What fuel source do unmanned probes use?

One thing to keep in mind is that space is a near-perfect vacuum, meaning there is essentially no drag force acting on a moving probe. By Newton's second law, the probe will not decelerate:

a = F / m

Clearly, when the force, F, is zero, so is the acceleration, a. The Voyager probes are moving at constant velocity, because no force is acting on them to slow them down.

The probes were originally launched on Centaur rockets, which burned liquid hydrogen with liquid oxygen. This initial burst of speed, coupled with a few well-planned gravity assists by the planets, imparted the probe with its current velocity.

After separating from the rocket, the Voyager probes utilized a radioisotope thermoelectric generator for its electrical power. The generator contains a sample of plutonium-238-rich plutonium(IV) oxide. The spontaneous radioactive decay of plutonium atoms in the sample generates a moderate amount of heat, which is converted into electricity by a thermocouple and heat sink.

As for your second question, I doubt you'd want radioactive plutonium in your car, especially considering just how big of a sample you'd need to power it.

Doom, Spumm, and CampbelP all have excellent answers, which pretty much cover how the probes work. I just want to point out one little piece of misinformation in their responses. The Voyager probes are actually slowing down as they leave the Solar System.

This is not due to friction, though. As they stated, the probes continue moving through space because of their initial momentum gained at launch, and through the gravity-assist flybys at the planets they visited. However, think of what happens to a baseball that you throw up in the air. It gradually slows down to a stop, and then gradually accelerates back downward. As the probes get further from the Sun, they are going "higher up" in its gravity field in the same manner as your baseball.

The thing about Voyager is that they're going faster than the Sun's escape velocity, so they will never completely slow down and stop like the baseball does. They are far enough away from the Sun that their slowing has decreased a lot, and they're almost remaining at constant velocity. They will continue outward until they are pretty much beyond the Sun's gravitational influence and are basically in orbit around the center of the Galaxy now.

To compare Voyager's various power sources to your car, imagine your car is out of gas. You put it in neutral, and get a very fast push start from some other car, which stops and leaves your car rolling along at a nice speed. You want to listen to the radio so you turn it on. Being electrical, it would normally get power from the engine's alternator, but you're out of gas, so it uses the battery in stead. Voyager's "battery" is the RTGs mentioned above - the radioactive decay of the plutonium is generating the electricity for Voyager's radio.

Voyager's on-board hydrazine thrusters are there for course corrections, but are not powerful enough to give it much speed. As mentioned earlier, it's not needed for this because the initial push of the launch rocket is why it's going so fast.

The gravity assists would be analogous to your coasting car going down a hill. At the bottom, you would be coasting faster than you had been at the top, even though you didn't use the engine at all.

They are using no power at all. At least none to keep moving through space. They are just coasting from the speed they got when they were launched by rocket in 1977. Since there is no air in space there is no friction, so once they get going, nothing slows them down.

For electrical power for their computer and radio they use plutonium. The same stuff in nuclear bombs. But it is in a RTG or radioisotope thermoelectric generator and not a bomb or even a reactor.

Probes inside the orbit of Mars generally use solar power.

Probes farther out (Including Voyagers 1 and 2, Galileo, Ulysses, Cassini, and New Horizons) usually use Plutonium-powered RTG's for heat and electricity, and to transmit. You probably don't want Plutonium in your car.

However, they don't use this power source for propulsion, like your car does. Most probes just use their initial momentum from launch, or momentum they gain from passing close enough to a planet to make use of their gravitational pull.

A couple of newer probes (like Dawn) use "ion engines" for propulsion, but they provide about as much thrust as gently blowing on your hand does. They just do it continuously for a very long time, eventually building up considerable speed.

Incidentally, one of the US Presidential candidates (three guesses as to who) has promised to try to ban all production of fissile materials. This would apparently include Plutonium.

That's going to make it quite a bit harder to find fuel for new space probes, so I kind of hope he loses.

The above posters are largely correct.

It should also be noted they use hydrazine rockets on these probes. Not for propulsion but to make course corrections or to reorient the craft as necessary.

You definitely would not want hydrazine powering your car. Nasty stuff and dangerously unstable (partly what makes it a good rocket fuel) unless handled very carefully.

Plutonium is even worse (considered one of the most toxic substances in existence...really, really nasty).

Solar is the best, because the sun is the best energy source. But it is also possible to use fossil fuels but the fuels would slow down the trip with the tank and everything. So I would have to say that the best energy source is from solar. Because you get nearly limitless power, it depends on how far away you are from the sun, and the solar panels are easy to carry through space.

I don't have a source, but I thought they used small nuclear reactors. I know some probes use nuclear to keep some of their instruments "warm" and working, but I don't know if it is the fuel source or not.

They use solar power. They will one day be too far from our sun to stay operational and they will stop sending us information.

There are solar cars out there, but they look fugly so I would say your stuck paying $4 a gallon.