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Why is it so hard to reach the Moon?

  1. Dec 27, 2008 #1

    fluidistic

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    Hi PF,
    What are the major difficulties the man faces when he tries to reach the Moon?
    Is it because he has to return to Earth, so that the difficulties are more than doubled?
    Because the distance between the Earth and the Moon is not that big compared to Earth's equator circumference (less than 10 times). But as there are so few men that walked on the Moon, it is clear that there are big difficulties. What are they?
    Thanks.
     
  2. jcsd
  3. Dec 27, 2008 #2
    Environment in space is terrible, plus we haven't bothered investing in cheaper alternatives to getting stuff out of our gravity well until recently.
     
  4. Dec 27, 2008 #3
    Its a lot easier to walk around Mount Everest than to walk up it.
     
  5. Dec 27, 2008 #4

    Janus

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    Tribdog hints at a part of the problem. The distance traveled isn't so much of a factor as the energy involved climbing against Earth's gravity. For instance, it takes more energy to reach the Moon, as close as it is, than the additional energy needed to continue to Mars.
     
  6. Dec 27, 2008 #5

    fluidistic

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    I think I get it. So it is much easier to travel from Moon to Earth than from Earth to Moon.
    So the big problem is the energy required to "get out" of Earth's gravity? (Of course I know it's impossible to leave entirely Earth's gravity, but one can go so far that it becomes of non importance compared to Moon's gravity)
     
  7. Dec 27, 2008 #6

    Astronuc

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    It takes a lot of energy just to get into orbit, e.g. at the level of ISS. NASA only sends 6-8 astronauts each mission. Manned spaceflight requires taking a lot of support, e.g. an oxygenated environment, food and water. The craft then needs to shield the astronauts from radiation, but there the trade of is mass. And, the spacecraft must take the fuel necessary for the orbital maneuvers, or in the case of the moon, the propellant (fuel) required for orbital transfers to and from the moon. Add to that the mass of the propellant storage and rocket motors.
     
    Last edited: Dec 27, 2008
  8. Dec 27, 2008 #7

    fluidistic

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    Interesting. I thought that fuel's storage would be a problem due to its mass. I didn't realize it was that hard to get away from Earth, even if the distance you want to go is not that much.
    I see that sending humans is really hard due to many factors.
    But even without humans it seems really hard to make an object soft land on the Moon. Check out http://en.wikipedia.org/wiki/Moon_landing.
     
  9. Dec 27, 2008 #8

    Astronuc

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    The problem with the moon, and for that matter Mars, is that the Moon has no atmosphere and Mars has a very thin atmosphere, so aerobraking is out of question.

    In the Moon or planetary mission, the landing craft would first establish an orbit, then at some point, decelerate from orbital velocity down to landing, at near zero velocity.

    In the Mars Rover and other mission, they use large balloons that soften (cushion) the impact of landing.
     
  10. Dec 27, 2008 #9

    russ_watters

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    Well, Mars certainly has enough atmosphere for it to be useful - we've used aerobraking before and the rovers had parachutes to slow them down (similar idea).
     
  11. Dec 27, 2008 #10
    On Scientific American Frontiers they had a fold up airplane that they wanted to use on mars. It was inside a capsule, a chute slows it down then the capsule pops open and the plane unfolds and flies around. so there is definitely enough atmosphere to matter.
     
  12. Dec 27, 2008 #11

    Astronuc

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    The atmosphere of Mars is useful, but a parachute by itself is not enough, hence the balloons to cushion the impact. The surface pressure is less than 1 kPa as compared to earth's surface pressure of ~100 kPa.

    The Rovers and Mars lander are small craft compared to an Apollo craft, and the parachutes are large compared to the payload.

    Some background:
    http://mars.jpl.nasa.gov/odyssey/mission/aerobraking.html
    http://mars.jpl.nasa.gov/odyssey/mission/aerobraking2.html

    Aerodynamic requirements of a manned Mars aerobraking transfer vehicle
    http://adsabs.harvard.edu/abs/1991afm....28..117B

    http://www.lpi.usra.edu/meetings/modeling2008/pdf/9025.pdf


    With respect to the Mars aircraft
    http://gltrs.grc.nasa.gov/reports/2003/CR-2003-212350.pdf

    http://marsairplane.larc.nasa.gov/platform.html
     
    Last edited: Dec 27, 2008
  13. Dec 27, 2008 #12
    They've gone to the moon many times. You would have thought it would've been a straightforward and relatively easy task to accomplish now.
     
  14. Dec 27, 2008 #13

    mgb_phys

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    It doesn't change the physics.
    A lot of people have climbed Everest but it doesn't make it easy
     
  15. Dec 27, 2008 #14

    D H

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    Exactly. That we have gone to the Moon doesn't necessarily make it easier to do so. It does give us a better idea of how hard the task is, and what to gotchas one needs to watch for. The same is true for climbing Everest.
     
  16. Dec 27, 2008 #15
    Very appropriately stated. Additionally, it's been so long since NASA has been geared up for lunar travel that the majority of personnel that were associated with the Apollo missions are long gone. Getting back into that frame of mind is kind of like when you have to pull out an instruction set for its Op codes to reprogram an old microprocessor whose architecture you’ve paid no mind for several decades. You can just sense the uncertainty and risk factors associated with lunar travel all over again.
     
  17. Dec 27, 2008 #16
    It was a lot easier before they took down the stairs.
     
  18. Dec 27, 2008 #17
    Plus we don't even have a rocket that could make it to the moon do we?
     
  19. Dec 28, 2008 #18

    russ_watters

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    It is easy. It's 50 year old technology. Von Braun was a superstar rocket scientist in his day, but there are thousands of engineers who could reproduce his accomplishments today. But wait - you don't equate easy with cheap, do you...?
    Exactly wrong: It is easier to climb Everest today! Any idiot with a decent bankroll and in half decent shape can do it! (and thousands have) Everest is, in fact, not a technically difficult climb. It requires little skill.
     
    Last edited: Dec 28, 2008
  20. Dec 28, 2008 #19
    Why is it so hard to reach the moon? Our arms are too short.
     
  21. Dec 28, 2008 #20
    What i'm saying is that if a job has been done like this enough then the necessary technology, building techniques and factories that would have costed a substantial initial investment to build would be in place now. Or what I should be saying is, whenever you fly on an aircraft, you're not required to pay for the whole million dollar vehicle to be built each time. Is it that hard to develop some reusable craft much like the shuttle to get to the moon?
     
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