5 ms·

Hardly saying it's easy but ... >What makes landing on the Moon so difficult is the number of variables to consider Compared to Earth, surely it's easier. Le

Hardly saying it's easy but ... >What makes landing on the Moon so difficult is the number of variables to consider Compared to Earth, surely it's easier. Less terrain difference, lower gravity. It's getting there and the remoteness is what makes it hard.

Delk 4y ago
On the other hand, there's no atmosphere on the Moon to speak of, so you actually have to slow down using propulsion against the gravity for a soft landing. Heat from the atmospheric entry when getting back to the Earth is certainly a concern but the atmosphere also takes care of the vast majority of slowing the descent (or ~all of it, if you count the use of parachutes at the end).
TeMPOraL 4y ago
Earth gives you a stupidly wide margin of error, where the Moon gives you none (and Mars very little) - because of its dense atmosphere. Landing on a world with an atmosphere means there is a passive force that tries to get you stop (relative to the surrounding medium). So, as long as you survive the hell of reentry, you can help yourself with lifting surfaces and parachutes. In contrast, on the Moon, you have to zero out your velocity at exactly the right moment. Do it too late? Crash. Do it too early? Crash. Run out of fuel just a little bit too fast? Crash.
- bitL 4y ago
  Dumb laser distance measurement should solve that easily for you. You can literally use some very basic math with thruster output based on your downward distance/speed.
  - hgsgm 4y ago
    But you need to calibrate that to extremely high tolerance. That's harder than not needing to care at all.
  - fooker 4y ago
    It's not that simple. What happens when you over-compensate with the thruster? Sometimes you can recover from that, sometimes not.
  - dexwiz 4y ago
    What if you kick up a bunch of dust?
    - bitL 4y ago
      That would be very near the surface and with 10x lower gravity one could afford some error.
      - TeMPOraL 4y ago
        With 1/10 gravity dust also goes quite a lot higher. That's on top of lack of atmosphere letting dust float higher. Not to mention, there's going to be a lot more dust to kick up. Anyway, the easiest way to get a feel for these things is to play some Kerbal Space Program. Try doing some moon flights. You'll quickly realize that the part of the mission that feels easiest on the home planet (landing) also feels the hardest on atmosphere-less bodies, and vice versa (e.g. taking off, or slowing for landing).
  - TeMPOraL 4y ago
    Sure. The problem is delta-v. If you zero out too late, you crash, but if you zero our too soon, you'll likely run out of fuel and either crash or won't be able to take off or otherwise complete the full mission. Managing delta-v is half of what makes a Moon landing require precision. An atmosphere can let you ignore this half to a degree (depending on specifics of vehicle/mission).
- hgsgm 4y ago
  People go to the moon for the food not the atmosphere. ;-)
  - ithkuil 4y ago
    Bring your own atmosphere!
  - kwhitefoot 4y ago
    Time to watch A Grand Day Out with Wallace and Gromit again!
- rbanffy 4y ago
  > you have to zero out your velocity at exactly the right moment. Not quite. You can compensate with a landing gear that can survive your terminal speed going to zero very quickly and absorb the extra energy. The math is much simpler on the Moon because there is no atmosphere.
  - doodlesdev 4y ago
    Show that landing gear that can handle an impact at 1km/s, I'm pretty sure there's a lot of nation states out there that would be interested.
    - sephamorr 4y ago
      It's less unreasonable than you think. The Ranger 1 spacecraft attempted to land a payload with an impactor before the US had figured out how to land more carefully.
  - swagasaurus-rex 4y ago
    There is no terminal velocity on a body with no atmosphere
kryptiskt 4y ago
Earth is pretty easy to do if you do it the simplest way, via atmospheric braking all the way down. You have to have a heat shield and parachutes, but if you have the equipment the atmospheric braking doesn't require any fancy calculations besides getting the entrance angle right (so you don't bounce off into space or burn up by going too steep). Mars is probably the worst of both worlds, you have a thin atmosphere that is a nuisance but not very good for braking.

Delk 4y ago

On the other hand, there's no atmosphere on the Moon to speak of, so you actually have to slow down using propulsion against the gravity for a soft landing. Heat from the atmospheric entry when getting back to the Earth is certainly a concern but the atmosphere also takes care of the vast majority of slowing the descent (or ~all of it, if you count the use of parachutes at the end).

TeMPOraL 4y ago

Earth gives you a stupidly wide margin of error, where the Moon gives you none (and Mars very little) - because of its dense atmosphere. Landing on a world with an atmosphere means there is a passive force that tries to get you stop (relative to the surrounding medium). So, as long as you survive the hell of reentry, you can help yourself with lifting surfaces and parachutes. In contrast, on the Moon, you have to zero out your velocity at exactly the right moment. Do it too late? Crash. Do it too early? Crash. Run out of fuel just a little bit too fast? Crash.

bitL 4y ago
Dumb laser distance measurement should solve that easily for you. You can literally use some very basic math with thruster output based on your downward distance/speed.
- hgsgm 4y ago
  But you need to calibrate that to extremely high tolerance. That's harder than not needing to care at all.
- fooker 4y ago
  It's not that simple. What happens when you over-compensate with the thruster? Sometimes you can recover from that, sometimes not.
- dexwiz 4y ago
  What if you kick up a bunch of dust?
  - bitL 4y ago
    That would be very near the surface and with 10x lower gravity one could afford some error.
    - TeMPOraL 4y ago
      With 1/10 gravity dust also goes quite a lot higher. That's on top of lack of atmosphere letting dust float higher. Not to mention, there's going to be a lot more dust to kick up. Anyway, the easiest way to get a feel for these things is to play some Kerbal Space Program. Try doing some moon flights. You'll quickly realize that the part of the mission that feels easiest on the home planet (landing) also feels the hardest on atmosphere-less bodies, and vice versa (e.g. taking off, or slowing for landing).
- TeMPOraL 4y ago
  Sure. The problem is delta-v. If you zero out too late, you crash, but if you zero our too soon, you'll likely run out of fuel and either crash or won't be able to take off or otherwise complete the full mission. Managing delta-v is half of what makes a Moon landing require precision. An atmosphere can let you ignore this half to a degree (depending on specifics of vehicle/mission).
hgsgm 4y ago
People go to the moon for the food not the atmosphere. ;-)
- ithkuil 4y ago
  Bring your own atmosphere!
- kwhitefoot 4y ago
  Time to watch A Grand Day Out with Wallace and Gromit again!
rbanffy 4y ago
> you have to zero out your velocity at exactly the right moment. Not quite. You can compensate with a landing gear that can survive your terminal speed going to zero very quickly and absorb the extra energy. The math is much simpler on the Moon because there is no atmosphere.
- doodlesdev 4y ago
  Show that landing gear that can handle an impact at 1km/s, I'm pretty sure there's a lot of nation states out there that would be interested.
  - sephamorr 4y ago
    It's less unreasonable than you think. The Ranger 1 spacecraft attempted to land a payload with an impactor before the US had figured out how to land more carefully.
- swagasaurus-rex 4y ago
  There is no terminal velocity on a body with no atmosphere

kryptiskt 4y ago

Earth is pretty easy to do if you do it the simplest way, via atmospheric braking all the way down. You have to have a heat shield and parachutes, but if you have the equipment the atmospheric braking doesn't require any fancy calculations besides getting the entrance angle right (so you don't bounce off into space or burn up by going too steep). Mars is probably the worst of both worlds, you have a thin atmosphere that is a nuisance but not very good for braking.