mfb said:
Well, the mission would have been planned differently.
Dawn achieved over 10 km/s delta-v with 425 kg of xenon for 625 kg of dry mass. Solar cells are impractical for a probe to Pluto, but a large RTG or a small nuclear reactor (or something subcritical in between) could work. Leave Earth with chemical rockets, accelerate with fly-bys, drift for a while, then use the ion drive to slow down. More expensive, needs more R&D, takes longer to reach Pluto, but certainly not impossible.
Not impossible, no. You could likely get to Pluto and park in orbit with the delta-v budget of the present mission and chemical rockets alone, but at the cost of increasing the length of the mission immensely. A 13 km/s delta-v with a Dawn type engine would require a reaction mass of roughly equal to that of the dry mass. So you would be doubling the mass of the probe (not counting the additional mass of the power source). So for the same Earth relative launch velocity you need to double the fuel.
If you are going to use the same launch vehicle, then you are going to halve your Earth-relative velocity with the subsequent increase in Earth to Pluto travel time. (though to be fair, if you decrease the Initial delta-v, you are also going to reduce the "fly-by" velocity upon reaching Pluto, which in turn reduces the delta-v needed to match Pluto's orbital velocity and decreases the mass of the probe, which effects the available delta-v upon leaving Earth.. The final result would have to take in the balancing out of all this.)
So, let's assume we have worked all this out. Now the next question is, will the benefits outweigh the extra costs? How much more data do we get by leaving the probe in orbit? Because of the distance, New Horizons can only transmit its collected data back to Earth at a slow bit rate and as a result, the data collected during the several hr flyby will take 16 months to transmit back to Earth. So you would end up with a situation of the probe collecting several hours of data, spending 16 months sending that data back, collecting another several hrs of data... Since the power source on the probe is presently expected to last until 2030, you might expect to get 11 cycles of this over the life time of the craft.( likely less, since we are assuming a longer trip time, some of the lifetime of the power source will be used up during the trip)
So, in the end you might end up with a couple of hundred hrs of data collected in all. Factoring in all the extra expense for the mission, is it worth it? Now for many of us, the answer would be yes, however, generally speaking, we are not the ones holding the purse strings, and the difference between what type of mission we might ultimately want and the one we get is what Congress will provide the funds for.