## Interstellar travel?

I’m in the middle of reading Neal Stephenson’s Anathem (which is awesome by the way – please don’t post any spoilers since I’m not done!).  Anyway, it inspired me to think a bit about interstellar travel from a realistic point of view.  In other words, I decided to try to put some hard numbers to things.  Specifically, I wanted to put some relativistic numbers on a trip to the nearest extrasolar planet (which, as of now, happens to orbit Epsilon Eridani and is 10.5 light-years from earth).

So, the first thing I did was to look up about how many “g’s” the human body can take without blacking out.  It turns out it can handle about 5.  Then I made the assumption that my hypothetical spacecraft would be able to travel at 3/5 the speed of light (I’ll let the engineers figure out how to do that).  If our ship accelerates constantly up to that speed, it would take about 42 or 43 days or so which is really pushing it for the humans.  So maybe we draw that out a bit more – say we double it.

Next, I drew a spacetime diagram and, in the absence of a good ruler but with an approximate straight-edge, estimated that the ship would arrive (with a lengthier deceleration since the human body has a harder time with that I believe) at its destination in about 18.5 years of earth-time.  To the astronauts onboard this would actually be closer to 14.8 years since they spent a great deal of time travelling at relativistic speeds.  Let’s assume that, considering the time and effort expended to get them there, they spend a good two years checking the place out before heading home.

All told, that means that they would be gone for a total of 39 years of earth-time which, to them, would be closer to 31.6 years.  That’s a long time – but not an entirely unreasonable amount.  Consider this: Saint Simeon Stylites sat on a pole for 37 years.  Considering we just celebrated the 40th anniversary of the Apollo 11 Moon landing, just think if we had actually sent Neil, Buzz, and Mike – who are all still alive – to Epsilon Eridani instead (with the proper equipment, of course)!

In short, aside from the obvious engineering hurdles, interstellar travel does not seem quite as far fetched when you actually look at the numbers.  It’s a long way away from happening – if it ever does – but it seems to me that if the human race put its mind to it (and doesn’t annihilate itself beforehand), this feat could be accomplished (for one particular engineering idea on how to keep the astronauts alive, see Arthur C. Clarke’s Rama series of books).

### 11 Responses to “Interstellar travel?”

1. That’s pretty cool, but I think you missed two things. First, acceleration and deceleration are the same thing. Duh 🙂 Second, try calculating how much energy it would take to accelerate 3 average people at 2.5 G’s for 3 straight months, and compare it to the amount of energy released in the largest declassified H-bomb explosion (50 Megatons equivalent, about 2.1×10^17 Joules).

2. quantummoxie Says:

Indeed, from a physics standpoint, acceleration and deceleration are the same thing. But, for some reason or another, the human body responds differently to one than the other (at least that was what I had come to understand from NASA biologists).

Your second point is an engineering issue. I didn’t say this would be easy to do. 🙂 However, that was my next step. Just out of curiosity (since you may have saved me some time), is that number for the original H-bomb or for modern nuclear warheads which are considerably more powerful than the originals?

3. 50 Megatons was the Tzar Bomba, the largest ever detonated. It was originally designed for 100-Megaton yield, but the Russians calculated that this would cause 25% of all nuclear fallout in human history (which was a lot even back then), and vaporize the release plane, so they scaled it back for the test.

I guess deceleration would be harder on this crew, since they would have 30 years of microgravity-induced bone and muscle loss.

4. quantummoxie Says:

Right. Maybe that’s why the human body does have such a hard time with deceleration. But I can tell you from personal experience that, when I fly, I have no trouble with take off, but I get wicked queasy on the approach to landing (which is a deceleration). So there may be more to it than just a loss of bone density. But who knows.

Interesting stats on the bombs. I have to think hard about how they could be used in Stephenson’s idea. Actually, what I’d have to do is put some numbers to it. His idea is to use them for propulsive purposes – detonate them next to a blast shield attached to one end of the spacecraft. The explosion causes the ship to accelerate. You’d need to detonate them at intervals in order to achieve the sustained acceleration. Deceleration could be accomplished gravitationally or using the reverse effect (with a blast shield on the front).

5. Well, Stephenson didn’t invent the idea. The engineering is practically finished, the project was halted for lack of funding. http://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29

6. I’m curious to learn why deceleration would be different from acceleration. If you remember the source, could you provide a reference?

7. quantummoxie Says:

Hmmm. Now that I think about it, maybe I confused the acceleration/deceleration thing with the body’s orientation. I believe the body can, for some reason, tolerate more g’s of acceleration upside down than it can right side up (this is likely due to blood pooling in the brain?). Realistically, of course, no human being could tolerate even 80-90 days of constant, mild acceleration greater than probably 1 or 1.5 g’s.

8. > Well, Stephenson didn’t invent the idea. The engineering is practically
> finished, the project was halted for lack of funding

I think more the wiki page you quoted had more correct explanation: “The Partial Test Ban Treaty of 1963 is generally acknowledged to have ended the project”.

For really fascinating details, I strongly recommend George Dyson’s “Project Orion: The True Story Of The Atomic Spaceship”.

Paul B.

9. quantummoxie Says:

Interesting. So this is a pretty old idea, then. It would be an interesting nuclear dis-armament idea, too – take all the world’s nukes and put them to use in a spacecraft. (I do realize this not even remotely realistic, practical, or even wise, but it’s an intriguing thought, nevertheless.)

10. An acceleration of 1 g would actually be perfect, allowing the astronauts (in the full meaning of the word…) to keep their bone and muscle mass. Speeds approaching c would still be reached in less than a year. Let’s do it!
And yes, resistance to so-called positive g is much higher (up to 5 for the untrained person, up to 9 with training and technical aids) than to negative g (down to -3). The visual phenomena occurring at the limits are called blackout and redout respectively:
http://en.wikipedia.org/wiki/G-force#Human_tolerance_of_g-force

11. quantummoxie Says:

Actually, that would be perfect since you could design it to ease them into weightlessness. Though that still leaves like 16 years of weightlessness which can’t be too healthy (unless you tried a design a la 2001 that simulated roughly 1g for the whole trip).