6. Small Comets

The existence of small comets would go a long way towards establishing that the Earth is an open system. These comets would, over geologic time scales, be responsible for supplying much of the planet's water. Without this extraterrestrial input, the Earth would be a barren planet.

For approximately the past 15 years, a debate has been raging in astronomy and allied disciplines over the existence of small comets. First proposed by Frank, Sigwarth and Craven in 1986, this hypothesis holds that the Earth is struck by millions of water-laden comets per year with an average size of 40 feet in diameter. The comets are composed mostly of water snow. As they approach the Earth they become a cloud of water vapor at approximately 800 miles altitude. This cloud of mist loses its coherence and enlarges while it descends towards the Earth. Frank et al. argue that small comets have, over geologic time, contributed much of the Earth's oceans. It is the supply of water from small comets that continually replenishes the water lost to subduction to the interior of the planet and prevents the surface of the Earth from becoming dry and barren.[83]

There have been several dozen articles for and against small comets. For example, one group of scientists used an optical telescope to search for small comets.[84] They found nothing, although their results did not rule out the existence of the phenomenon. Frank and Sigwarth conducted their own optical search using the same telescope and they found small comets.[85] Both results were published in a prestigious peer reviewed journal and so cannot easily be discounted. I evaluated the debate by focusing on one detection method and reading the sequence of articles very carefully. By focusing in depth on a specific facet of the debate, I hoped to reach definitive conclusions which, although limited only to one detection method, would nevertheless be revealing.

In 1997, a group of radar scientists and astronomers (Knowles et al.)[86] conducted a radar search for small comets. They used the United States Naval Space Surveillance System radar. This radar has transmitter and receiver sites across the southern United States and a central data processing facility in Virginia. It is designed to catalog and track essentially all the world's Low Earth Orbit satellites. The authors state that this is the best available radar in the world for locating small comets because it searches an extremely wide volume of space with very high sensitivity. Some narrow aperture radars, (radars designed to search a small volume of space very carefully), are more sensitive, but these might miss the small comets because they do not have a wide enough field of view.

During the approximately 1 month of searching with the radar, some 12,000 unidentified targets were observed. Of these, none fit the theoretical characteristics of small comets. 50 were signal noise, and the rest were either new satellites, moving too slowly to be small comets or were in gravitationally bound orbits (not unbound infalling objects).

Frank and Sigwarth were given the opportunity to reply to the negative radar results. Knowles et al. then replied to the reply. This author has carefully analyzed the exchange and found that none of Frank and Sigwarth's objections are valid. This analysis is included as Appendix I. The results of the Naval Space Command radar observations seem to be strong empirical evidence against the existence of small comets.