|Quoting solarflyer22 (Reply 23):|
I didn't realize how much speed and time is required but the key really is the deep space engine. This is very solvable problem but I don't know where NASA is in the development of a Ion thruster. It is lighter, builds up a higher speed and can slow it down in time.
The key is the engine. NASA launched Deep Space 1 and 2 with the Ion thrusters and both worked. With a limited amount of electricity (a few watts) and a few pounds of metal you get Ions zooming out the back at the speed of light. The notion of heavy lifting chemicals to simply explode into space to build up speed is not going to cut it for deep space travel on a budget.
The problem with ion thrusters is the power requirement. DS1 was able to devote 2100W to the ion drive, but that was from solar panels, and it stayed fairly close to the sun, where solar power is useful. New Horizons, powered by a GPHS-RTS
(solar being useless much beyond the asteroid belt), had about 250W available at launch, and 200W-or-so by the time it got to Pluto. Yes, you can build bigger RTGs, or use several RTGs to boost available power (the RTG on New Horizons was a spare from Cassini, which had three), but Pu-238 availability is a massive problem. Right now the US stockpile plus short term production available to NASA should be enough to build three 125W MMRTGs (smaller than the GPHS-RTGs), like the one on Curiosity, in the next five or six years, and one of those is already allocated to the planned 2020 Mars Rover (and that's literally it). Most of NASA's Pu-238 over the last couple of decades was purchased from Russia, but they're pretty much out too, and no longer selling it either.
Very limited Pu-238 production has been restarted, and over the next few years there is some hope that we can get to the point of producing 1-2 kilograms per year. Note that a (250W) GPHS-RTG needs nearly 10kg of Pu-238, while the smaller MMRTG needs only half that. So if you wanted 1000W for an ion thruster, you'll need the planned/hoped-for production for the next 25-or-so years.
So ion thrusters are great, you just need to figure out how to power them. An actual nuclear reactor (as opposed to an RTG) would be plausible, but is likely impossible from a safety/political standpoint. FWIW, The Soviet Union actually new two types of "real" reactors, the BES
-5s (2KW) and the TOPAZ (6KW), although both were intended only for (relatively) short terms (on the order of six months - its unclear how practical a multi-decade lifespan for something on that scale would be). The soviets launched a total of 31 BES
-5 and two TOPAZ's, and had a number of accidents with them (not least was Kosmos 954, which scattered radioactive contamination over 50,000 square miles of northern Canada). In the mid-sixties, the US launched a single experimental SNAP-10A reactor, which produced some 600W, before failing after some 43 days (although the failure was not directly related to the reactor). All three of those were fueled with U-235, which at least has the advantage of reasonable availability (FWIW, the BES
-5s used about 50kg of U-235 - reactors that small are horribly inefficient users of U-235 - a typical gigawatt-class PWR has about 100t of Uranium total, about 3% of which will be U-235).
Even if you assumed a very small ion thruster, you've still got a very limited power budget to work with - you have to run the rest of the spacecraft as well as the engine. And smaller ion thrusters tend to have lower ISPs, so you're increasing your fuel/mass requirements, which will impact your launch costs and the amount of time you need to slow down.
Again, this is not impossible, it's just very difficult to see how we get there.
Oh, and exhaust velocities are nowhere near the speed of light, typically in the 20-50km/s range.
*The exact amount available for all uses is unclear, but is unlikely to be more than twice the ~12kg (plus a kg or two of new production) available to NASA (note that NASA may also be planning to do a bit of stretching with decayed "old" Pu-238 as well, a limited amount of which is available).