Someone going by the handle of Speising made a comment on my post Follow-Up: Energy Requirements of Interstellar Travel, and the reply I composed grew so large that I decided to make it into a full post.

The comment was:

“So what about ram-jet like ships? probably quite useless (to vulnerable) as carriers for an invasion force, but they do not have the problem of carrying all that fuel with them.
also, of course, If we assume ET doesn’t want to spend 200 or more years making a round trip to Earth… doesn’t necessarily apply for ET’s with, eg., longer life spans than ours.”

Thanks, for the comment, speising.  Basically, you’re talking about a Bussard Ram Jet.  There’s a few problems associated with that.

You’d be scooping up hydrogen to use as a fusion fuel, but hydrogen’s not a particularly good fuel for fusion, believe it or not.  The proton-proton chain, which is the primary source of energy production in stars less than 1.3 solar masses, is a very slow process (like an average of one billion years per reaction in the first step), which is a good thing otherwise the sun would have burned out after just a few million years.

You could theoretically use the CNO cycle for hydrogen fusion, but the confinement and cooling requirements would likely be insurmountable.  We’re talking about temperatures and densities greater than that of the core of the sun.

Also, the interstellar medium isn’t as dense with hydrogen as Bussard thought it was, and you probably wouldn’t be able to scoop up enough fuel.

All this completely ignores the shielding requirements, which I never even went into in my earlier posts, mostly because I concluded interstellar travel was already impractical before even getting to the shielding requirements.  Traveling at speeds even at one tenth the speed of light, every particle of dust floating in space is going to impact your space craft with a lot of kinetic energy.

Let’s assume a particle of cosmic dust floating in interstellar space with zero velocity relative to the Earth.  Let’s also assume this particle is medium sized cosmic dust, say 300 micrometers in diameter, and let’s further assume it’s density is average for cosmic dust, 2.0 g/cm^3.  This particle has a mass of only 2.82X10-8 kg or .028mg.   If our vessel is traveling at 1/10^th the speed of light relative to Earth, that particle of cosmic dust is going to impact our spacecraft with a kinetic energy of 12 Megajoules.  To put that into perspective, lets assume a typical automobile mass of 1500kg (3300lb); that particle of dust is going to impact our spacecraft with the same kinetic energy as a car traveling at 454km/h (284mph).  How are you going to protect against that kind of collision, and what do you do if you run into a particle that was 10 or 100 time larger?  300 micrometers is pretty small; a strand of human hair is 100 micrometers wide.

In regards to the other part of your comment,

“If we assume ET doesn’t want to spend 200 or more years making a round trip to Earth… doesn’t necessarily apply for ET’s with, eg., longer life spans than ours.”

I’ll just add that even if an alien species were to have a significantly longer life span that humans, it wouldn’t necessarily follow that their perception of the passage time or their value of time were different than ours.  If science found a way to extend you lifespan to 1000 years, would you be interested in spending 200 years in a submarine without port if there was an alien planet at the end of the trip?  I think 200+  years is still a long time, no matter how many years you have ahead of you in life.