Expert: Ed - 12/30/2010

Question
Why is it extremely hard to predict when Haylee's Comet will return?

Answer
Emily, Halley's Comet has a well-understood orbit.  It will come back in the summer of 2061, unless we develop super-spaceships before that year, and we decide to travel out in space and make the comets do something else.  Who knows, maybe 30 or 40 years from now we will send spaceships out there, and try to put some robots or colonists on it, for some scientific reason.

Today, we can observe exactly where it is, at all points in its orbit, if we choose.  

The orbit of Halley's Comet has varied between about 76 and 80 years, because at times it is pulled slightly in various directions by various planets, asteroids, and moons.
http://space.about.com/od/comets/a/halleyscomet.htm

I hope this answers your question.  Keep looking up!
--Ed