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Comment by cryptoz | Hacker News Reader

Yes it would be possible. Project Orion in the 1950s demonstrated that using nuclear pulse propulsion, you could accelerate to (max) 0.1c for intersteller missions. If we were to actually spend a little bit of money and effort on real space travel - which we're about to, if SpaceX and Plantery Resources succeed - I think it's completely reasonable to think that we will launch (and perhaps arrive) on these planets in our lifetimes.

It's optimistic, sure, but it's possible.

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0.1c would make this a 220 year journey each way. So what you're really saying is: no, it's not possible reach these planets in our lifetime.

I was imagining we would also be researching biology at the same time, and would be able to live that long or sleep during the trip. If we're able to move at .1c I can't imagine we'll still die as quickly as people born in 1900.

I saw a presentation on this at a DARPA meeting a couple years ago. It is exactly the kind of thing that could make long-term space travel possible.

http://www.wired.com/dangerroom/2007/12/zombie-mouse-ma/

There's a huge gap between "20 years from now, we won't die as quickly as people born in 1900" and "20 years from now, people will live four times as long as they do in 2013." The former is quite likely. The latter is pretty far out there.

Is that fast enough for relativistic effects on-board? It might be 220 years for those left behind, but for the people on the ship it might be less than a lifetime. They'd probably still have to raise children on the way to take care of the exploring when they got there.

218 years, instead of 220.

http://www.wolframalpha.com/input/?i=time+dilation+calculato...

There are no substantial relativistic effects at 0.1c; the time dilation factor is sqrt(c^2 - v^2), so about 0.995 in this case.

you have to take into account that the closer you get to light speed, the slower time goes. In other words, people on the space ship may age slower, meaning that 220 years may be a lot less to them. What is the equivalent of 1 earth year on a spaceship going at 0.1c, that is the question.

The relativistic correction is

Time_on_Ship = Time_on_Earth * (1 - (v/c)^2)^(1/2)

If v <<c we can use the Taylor approximation

Time_on_Ship =~ Time_on_Earth * (1 - (1/2) (v/c)^2)

Distributing

Time_on_Ship =~ Time_on_Earth - Time_on_Earth (1/2) (v/c)^2

With the current number, assuming instant acceleration to simplify the calculations:

Time_on_Earth = (22/.1)

Time difference =~ (22/.1) (1/2) * .1^2 years = 1.10 years

The important detail is that for small speeds, the correction is quadratic, so the difference is small unless the speed is very close to c. In this case the correction is only 0.5%.

Good stuff!

Thx Gus

The Lorentz factor doesn't really make an appreciable difference until you get much closer to c. For instance cutting the trip time in half would require .866c.

.99c would cut the trip down to ~30years.

> .99c would cut the trip down to ~30years.

Surely you forgot to take into account that increasing the speed shortens the trip as well as dilating time?

Travelling at 0.99c would take 22 years to reach that from an observer's point of view, which would be 3 years for the people on board

You're completely correct. I didn't take that into account. No more comments about relativity before coffee.

BTW How do we know that interstellar space is empty, that there is no dust, stones, etc.

Probably even very small objects make high-speed flights impossible because of collisions.

I believe we know that it's not empty. Take the asteroid belt: there are millions of objects there, yet we fly through it with interplanetary probes, no problem.

We know that space is not empty, but it's empty enough.

I've heard our asteroid belt described like this:

The odds of randomly happening on an asteroid while traveling though the asteroid belt is about the same as randomly happening on a ship in the Pacific Ocean.

Why max to 0.1c? Is there friction in space or so?

Sort of... the faster you go, the more energy you need to accelerate even faster. The Orion design, IIRC, basically involves sitting on a big iron plate and tossing nuclear bombs under it. Each bomb can only impart a certain amount of energy and at some point you're not going to accelerate any faster unless you shed some mass.

You will shed some mass in the form of the bombs you're blowing up, and whatever you break off the bottom of the plate you're blowing them up against, and I think the design requires a front shield too which will also deteriorate over time and shed mass, but those all probably balance out around the 0.1c velocity.

Last I read up on such ideas, the better design was to use a sail in front of the ship to catch the push from each explosion, then you only need the one radiation shield. I think it also allows for making a larger surface to catch the explosive push, and it depends on materials absorbing loads through tension rather than through compression.

Part of the problem is that you need to decelerate at the end. Maybe it's possible to use a gravity assist to decelerate from 0.1c inside the destination solar system, but otherwise you need to turn around and thrust the other way at about the half-way point.

Another part of the problem is the faster you want to go, the more fuel you need, which makes your ship more massive.

Use a mag-sail as a "brake" to slow down at the destination star.

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