I read recently that the space shuttle's top speed is approximately 8km/sec. However, since space is a vacuum and there is no air friction or anything like that to slow it down why wouldn't continually adding thrust lead to continual acceleration. What is it that causes the shuttle to top out at the current top speed?
Several reasons, One the amount of energy carried is limited
two being gravitationally bound to earth in the beginning and the sun afterwards, that's the reason space crafts that reached far into the solar system weren't send directly, but had a planned trajectory to get past several planets (usually it's earth -> Venus->earth->Jupiter -> ect depending on the planetary alignment)
What you are talking is theoretically true, but todays space craft waste most of the energy they have during lift off (and you can carry only a limited amount since if you carry more you need a bigger rocket that consumes and it is a vicious circle), other alternatives, such as plasma, or solar sail, offers this possibility a little continuous trust/acceleration trough a long period of time, while the acceleration is nothing compared to chemical power, overtime say one month you'll reach speeds faster than the current space crafts if you add to that the gravitational assist you end up with a pretty speedy space craft. right now these technologies are being tested and evaluated .
