Originally Posted By: paul
Acceleration to near-C, at a constant 1G, takes about 1 year

how much fuel will that acceleration cost?

Depends on the mass of the vessel, and how close to C you try to get.

Originally Posted By: paul
what happens when you encounter a grain of sand traveling at
near c.

Depends. If you have a waffle shield, it gets a hole in it and the particle is deflected. If not, your ship experiences a collision with the force of a small nuclear explosion.

Originally Posted By: paul
how much energy would it cost to detect that grain of sand
at a distance far enought to allow for a course change.

Why bother? Deflecting a particle of sand is a much lower-energy option than moving the whole ship, and if a passive shield is used, requires no pre-detection of anything other than large objects.

Originally Posted By: paul
and we dont yet know if there is anything in the void between the solar systems that would allow for a course change.

You don't need anything to "allow for" a course change - its all Newton's 3rd law: every reaction has an opposite and equal reaction. So you throw some propellant out with a force of 1N, your ship'll experience an equal force in the opposite direction and thus be accelerated in a direction opposite that of the propellant.

That propellant doesn't need anything to "allow" for it to provide that opposite reaction.

Originally Posted By: paul
how do you know that there are even enought ions in the void?

Why do I need ions?

BTW, the density of hydrogen in interstellar space is already known, to a point where we've mapped its density both locally, and across much of the milky way: