Interstellar Travel

How Long Might a Generation Starship Last in Interstellar Space?
Silent Abyss

I'm only posting this video as a curiosity. It showed up on YouTube the day after I mentioned Nemesis by Isaac Asimov in a post. I only listened to few minutes. Who's got three hours to spend listening to someone drone on endlessly about generation ships? I dunno, maybe I could listen to him if I was busy doing something else, but usually, if I am working on something I will listen to music, which requires no thinking, as opposed to listen to someone talking. I do listen to audio books when I go for a walk. So far I've just been listening to free audio books on YouTube Music. Some are wretched, but some are okay. You have to be careful because if you search for Audio Books (or is it Books on Tape?), half of the items it returns will be music by a band with that name.

Nemesis did get me restarted thinking about interstellar travel. Right now the only way we know how to go anywhere in space is to throw things in the opposite direction. That can get us to the Moon, and can get robots to Mars, but rockets are not going to get us to the stars. Tsiolkovsky's rocket equation puts a limit on how fast you can go, but it is based on your rocket's exhaust velocity. If you can get that velocity up some large fraction of the speed of light, you could reach the stars, but getting that high velocity would be a bit of a trick. We can do it with particle accelerators, but they are only working with particles. We would need to do it with a continuous stream of matter.

Of course, none of this matters if we don't have a destination. Astronomers have detected numerous exoplanets, that is planets orbiting another star, but we have scant information about the planets themselves. One of these days astronomers are going to find one that looks like it might be worth going to visit. Probably want to send a probe before we send a manned mission. Also give us a chance to test our particle beam rocket engine.

Even with a super particle beam rocket engine, it will probably take years to get to the star. And it will  be even longer before we hear anything back from our probe. And will we even be able to hear it? Our current deep space network has limited bandwidth, and we are only dealing with small fractions of interstellar distances. Lasers might work, but I suspect it would have to be one heck of laser. Sending actual physical mail might work better, but you would need the probe to return home to deliver the package.

Problem: if you use half of your mass as reaction mass to get you to your cruising velocity, then you will need half of your remaining mass to slow down to visit your destination. Now you've collected your data and you want to return it to Earth, you will need half of your remaining mass to accelerate to cruising speed, and then half of you remaining mass to decelerate.

100   tons x 50% = 50   tons

 50   tons x 50% = 25   tons

 25   tons x 50% = 12.5 tons

 12.5 tons x 50% = 6.25 tons

Start with 100 tons and you will only have 6 tons for your spacecraft. But using only 50% of your mass is probably highly optimistic. You are more likely to need 90%, which means you will only have 10% of your starting mass that you began that stage with. So your fuel consumption will look like this:

100   tons x 10% = 10    tons

 10   tons x 10% =  1    tons

  1   tons x 10% =  0.1  tons

  0.1 tons x 10% =  0.01 tons

Start with a 100 ton rocket and you return package will only weigh 20 pounds. Somehow I don't think you are going to be able to make a nuclear powered particle accelerator rocket engine and pack it in a 20 pound package with all the necessary sensors and recording equipment. So a ton in this case is liable to mean a zillion pounds.

On the other hand, some whiz kid might conjure up a warp drive, an astronomer will discover a whole raft of possibly habitable planets, and all this speculation can be ignored.