Without RTG, missions beyond Jupiter are not possible. There is the Juno probe launched recently, which runs on solar panels, but those things are pretty hefty

Without RTG, missions beyond Jupiter are not possible. There is the Juno probe launched recently, which runs on solar panels, but those things are pretty hefty (60m^2 surface area) and a much greater weight than an equivalent RTG. This means limited missions up to Jupiter (or perhaps Saturn) and no further exploration of the Kuiper belt.