Preparation H. It’s wonderful stuff.
Back on topic - so [sup]235[/sup]U is not all that radioactive to begin with, but is wicked crazy for fissioning once you mix in some neutrons? Sounds like our interstellar project is back on track, then. 
Because as you slow down the ramjet scoops less and less fuel.
Sort of like acceleration in reverse. What’s the problem again?
But with the hydrogen scoops, when you’re trying to accelerate, you have to accelerate the hydrogen from zero momentum, whereas when you’re trying to slow down, the hydrogen is already going in the direction you want to be going. I wonder if you couldn’t just put out really wide sails that would run into the interstellar hydrogen and provide fuel-less deceleration.
The same thing would go for fuel ships you send out beforehand: if you make them of sufficient weight, and used a really long elastic band (to prevent shock to the inhabitants,) you would achieve additional braking by the mere fact of running into them to gather the extra fuel.
You could. You’d achieve a lot more deceleration by using it for braking thrust, though, rather than just deploying a “Bussard parachute”. 
The ramjet obviously needs some on-board fuel just to get going - like any other ramjet vehicle. And there’s no reason why it can’t use the fuel scoops to refill its tanks before actually using the engine to brake - then you have a fuel reserve for when you drop below ramscoop speed.
You can, but the change in velocity limited at best to something less than twice the speed of the planet. The actual analysis is somewhat more complicated than this but if you’re approaching any planetary sized object at 0.1c then the effect of a gravity swing-by is obviously negligable. Something the size of a galactic mass singularity would be necessary to effect a significant reduction in speed without subjecting the ship and its delicate human cargo to stresses far in excess of material limits.
Actually, even given a propulsion system with the very high specific impulse of I[sup]sp[/sup]=10,000 with sufficient thrust to actually move a massive object (roughly that expected from D-D or D-T fusion) you’re talking about a ratio of deceleration to acceleration propellant of somewhere on the order of 10000:1 or more (neglecting the mass of your actual cargo under the assumption that it is negligable compared to propellant mass) to attain that speed. Unless you can come up with some propulsion system that is massively more effective (i.e. a higher effective exhaust velocity for a given thrust level) than any known or speculated mass propellant (see the Tsiolkovsky rocket equation which very succintly lays out the effectiveness of a rocket motor) you just can’t carry enough fuel to reasonable decelerate. You can stage your system as iamthewalrus(:3= suggests (which is analogous in concept to multistage orbital boost systems used today) but the payload mass is such a small fraction of the overall starting mass–even a fraction of the mass required for deceleration–that it doesn’t really help much. Prepositioning propellant to be captured en route doesn’t really help, as you have to accelerate and/or decelerate them too, as well as the added complexity of rendezvousing with them. And external thrust source eliminates the problem of carrying propellant, but no feasible system for interplanetary, much less interstellar, distances has ever been credibly proposed. (Massive interstellar lasers are a patent absurdity given both the divergence of a laser beam and the inefficiency of laser, and as for Bussard ramjets, the less said the better; they belong in Larry Niven stories, not plausible speculation for interstellar transit.)
The only conceivable propellants to accelerate to relativistic speeds using any conceivable scheme are lightweight or massless fundamental particles. When you get that matter-antimatter photon drive all worked out let me know, and I’ll give you a hand with the patent application. Aside from that, you’re going to find yourself limited to a tiny fraction of a percent of c. (And before someone pulls out the Orion nuclear pulse propulsion, take a look at both the effective velocity and geometric efficiency of such systems; you’ll find that while providing magnitudes greater thrust per propellant mass than any conventional chemical propellant, they’re actually horribly inefficient and effectively limited to about a percent or two of c even given an unlimited supply of propellant bomblets.)
And as Shagnasty notes, there are many other problems with transiting at anything like relativistic speeds. Not only will the slightest speck of dust have the relative energy magnitudes greater than the largest thermonuclear weapon ever built, but you also have the problems of thermodynamic and resource sustainability; you have to provide enough energy for a recycling lifesystem to operate (and replace resources that are not perfectly recycled in usable form) and yet radiate away unrecoverable waste heat into space, which will require massive radiating surfaces for any complex system. You’ll end up with a ship the size of a small moon, at least, and pushing before it a literally massive shield to deflect oncoming particles.
We may one day travel to other star systems, but baring the development of some kind of highly speculative “warp drive” or the discovery of usable trans-spacetime-plenum conduits (or whatever space opera term you care to use), “we” should be loosely defined as “our highly modified successors”; semi-sentient robots, or organic organisms capable of surviving the environment of interstellar space without complex plumbing, or whatnot. It’s unlikely, to a point of certainty, that we’ll be running around in colorful spandex uniforms shouting “Fire at will!” and banging the occasional Orion slave girl. Colony ships moving at sublight speeds are only marginally less likely than USS Enterprise.
Stranger