|MadSci Network: Astronomy|
> Are there any factors which could change the orbit of Halley's comet?
Sure -- but only by very small amounts. Each time a comet passes through the inner solar system, its icy exterior is heated by the Sun. Portions of its surface sublimate (turns from solid into gas), creating little geysers. As gaseous material shoots off the surface, it pushes the comet a little tiny bit in the opposite direction. This "rocket effect" can change a comet's orbit by very small amounts -- less than one percent in period, for example.
> What is Halley's track?
I suggest you look at the following page, which shows a picture of the orbit and has links to more: http://csep10.phys.utk.edu/astr161/lect/comets/halley.html
> Why isn't there any air in the cosmos?
Hmmm ... how best to answer this question? Space is so big, and the amount of matter in it is so small, that the average density of material is miniscule -- about one atom per cubic meter or so. Only rarely does matter clump together to form clouds of gas, stars, or planets.
I don't think that answers your question very well, but I don't know if there _is_ a good answer.
Try the links in the MadSci Library for more information on Astronomy.