[Benjo]

"For space travel to be truly practical, we need to come up with some sort of reactionless (or almost) drive. We're never going to get anywhere by the high-tech equivalent of throwing rocks out the back to make the car move forward."

Here's the thing I've never understood. If someone were to come up with a new kind of space drive hundreds or thousands of times faster than what we have today, what happens when the spacecraft hits dust or other fine but ubiquitous space debris, as it surely must at some time or another during a trip to the nearest star? At speeds approaching some large fraction of the speed of light won't a collision with even so much as a grain of sand be like taking multiple rounds from a 120 mm cannon?

[NicknamedBob]

"At speeds approaching some large fraction of the speed of light won't a collision with even so much as a grain of sand be like taking multiple rounds from a 120 mm cannon?"

Yep! That's why you pack the luggage in the forward compartment. Seriously, sorry, I meant to say seriesly, the apollo missions had heat shields for re-entry when they went up, didn't they?

Just put the extra toothpaste, and the folding lawn furniture in the front. (In case you hit a deer.)

[thedugal]

Well, space junk is a serious problem, however the speed of your vehicle is basically irrelevant. You could be sitting perfectly still and a piece of space dust travelling a few zillion miles per sec could hit you. The dust particles are tugged by gravity and after awhile out there they can really pick up a decent velocity, even better than an unladen swallow. I think it's basically like lightning though. If it hits you, yeah, you are shagged sure as shiola, but, it's not like people are falling over dead everyday from bolts to the head.

[Restorer]

If someone were to come up with a new kind of space drive hundreds or thousands of times faster than what we have today, what happens when the spacecraft hits dust or other fine but ubiquitous space debris, as it surely must at some time or another during a trip to the nearest star? At speeds approaching some large fraction of the speed of light won't a collision with even so much as a grain of sand be like taking multiple rounds from a 120 mm cannon?

At any significant fraction of the speed of light, even atoms of hydrogen become rather dangerous. As others have pointed out, this danger would necessitate some sort of shield. Luckily, this would probably be available as a byproduct of a reactionless drive. There are even designs that sweep up particles and atoms in front of the the ship and use it as fuel. Bussard ramjet, if I remember correctly.

Not that anybody's demonstrated a working model yet. :)