Question

How might a collision between Neptune and Halley's Comet affect Neptune's orbit?

Answer 1

The effect would be minuscule ... imperceptible and essentially unnoticeable,
a lot like a collision between Earth and a few tons of meteoroids.

-- Neptune's diameter is a little more than 2000 times the comet's diameter.

-- Neptune's mass is a little more than 465 billion times the comet's mass.