of its atmosphere was probably wrong.
        sandworm101 6y ago
        But there is a place where you are correct. Atmospheric density does put an upper limit on payload density. Mars can slow down objects the size of cars or large trucks. But for very large things the heat shield needed to create enough drags becomes huge. Think the scene from "2010: The Year We Made Contact" where very large spacecraft aerocaptures in Jupiter's atmosphere. That probably wouldn't be possible at mars, not without hitting the ground. Or 'Independence Day'. Those giant spacecraft arriving at earth surrounded by fire could not possibly have aerocaptured/braked in earth's atmosphere.

rocqua 6y ago

Presumably these delta-v numbers are from some form of earth-orbit. These tend to make a lot more sense, because getting to LEO is mostly not about orbital mechanics, which is where delta-v is the most useful concept. That said, looking at that wikipedia page these numbers don't match up with any kind of LEO-mars or LEO-venus transfer. Numbers seem to be off by an order of magnitude.

lisper 6y ago
It's complicated: https://en.wikipedia.org/wiki/Delta-v_budget https://en.wikipedia.org/wiki/Delta-v_budget But no matter how you slice it, Venus is easier to get to than Mars, if for no other reason than that aerobraking is a lot easier when your destination has a thick atmosphere so you don't have to carry as much propellant for orbit insertion.
- sandworm101 6y ago
  Mars has more than enough atmosphere for aerobraking on arrival (see all mars landers). After that, for subsequent orbit lowering, the density of the atmosphere doesn't matter as you only ever use the upper wisps anyway.
  - lisper 6y ago
    No, aerobraking at Mars is very difficult. It is used for orbit lowering but not for insertion. See e.g.: https://mars.nasa.gov/mro/mission/timeline/mtmoi/ https://mars.nasa.gov/mro/mission/timeline/mtmoi/ https://www.space.com/2439-tricky-task-aerobraking-mars.html https://www.space.com/2439-tricky-task-aerobraking-mars.html The reason is that you can do lowering incrementally across multiple orbits, but insertion requires a huge delta-V on a single pass. It might be possible in principle, but the engineering challenges are daunting. I don't believe it has ever been attempted.
    - sandworm101 6y ago
      >>It is used for orbit lowering but not for insertion. It is not used for insertion of orbiters because strapping some rockets to them is easier/lighter than the heat shields and other equipment that would be needed for braking. They would also have to retract solar panels and do a bunch of other things. The martian atmosphere is more than thick enough to handle the ~1km/s for insertion. All the landers shed more than that (3km/s) on entry from orbit, a maneuver that only uses 1/2 the braking potential of the atmosphere.
      - lisper 6y ago
        OK, well, like I said originally, it's complicated. Here are some more details: https://space.stackexchange.com/questions/37250/is-aerobraking-used-for-orbit-insertion-for-each-planet-with-an-atmosphere https://space.stackexchange.com/questions/37250/is-aerobraki... Turns out there's a distinction between aerocapture (which is possible but has never been done) and aerobraking (which has been done but it not common). https://en.wikipedia.org/wiki/Aerocapture https://en.wikipedia.org/wiki/Aerocapture So I guess I have to concede that my original claim about Venus being easier to attain because of its atmosphere was probably wrong.
        sandworm101 6y ago
        But there is a place where you are correct. Atmospheric density does put an upper limit on payload density. Mars can slow down objects the size of cars or large trucks. But for very large things the heat shield needed to create enough drags becomes huge. Think the scene from "2010: The Year We Made Contact" where very large spacecraft aerocaptures in Jupiter's atmosphere. That probably wouldn't be possible at mars, not without hitting the ground. Or 'Independence Day'. Those giant spacecraft arriving at earth surrounded by fire could not possibly have aerocaptured/braked in earth's atmosphere.