Could the impact of a sizable asteroid or Comet change the Earths distance from the sun?
No asteroid or comet has enough mass to change Earth’s orbit in any appreciable way. For example, asteroid 433 Eros (the largest asteroid that crosses Earth’s orbit) has a mass of about 1 billionth that of Earth. That means that If it were to hit the Earth head-on (which it could never actually do, because Eros and Earth orbit in the same direction), Earth would lose about 1 billionth of its momentum. That would move the Earth only 1 km closer to the Sun.
