Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

G-Forces in Space and the Human Body

  1. Jun 21, 2012 #1
    Having an infuriating time trying to explain physics to co works. The debate is as follows: to get to Pluto in a quick time, say less than a year, you would have to travel extremely fast. If it were possible to reach such a speed via rockets/ion drive the human body would be crushed under the extreme g-forces created to go you to that speed. Even in space if you accelerate you feel G-Forces. My co work is saying because you are strapped in to a seat and in zero g you will move at the same rate and will not feel any pressure.

    so to sum it up I need consensus on the following.

    1 Accelerating in space create G-Force on those in a space ship that you would feel.

    2 At extreme accelerations in a short space of time the human body would be crushed.
  2. jcsd
  3. Jun 21, 2012 #2


    User Avatar

    Staff: Mentor

    Yes, those are correct....but you might want to check the distance to Pluto. It is achievable in probably a few weeks or months at 1g.
    Last edited: Jun 21, 2012
  4. Jun 21, 2012 #3
    I was considering the New Horizons satellite travel time of 12 years to get to Pluto stops included. Would it be possible to get a more in depth explanation to the reasons behind G-Forces in space/vacuum as "You are correct" will not be a significant explanation for these fellows.
  5. Jun 21, 2012 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    The first statement is correct. Pluto is a long ways away. The second statement is not. A spaceship could get to Pluto in 18 to 20 days by accelerating at 1g all the way, but reversing direction halfway between so it can come to a stop. If you don't care to come to a stop, it's only 13 days if one accelerates at 1g all the time.

    You're right. There's no way using local experiments that can distinguish between being stationary on the surface of the Earth and being in a spaceship accelerating at a constant 1g. This is the equivalence principle, an guiding concept behind general relativity.

    So if a spacecraft could get to Pluto in just 18 days, why haven't we done that? Why is it taking New Horizons almost a decade to get to Pluto? The answer isn't the crushing acceleration. It's instead something called the ideal rocket equation. Build a rocket that can accelerate at 1g for 10 minutes: That's fairly easy. Build one that can accelerate at 1g for 30 minutes: That's a much harder task, much harder than just three times as hard. It's exponential rather than linear. Build one that can accelerate at 1g for 18 days: The amount of fuel needed would rival the mass of the Sun.

    New Horizons is getting to Pluto by falling there. That's why it is taking so long. Every spacecraft spends almost all of its time falling, hardly any time thrusting.
  6. Jun 21, 2012 #5
    Now I'm confused. The issue was if you were in a shuttle in space and you went from zero to say 50gs of force in a minute it would kill you.

    The assumption being you go from park to boom then float to pluto.
  7. Jun 21, 2012 #6
    You're right that extreme accelerations will kill you just as effectively in space as they will on Earth. If you accelerate in a car you feel pushed back into your seat; if you accelerate in a spaceship you are pushed back into your seat in an identical fashion. Gravity has nothing to do with this effect.

    So you're right; if you want to do one short blip of acceleration at earth and then coast to Pluto, the g forces would probably be deadly. Playing around with the numbers, if you want to coast to Pluto in 2 weeks you need to be going around 5000 km/s. If you want to accelerate to that speed over the course of a one minute rocket burn that's an acceleration of about 80,000 m/s^2 or around 8000 gees. At a guess this acceleration would squash you so flat as to be unrecognizable.

    The above posters are pointing out that you can still get to Pluto quickly as long as you do a longer, gentler rocket burn. I guess this is tangential to what you are trying to learn about.
  8. Jun 21, 2012 #7

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    On the other hand, if you want to coast to Pluto in a year, that's a Δv of about 168 km/s. Limit the acceleration to 1g and a rocket would only need to be firing for 4.75 hours or so. No problem! Big problem. We can't make a rocket that accelerates at 1g for 4.75 hours or so.
  9. Jun 21, 2012 #8
    Thanks gents. You've educated 2 architects and one designer. Further question for me personally. If you were to eliminate the oxygen in your lungs IE oxygenated liquid and kept your body in a pod with liquid would you be able to withstand more G's theoretically? This is experimented with in deep sea however I'm not sure about space.
  10. Jun 21, 2012 #9

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Theoretically, yes. There's still an upper limit due to the brain. An acceleration of 100 g for even a short period of time will cause a concussion. I don't know what the limit would be for a sustained acceleration.

    NASA and the DoD experimented with liquid breathing back in the 1960s and 1970s (and I'm sure Russia did, too). Nothing came of it. It's far easier just to keep the acceleration within limits. G suits, now that's a different matter, but that's not liquid breathing.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook