Friday, November 18, 2005

The Real Prospects for Interstellar Travel (Part II)

Basically, conventional rockets won’t be able to get us up to light speed. There are other ways to work the problem, however:

1. Don’t go so fast: Build a generation-ship. Generation ships are ships that only accelerate up to a small fraction of the speed of light, and then coast sloooowly towards another star system. If you had that fusion propulsion thing worked out, and only wanted to go 5% the speed of light, you could get mass ratios of 148 or so. A base built on a comet that uses its water as a fuel source might be a potential example of one of these. Of course, they’re called generation ships, because they would have to accommodate multiple generations of astronauts! A trip to Alpha Centauri no longer takes 5 years, but 100!

2. Bussard Ramjet – It might be possible to scoop fuel from interstellar space. A bussard ramjet works by accelerating conventionally up to some fraction of the speed of light, where it then turns on a giant magnetic scoop. This creates a huge magnetic field in front of the vehicle, several tens to hundreds of miles wide. Through some sort of ionizing laser or radio pulse, the ramjet ionizes the interstellar hydrogen in front of it (if it isn’t already ionized) and draws it into the vehicle, where it is fusioned and expelled out the back, faster than it was drawn in. In order to work, the fuel must be expelled at a greater velocity than it is taken in. The interstellar medium is very sparse, however, and so this vehicle must attain a good running start.

3. Don’t shoot the fuel from the spaceship – shoot it to the spaceship! If you fire the fuel at the spaceship in the form of a neutral particle beam, you don’t have to keep it onboard, or use an onboard power source to accelerate it. The fuel can be accelerated from a gun located back in the home star-system. The fuel exhaust velocity can be however high you want it to be, due to the fact that you can put any amount of external energy into it as it leaves the accelerator gun. The vehicle would grab the fuel by ionizing it and magnetically reacting against it, as it comes in, thus gaining velocity. The vehicle’s design could then focus on bearing and supporting payload and astronauts, rather than packing in fuel energy. The amount of fuel required to accelerate the spacecraft no longer depends exponentially on the speed you want to achieve, but rather in a different non-linear way, so that the limitations that rockets have are no longer relevant.

The governing differential equation is dv/dt = mdot(ve-v)/m, assuming complete deceleration of the particle beam by the spacecraft. Mdot is the rate of mass expulsion at the solar system. Ve is the velocity of the particle beam. M, and v are mass and velocity of the spacecraft. This doesn’t take relativity into account.


Interstellar travel is difficult to accomplish, and requires a lot of technology that we don’t have yet. Does this mean we can’t do it? No, I don’t think that it does. For one thing, even if we’re limited to the technology that we know of today, we can still, just barely, make these sorts of voyages. And there’s no telling what we will discover, or what refinements to our technology will be made in 1000, 2000, 10,000 years of human history. There’s no way that the ancient Chinese could have made it into orbit or to the moon, even though they were the inventors of the rocket. There’s no reason to suspect that we’ve made the last inventions or discoveries in science or technology, or that we’re converging on the “end of human knowledge”, as some would like to believe.

But I also don’t expect it to happen in our lifetime, or our children’s lifetimes. It will take a long hard climb up in our space travel capabilities. Before we start launching spacecraft to the stars, we first have to be capable of conquering the solar system. We have to learn to crawl, before we can run. We have to be able to make 3 year voyages, 5 year voyages, 10, ect before we can start zipping off to the stars in 20+ years trips. We have to be able to construct decent bases and stations in space before we can start constructing giant accelerator guns to propel starships. We have to be able to make 30 kps dv missions before we can make 3*10^5 kps dv missions. So we have a long way to go, and I think that space travel in our lifetimes, in this generation, and those immediately after ours will have more to do with conquering the solar system than with zipping off to other star systems.


Anonymous Anonymous said...

Our first interstellar voyage may in fact be accidental. If Alpha Centauri is a mere 4-5 light years away and the Oort cloud extends to an estimated 3 light years, human exploration of our outer solar system may allow us to slip into the next with then conventional means.

Monday, November 21, 2005 11:18:00 AM  
Anonymous viagra online said...

What an interesting information in fact I was watching and important documentary in History channel related to Interstellar Travel, it was perfect because it also related to the time machine.

Monday, May 02, 2011 8:23:00 AM  

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