In order to launch something so far out you have to "slingshot" off the other planets. A launch window like that only comes once every so often.
"For example the Voyager missions which started in the late 1970s were made possible by the "Grand Tour" alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century."
I was thinking the same thing as yock, but if I had the resources to fund such a project I'd want to give the probe a power source that would allow it to accelerate continuously. I think we've got both the electric generator and propulsion technologies to do that now, where weren't available in the 70's. That would let the probe reach Voyager's distance much more quickly, and we'd be flexible in choosing a launch date.
I'd also want to use some of that power to make the probe 'bright'. It could transit telemetry back to us with more power so we could keep contact with it longer, and it could also broadcast its presence in every other direction too. That way if there is anyone out there, they'll have a chance to find the probe. With the Voyager probes, even if the galaxy is teeming with intelligent space-traveling life it's unlikely that anyone will every find the tiny little things.
The Dawn spacecraft use ion drives. "With the propellant it carries, it can perform a velocity change of over 10 km/s, far more than any other spacecraft has done with onboard propellant after separation from the launch rocket." Voyager 2 got a 20km/s boost from Jupiter, and almost the same from Saturn, about 4km/s from Uranus, and about 10 from Neptune. (See http://www2.jpl.nasa.gov/basics/bsf4-1.php )
In other words, even the best rockets we have now are not as good as that rare lineup of all four gas giants in this system.
Also, we don't have a better energy source. Voyager used RTGs, and that's still what we have. In any case, power isn't the limiting factor, it's exhaust velocity and amount of fuel.
Looks like Uranus gave a tiny boost and Neptune a negative boost -- not through any intrinsic property of the planet, mind you, just the constraints applied when you want to tour the whole solar system without burning any fuel.
That's the same graph as the JPL link I pointed out earlier, but clearer and with more description.
It shows that my earlier reading was incorrect. It looks like the delta-V from Jupiter is 11km/s, Saturn is 8km/s, Uranus is 1km/s and Neptune, as you point out, is a negative delta V w.r.t distance from the Sun. Note that the comment says it's because Voyager 2 deflected out of the plane of the ecliptic as a consequence of doing a flyby of Triton, and not because of fuel constraints.
In any case, I had forgotten that Voyager I did not do the grand tour of the giants. I haven't been able to find a similar velocity profile for Voyager I.
I suspect an ion drive system could do it. You still need to slingshot somewhat, and you would gain flexibility if you didn't try to fly by all the planets on the way out. New Horizons [1] looks to hit the Kuiper belt in 10 yrs so considerably faster than Voyager.
Could you elaborate? Are you talking about velocity outwards from the sun or including the orbital term as well? Are you talking about the Voyager I speed now, or just after the Jupiter gravity assist?
From what I can find, the best ion craft we have now (Dawn) is about 1/2 as effective as a Jupiter flyby, and 1/6 as effective as the series of flybys that Voyager did.
Dawn isn't the best we can do, though, because it's solar powered. Even in the inner solar system, solar power can't give you nearly as much power as you can get from a decent radioisotope thermoelectric generator, and in the outer solar system you can fuhggedaboudit.
If we wanted to send a probe to the middle of nowhere right now and had a billion or two to do it we'd strap our best ion drive to our best RTGs and then send it via a Jupiter slingshot. No idea how fast we could get it. Of course it would continue to accelerate until the power or the propellant ran out.
The New Horizons RTG puts out 300W of electrical energy (and more as heat). The solar arrays on Dawn can supply "10 kW at Earth and 1.4 kW at Ceres". Wikipedia says the NSTAR thrusters on Dawn requires 2.3 kW.
How do RTGs help?
Let's say they somehow do (perhaps I have the numbers wrong). How much more delta V would you get using an RTG? Twice the delta V as solar panels? Note: momentum transfer goes as the square root of the energy, so you would need 4x as much power as Dawn uses. On the other hand, you wouldn't be carrying heavy panels.)
If so, 20km/s is about what a Jupiter or Saturn flyby gives. So RTG+ion thruster+Jupiter is about equivalent to the Jupiter+Saturn part of Voyager. Except Voyager also used the outer gas giants.
My point is that to get the same speed you'll need at least both of Jupiter and Saturn lined up right, and preferably at least one of Uranus or Neptune. Your proposal isn't enough.
I must admit that you've surprised me with the numbers on solar vs RTG. (I've learned something today!) But of course you can always strap multiple RTGs together. According to wikipedia's article on RTGs the most efficient ones produce about 4 watts per kilogram, so you'd need a couple of tons of 'em to equal Dawn's 10 kW at Earth, well in excess of Dawn's mass. Solar panels still rapidly become useless (like the square of the distance from the sun) as you get further out, of course.
Well, you don't have to go as fast as the Voyagers to end up out on the edges of the Solar System in a few decades. Even so, it's interesting that the New Horizons Pluto probe was launched with a higher speed relative to Earth than the Voyagers but will ultimately end up traveling slower because it won't slingshot by as many planets.
But Voyager 1 (the faster one) didn't even go the grand tour. It just flew by Jupiter and Saturn. That constellation should happen more often. And as you can see with the Pioneer missions even Jupiter alone can make you quite fast.
"For example the Voyager missions which started in the late 1970s were made possible by the "Grand Tour" alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century."
source: http://en.wikipedia.org/wiki/Gravitational_slingshot#Limits_...