I have seen this question posed often recently, without a satisfactory answer in my opinion:
Why won’t New Horizons orbit Pluto?
The answers I read are always along this line:
I would like to know the approximate scale of these factors, for example:
This mission study came up with a 900 kg nuclear-electric-propulsion spacecraft launched on an Ariane V with a C3 of 100 km2/s2 and a Jupiter gravity assist along the way. 1.05 kW electrical power at Pluto from RTGs is required. That would be four "classic" NASA RTGs, or about nine MMRTGs. It has a 20 kg science payload. (New Horizons has ~30 kg of instruments, one classic RTG, with a launch mass of 478 kg.)
There are some optimistic assumptions (only 20% dry mass margin on most components?!), but this gives a general idea of the scale of such a mission. It is not out of the question, but it would definitely be in the flagship cost class, comparable to Cassini. Cassini has three classic RTGs, but uses chemical propulsion.
This is probably the easiest to answer if we take the same mission profile and just track from the Pluto flyby backwards. I'll make some rather broad assumptions and first order approximations, like that NASA had their NEXT ion thruster developed to the highest technology readiness level (they did exist when New Horizons launched, but not at TRL required to actually launch on a deep space voyage of discovery).
I'll also spare you with pesky details and just assume that if we can bring New Horizons close to a dead stop somewhere at Pluto and relative to it from ~ 13.79 km/s, that would do. Delta-v to insert into Pluto's orbit from then on won't be that much compared to everything else we need to change, part of the job would simply be falling towards it on its own gravity, then circularize at some desired altitude. I'll also assume that the orbital space is free of debris. Brace yourself, it's going to be a dirty, Mos Eisley Spaceport type of a job;
So we got with propulsion that's great for the job but wasn't really available at the time, with 205.7 kg of plutonium for our RTG that nobody really had available at such quantity, and a few rather generously small estimates for the mass of additional systems and xenon fuel needed from probe's launch mass of 478 kg to 26,300 kg. And that's just the beginning of the story of our makeshift Pluto orbiter that now launches $(22,190\text{ kg} \times 13.79\text{ km/s} / 236\text{ mN})^{(1/2)}$ or about 6 years and 4 months sooner to get to Pluto at the same time as New Horizons will, assuming nothing breaks and our NEXT thruster's performance doesn't degrade during this additional time of continuous 236 mN thrust and consuming 6.9 kW of our RTG generated power.
Again, this is all generously underestimating the problem and throwing at it technology that wasn't yet ready, requiring parts and consumables that weren't available and would have to operate for over six years longer without degradation in performance (but I did account for Pu-238 decay rate with half-life of 87.7 years), i.e. if New Horizons was made in Mos Eisley Spaceport into an orbiter, it would be 55 times its launch mass as a flyby probe.
I'm not sure how or on what you could hurl such mass off the Earth and give it such a kick to still get there in time, but that's a different matter. Note that over six years longer mission also means that the Earth and Pluto don't align as nice and that comes with even more problems. But you now have an orbiter, and with enough power for the upcoming centuries to even transmit science and telemetry data back at a much faster rate. Oh, you might want a better transceiver antenna too, bigger and perhaps one that can gimbal independently of the orbiter itself, unlike the flyby version of New Horizons, so you can receive and transmit towards the Earth without having to take your eyes off the target of your observations. I hope Pluto is worth it, chances are that it you made it into its orbit, you'll be there a long, long time. ;)
Others will give you different estimates, depending on their choice of propulsion and power systems, but that's my view on the matter. I didn't read suggestions made in your link, because I wanted to see how that plays out on my own. This is now an updated edit with recalculation for required thrust time (not sure where I got those numbers I plugged in before, but they were suspiciously enough off to warrant a second look). Hope this helps.
To bottom-line TildalWave's answer:
This Slashdot post contains a calculation of launch weight for an orbiter, using these constraints:
The Space Shuttle Main Engines, one of our most efficient rocket engines, has an Isp of 4.436 km/s. By the rocket equation [this means that, to change velocity by 11 km/s using this engine, a spacecraft would need a ratio of wet mass to dry mass of exp(11/4.436) = 11.9. In other words, to stop the New Horizons probe at Pluto, we'd need to have sent along an extra 10.9 times its mass in fuel. And that's ignoring the mass of the engine and tankage, which makes things worse.
Fuel boil-off... is an additional problem: it means we couldn't use the liquid-hydrogen/liquid-oxygen propellant used by the SSMEs, but some more stable (and less efficient) propellant, which further increase the required fuel mass.
...New Horizons was launched on an Atlas V 551, which has a capacity of 19t to LEO. To send the probe plus 10.9 times its mass in fuel would therefore take an equivalent capacity of ~11.9*19 = 226t to LEO. The Saturn V, the most powerful launcher ever made, had a capacity of 118t to LEO. So you'd need two Saturn V launches, rendesvousing in orbit, to get a spacecraft with enough fuel to fly to Pluto and stop there. (Probably 3-4 launches, when you consider the other problems described above.)
