Forces of change

John Matese and Daniel Whitmire of the University of Southwestern Louisiana, Lafayette, in a recent paper say that tide-like variations in the pull exerted on the orbit of the solar system by the rest of the galaxy causes comets to pummel Earth once every 30 million years. Most comets orbit the sun in the Oort cloud, which lies 100,000 times farther out than the Earth. If the orbits are disturbed, some comets may fall into the inner solar system and crash onto Earth.

The team has calculated orbits into which comets from the Oort cloud could be tugged by the tides. There are two tidal forces, one which tugs matter in a direction perpendicular to the plane of the galaxy, and one