You could represent the orbit of a close approach comet as a highly HYPERBOLIC geocentric orbit and this should be reasonable for several days I would have thought. However I suspect that most sat programs don't deal with hyperbolic orbits. There are many programs available that allow you to plot solar-system objects as graphics or ephemerides, but don't ask me about them. I just know they exist. Cheers, Rob rmn@aaocbn1.aao.gov.au