The first step in understanding anything about a comet is to understand where it is. And the knowledge about where comets are, how they move, where they come from, and how they start in toward the inner solar system has changed dramatically over the years. Comets provide a good framework for understanding the distance scale of the solar system, as well as contemplating individual objects and why they behave the way they do.

When someone reports a suspected comet discovery, the first thing astronomers do is to make precise astrometric (positional) measurements of the object as it moves along its orbit. They want to make these accurate measurements of the comet’s position over a reasonably long arc – that is, over three or more measurements over the span of several days or a week – in order to calculate a preliminary orbit for the object. To do this, they need more than just careful positional measurements. They also consider the comet’s center of mass, gravitational perturbations that may be influencing the comet by planets or asteroids, and the modeling of nongravitational forces – that is, any outgassing from the comet itself that would influence its orbit.