Celestial Mechanics is a field of Science in which a lot of experience has been accumulated over the past centuries: Kepler, then Newton found the basic theoretical laws that every object in the Solar System follow. Since that time, many orbits have been computed, the General Relativity has been dicovered, many Space probes are navigated from planet to planet with very high accuracy. What I want to stress is that if there is ONE field of Science in which we are comfortable, it is celestial mechanics.
The following figures show two views of the Solar System (top view, and 30 deg over the plane of the ecliptic). The positions of the planets and Hale-Bopp have been plotted with a 1-week interval from November 28, 1996 (positions marked by the name label) until July 1997. The inclination of the comet's orbit is almost 90 degrees, meaning that the plane of its orbit is perpendicular to the Earth's orbit. To make it easier to visualize these in 3D, the Earth's orbit plane (the ecliptic) is marked with the dark green circles. The different positions of the comet have been connected by "vertical lines" to the ecliptic.
The case of Hale-Bopp is simple: just like two cars cannot collide if the roads they are on have no intersection, Hale-Bopp cannot collide with the Earth because their orbits do not cross each others.
The point where the two orbits are the closest is in the region where the comet's orbit crosses the plane of the ecliptic (marked by an arrow in the lower panel). Even if the orbit were slightly different and were crossing each other at that point, we would still be safe: Figure 2 shows that the Earth will be in that point in early January 1997, almost 4 months before the comet. When the comet will be there, in late April 1997, the Earth will be at 1/3 of its orbit away.
The Earth's orbit crosses the orbits of several comets. For instance, every year around August 13, we go through the orbit of Comet Swift-Tuttle. That comet left enormous quantities of tiny dust grains behind it, the last remnants of its tail. When the Earth passes there, it collects these grains that fall in the atmosphere, and heat up until forming a meteor, or shooting star.
For more information on the collisions with comets, have a look at the excellent Asteroid and Comet Impact Hazards page.
Creation: Thu Nov 28 16:53:34 1996
Creation: Thu Nov 28 16:53:34 1996 -- Hits: