About human space travel times.

I know this has popped up as an aside in several previous threads, but search is not my friend.

At the maximum acceleration a human body can stand, how far could you reach any nearby star of interest? Assume the impossible: That you carry enough fuel to maintain acceleration all the way to the half point and then to stop all the rest of the way.

What is this acceleration, btw? What are the assumptions we are making to call this acceleration survivable (for a long time)?

How does this acceleration compare to what our actual machines, probes and robots can withstand? What would be the transit time for this acceleration?

What is the maximum acceleration we are now getting from our modern rockets? And again, what would be the transit time for this acceleration if you had the fuel to keep it burning all the way there?

Thanks in advance, I know there are many here who know these from the top of their heads.

Interesting question. I know the human body, with training, can withstand 9 G’s - briefly.

I’ve experienced 3 G’s and found it to be quite uncomfortable.

If you are talking about long term sustained acceleration, I’d say no more than 1 - 1.5 G’s. Which would be 10-15 m/s^2.
What kind of travel times are we talking about? 10hrs? 10 days? 10 years? It makes quite a difference - even in the hypothetical.

According to the math on this page, from Earth to Alpha Centauri (4.37 Ly) takes about 6 years from Earth’s point of view, 3.58 subjective, if you can accelerate at 1 g constantly. If you bump that up to 2 g, it would take 5.25 and 2.32 years, respectively. Even 2 g seems high, though. I’m not sure if humans can survive that over a time period of years.

Another thing to consider, though: in the Honor Harrington universe, maximum ship velocities (relative to the background of particles in the galaxy, I assume) is limited to about .8 c. The idea is, if you at a high fraction of light speed, you wind up hitting particles which are traveling at high fractions of light speed relative to you, which would probably do bad things to your ship and anyone living inside. I don’t know if the math checks out or not, but something worth considering.

My quick calculations suggest that if you were to accelerate for 10 yrs and then decelerate for 10 yrs at 15 m/s^2 you could make it about 300 light years.

I figure a 40 yr round trip is pretty much the max of Human endurance.

I did the math for 2 g acceleration under Newtonian rules, and got a result of about a 3 year trip to AlphaCent. However, even 1 year at 2 gs would put you up past the speed of light. :smack: So, it’s hard to answer the question without getting into relativistic effects and particle bombardment.

Darn it. Good point. Cap the speed at whatever reasonable limit makes sense.

Great link. That however, doesn’t seem to account for the stated problem of running at FTL speeds, or does it?

Don’t despair: The Newtonian calculation actually gives the correct result, if you’re interested in the shipboard time. If you want the time relative to Earth, it gets a bit more complicated.

Unfortunately, reasonable estimates vary quite widely on this, because we have never been able to do experiments on the effects of particle bombardment on living tissue at relativistic speed.

Let’s agree on .8c since it was good enough for some Sci-Fi writer.
ETA: btw, it seems to me that acceleration is pretty much an irrelevant issue since we hit c very early on the trip. Does it amount to much once we get cap speed?

I’m not an expert, but if your equations get you travelling over light speed, they are clearly wrong. Perhaps the calculations assume Newtonian physics, and you should be using relativistic physics.

Not if you active your warp drive.:stuck_out_tongue:

But, anyhoo, I am curious about travel that is realistic in speed like the Apollo program, or our deep space probes. 0.8c would be nice, but we have no way in hell of achieving that yet…

I’m sure better physicists than I will be along to clarify, but even when you’ve accelerated to near light speed relative to your origin, velocity is still relative. The above scenario describes some absolute frame of reference - which basically gets you back to the old ether theories.

So accelerate yourself to .8c relative to earth and then “coast”. You can consider yourself as motionless as you were when you woke up that morning, and truthfully say that the earth is now moving away from you at .8c. And then start worrying about all the particles the earth is going to hurtle itself into.

I do not think it works like that in this case. The spaceship is clearly the one who underwent an acceleration so in this case we know the spaceship is moving. Particles are going to be zapping your ship in a serious fashion when you get going at relativistic velocities.

Not if once you hit .8c you coast all the way to the limits of the known universe :slight_smile:

But to disagree with audient, everything “large” in nearby space is going at subrelativistic speeds, so accelerating to .8c relative to earth means we will be accelerating to basically .8c relative to everything else we know of in our Local Group of galaxies.

So, anyone with the tools to estimate a travel time once you cap top speed at .8c? Does acceleration make much of a difference in transit time?