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Sure, if you ignore all the other problems. Why not fly to the Venus and test there? Why not on Saturn? The reason is simple, the cost of doing so outweighs the

by nickik 10y ago

Sure, if you ignore all the other problems. Why not fly to the Venus and test there? Why not on Saturn? The reason is simple, the cost of doing so outweighs the benefits of doing so. Look, its simply almost as expensive to make such a test on the moon, as it is on mars. If mars is your goal, then testing on the moon makes absolutely no sense. People seem to believe that because the moon is closer, it means its much easier there. The reality is that the deltaV is not that different, plus mars gives nice tools to actually land, like an atmosphere.

dragonwriter 10y ago
OTOH, if you are "rapidly" (insofar as that is possible in space technology) iterating on some aspect of the system that is relatively insensitive to the differences between the Moon and Mars, the Moon being closer shortens your cycle time considerably. Delta-V isn't the only cost you might be concerned with.
- nickik 10y ago
  True, but nobody has the money to develop something that needs multible moon test mission before it ever gets used productively. Also, SpaceX has actually thought about the problem quite a bit and until somebody payed them they clearly said they are not going to do anything moon related. They clearly did the same calculations. Moon is only worth it, if you actually want to go to the moon.
  - gravypod 10y ago
    > True, but nobody has the money to develop something that needs multible moon test mission before it ever gets used productively. No one would be willing to fund anything that hasn't gone through at least one viability test on the closest short term analogous scale.
    - nickik 10y ago
      You can repeat that as often as you want, that does not make it true. SpaceX clearly did not think that for their mars architecture needed a moon testing step. Mars concepts such as Mars Direct did not include such a step. Unless you have extreme time pressure its much, much, much preferable to test on Earth and Mars, rather then the Moon. Earth because it's by far the cheapest, and it shares a reasonable amount with Mars, and Mars because that's where you want to go anyway. The only advantage that the moon has is that it is closer than mars in pure distance. That advantage is eradicated when you consider the much more important DeltaV.
      - palunon 10y ago
        Isn't Earth closer to Mars half a (lunar) month ?
        Symmetry 10y ago
        No. The Moon is in orbit around the Earth and both the Earth and Mars are in orbit around the sun. The Moon varies between 360,000 and 400,000 km away and the closest Mars gets to us is 54,600,000 kilometers. Those numbers are a bit of an exaggeration because how much fuel you need is just as important as absolute distance/time to travel and for that you need 4 km/s of delta-v to get from Earth orbit to Moon orbit and 5.7 km/s to get to Mars orbit.
        palunon 10y ago
        I meant that the Earth is closer to Mars than the Moon is, not that Mars is closer to Earth than the Moon is (of course it isn't, that would make for a big, big blood moon)
        Symmetry 10y ago
        Oh, in that case then the relevant metric is probably delta-v in which case the Moon is always a lot closer to Mars than th Earth is. See here: http://i.imgur.com/SqdzxzF.png http://i.imgur.com/SqdzxzF.png
ctdonath 10y ago
The difference between a human-cargo test run of days vs months/years is substantial.
- gravypod 10y ago
  Exactly and that's not mentioning the benifit of being able to easily recover a failed prototype or even just analysis of a failed prototype. Imagine trying to diagnose a problem that comes up 10 months into the program after the probe is no longer in a direct visual path or while communications via microwaves start taking 10-20 seconds from point to point. There are too many questions to be answered before we can just say "you know while we're at it lets just risk the lives of 10-20 people, who cares right?"
  - JoeAltmaier 10y ago
    On the other hand, 10-20 people in a bus out of control on a mountainside can get in the news, even if it ends in tragedy. But we don't color our whole view of auto travel because of it. We don't make policy or even many business decisions because of it. Space travel can be dangerous. We will likely lose some crews. Rescue will always be too far away to matter. But I think folks will do it anyway.
    - TeMPOraL 10y ago
      Of course they will. Say to me, "There's too much danger." Say we could be lost, And I reply, "I'm no stranger To danger. That's the cost." https://www.youtube.com/watch?v=Oet1j9EA2fs&index=10&list=PL8394E70A4DB81B5D https://www.youtube.com/watch?v=Oet1j9EA2fs&index=10&list=PL... (Incidentally, this lovely song is a good answer to a lot of comments on this story.)
  - el_isma 10y ago
    I don't understand why the human cost of space travel is always weighted so heavily. All exploration costs lives. How many died conquering America? Exploring the poles? The Everest? To this day people die in the Everest. It's their choice to risk their lives in such a way. Yet I don't hear people saying "don't go there". Space is riskier, but also has more rewards.
jobu 10y ago
> plus mars gives nice tools to actually land, like an atmosphere. Actually Mars is a pretty difficult place to land. The atmosphere is just thick enough to require heavy heat shields, but too thin for parachutes to be sufficient. Here's a Wired article that talks about some of the issues - https://www.wired.com/2011/11/landing-on-mars/ https://www.wired.com/2011/11/landing-on-mars/
- lutorm 10y ago
  Still, you can get rid of most of the velocity for a Mars landing using aerobraking alone. To land on the Moon you'll be bringing all of that delta-V with you as fuel. Landing a Dragon on Mars already takes a lot of propellant: https://youtu.be/ZoSKHzziLKw https://youtu.be/ZoSKHzziLKw
- Symmetry 10y ago
  Even if Mars's atmosphere only kills 90% of your orbital velocity instead of the 99% you'd get on Earth[1] you're still saving a humongous amount of fuel, far more than the mass of the heat shield you have to carry along. If you're designing a new heat shield for each mission then the added complexity might make that not worth while but SpaceX is reusing the same heat shield design they've been testing on Earth so that isn't a concern. [1] numbers vague guesses

by nickik 10y ago

dragonwriter 10y ago

OTOH, if you are "rapidly" (insofar as that is possible in space technology) iterating on some aspect of the system that is relatively insensitive to the differences between the Moon and Mars, the Moon being closer shortens your cycle time considerably. Delta-V isn't the only cost you might be concerned with.

nickik 10y ago
True, but nobody has the money to develop something that needs multible moon test mission before it ever gets used productively. Also, SpaceX has actually thought about the problem quite a bit and until somebody payed them they clearly said they are not going to do anything moon related. They clearly did the same calculations. Moon is only worth it, if you actually want to go to the moon.
- gravypod 10y ago
  > True, but nobody has the money to develop something that needs multible moon test mission before it ever gets used productively. No one would be willing to fund anything that hasn't gone through at least one viability test on the closest short term analogous scale.
  - nickik 10y ago
    You can repeat that as often as you want, that does not make it true. SpaceX clearly did not think that for their mars architecture needed a moon testing step. Mars concepts such as Mars Direct did not include such a step. Unless you have extreme time pressure its much, much, much preferable to test on Earth and Mars, rather then the Moon. Earth because it's by far the cheapest, and it shares a reasonable amount with Mars, and Mars because that's where you want to go anyway. The only advantage that the moon has is that it is closer than mars in pure distance. That advantage is eradicated when you consider the much more important DeltaV.
    - palunon 10y ago
      Isn't Earth closer to Mars half a (lunar) month ?
      - Symmetry 10y ago
        No. The Moon is in orbit around the Earth and both the Earth and Mars are in orbit around the sun. The Moon varies between 360,000 and 400,000 km away and the closest Mars gets to us is 54,600,000 kilometers. Those numbers are a bit of an exaggeration because how much fuel you need is just as important as absolute distance/time to travel and for that you need 4 km/s of delta-v to get from Earth orbit to Moon orbit and 5.7 km/s to get to Mars orbit.
        palunon 10y ago
        I meant that the Earth is closer to Mars than the Moon is, not that Mars is closer to Earth than the Moon is (of course it isn't, that would make for a big, big blood moon)
        Symmetry 10y ago
        Oh, in that case then the relevant metric is probably delta-v in which case the Moon is always a lot closer to Mars than th Earth is. See here: http://i.imgur.com/SqdzxzF.png http://i.imgur.com/SqdzxzF.png

ctdonath 10y ago

The difference between a human-cargo test run of days vs months/years is substantial.

gravypod 10y ago
Exactly and that's not mentioning the benifit of being able to easily recover a failed prototype or even just analysis of a failed prototype. Imagine trying to diagnose a problem that comes up 10 months into the program after the probe is no longer in a direct visual path or while communications via microwaves start taking 10-20 seconds from point to point. There are too many questions to be answered before we can just say "you know while we're at it lets just risk the lives of 10-20 people, who cares right?"
- JoeAltmaier 10y ago
  On the other hand, 10-20 people in a bus out of control on a mountainside can get in the news, even if it ends in tragedy. But we don't color our whole view of auto travel because of it. We don't make policy or even many business decisions because of it. Space travel can be dangerous. We will likely lose some crews. Rescue will always be too far away to matter. But I think folks will do it anyway.
  - TeMPOraL 10y ago
    Of course they will. Say to me, "There's too much danger." Say we could be lost, And I reply, "I'm no stranger To danger. That's the cost." https://www.youtube.com/watch?v=Oet1j9EA2fs&index=10&list=PL8394E70A4DB81B5D https://www.youtube.com/watch?v=Oet1j9EA2fs&index=10&list=PL... (Incidentally, this lovely song is a good answer to a lot of comments on this story.)
- el_isma 10y ago
  I don't understand why the human cost of space travel is always weighted so heavily. All exploration costs lives. How many died conquering America? Exploring the poles? The Everest? To this day people die in the Everest. It's their choice to risk their lives in such a way. Yet I don't hear people saying "don't go there". Space is riskier, but also has more rewards.

jobu 10y ago

> plus mars gives nice tools to actually land, like an atmosphere. Actually Mars is a pretty difficult place to land. The atmosphere is just thick enough to require heavy heat shields, but too thin for parachutes to be sufficient. Here's a Wired article that talks about some of the issues - https://www.wired.com/2011/11/landing-on-mars/ https://www.wired.com/2011/11/landing-on-mars/

lutorm 10y ago
Still, you can get rid of most of the velocity for a Mars landing using aerobraking alone. To land on the Moon you'll be bringing all of that delta-V with you as fuel. Landing a Dragon on Mars already takes a lot of propellant: https://youtu.be/ZoSKHzziLKw https://youtu.be/ZoSKHzziLKw
Symmetry 10y ago
Even if Mars's atmosphere only kills 90% of your orbital velocity instead of the 99% you'd get on Earth[1] you're still saving a humongous amount of fuel, far more than the mass of the heat shield you have to carry along. If you're designing a new heat shield for each mission then the added complexity might make that not worth while but SpaceX is reusing the same heat shield design they've been testing on Earth so that isn't a concern. [1] numbers vague guesses