Had earth have been denser planet than it is...

In article ,
Christopher wrote:
(The archetypal example of big rocket with tiny payload because of high
delta-V requirement was the original Pluto Fast Flyby proposal...

For a Pluto Fast Flyby wouldn't it be more feasable to have a small
nuclear thermal rocket sent on its way from LEO...

There was no possibility of such a thing being available -- and thus
feasible in any practical sense -- for the mid-90s launch that PFF wanted.
The Titan IV plus upper stages was perfectly feasible, essentially off
the shelf... just horribly expensive, at a time when spending the better
part of a billion on one planetary mission simply wasn't in the cards.

...the rocket after
the probe has been uncoupled could be sent on a crash course with
Jupiter or Saturn or Uranus or Neptune, so their isn't an old reactor
floating around in solar orbit, as a future menace.

As has already been noted, it's very unlikely that there would be a planet
in a convenient position for this, and in any case the reactor (like the
probe) would be far beyond solar escape velocity by cutoff. It would end
up in interstellar space, far away from anything that might be harmed.
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MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |