a fun-back-of the-napkin-calculation wrt interstellar space travel. Suppose, you have the technology to accelerate a spacecraft to 0.5 lightspeed. The distance to travel is 10 lightyears, which takes some 20years.
Now consider that: You need a fairly large vehicle, which is not the size of your typical dunebuggy, right? I estimate the the frontal area in the direction of travel to be something like 10mx10m =100m2. This computes to a volume travelled of roughly 50000km3, threogh which the spacecraft ploughs through. Not much, You probably say. Our nearby friend, the moon, is 300 000km away. --- Now comes the funny part. Interstellar space is not empty. We just do not know, how empty it is. Now lets assume that there are some particles of 1gram in this volume, which has to be ploughed through. Is this a problem? Well. Consider a cannonball with mass 100kg and speed 1000m/sec (Mach3). Mightier than the mightiest kinetic weapon ever designed. Its kinetic energy is 5e7Wsec. This is quite abstract, and one has to figure out what that means. A bullet like that would destroy ANY fortress ever designed on this planet. Now back to interstallar space. Let us assume, that a 100kg bullett is NOT found in every 50000km3. Make it tiny. Say 1 gram. What would be the impact? According to my back-of the napkin calculations this tiny particle would have an impact of 2e5 times the one of this ancient bullet. To match the impact, and i'm being a bit generous here: A particle of mass 1e-8kg = 1e-5g =10ug would have that same impact as this magical bullet. How big would that particle be? Assuming a density of 1 (Water), one liter -10cm3- would be 1000g, one centiliter -1cm3- would be 1g one milliliter-1mm3- would be 10ug. ------------ So here we are:: ANY PARTICLE OF SIZE 1mm3 in the way, over a travel-distance of 10 lightyears, would completely destroy the spacecraft! (actual numbers actually go to the worse side. optimists are in the defensive here.) We do not know what actually is the case in interstellar space. How could we know what particles are in interstellar space? Only indirectly, through probabilistic ectrapolation of wht we know already. Ignorance of basic numbers probably is a bad advice.
