The Small‐Comet Hypothesis

Abstract

According to the small‐comet hypothesis, small comets strike the Earth approximately 20 times per minute, each small comet nominally containing 100 tons (105 kg) of water‐ice. The primary observations interpreted as evidence for these small comets are dark spots in the Earth's atomic‐oxygen UV dayglow seen by the UV imager on the Dynamics Explorer (DE) satellite. These small comets must disintegrate near Earth and then sublimate within a few seconds, the water vapor expanding to form clouds of water vapor, nominally 50 km in diameter, that temporarily block the spacecraft's view of the dayglow, thus producing the dark spots. In this review we examine problems in basic mechanisms underlying the small‐comet hypothesis. These include inconsistencies with known geophysical phenomena, conflicting results from independent searches for evidence of the presence of small comets, and inconsistencies within the small‐comet hypothesis itself. No other geophysical interpretation that can account for the DE dark spots has been advanced. The only viable alternative in the literature is a nongeophysical one—the instrument‐artifact hypothesis—in which it is proposed that the dark spots appear randomly in the DE pictures. Tests of this hypothesis, both qualitative and statistical, show that it, neatly and economically, explains the DE dark‐spot data. Owing to the weight of accumulated evidence against the small‐comet hypothesis, the lack of credible supporting evidence, and the plausibility of the instrument‐artifact hypothesis, it is unlikely that the small‐comet hypothesis is valid.

An optical search for small comets

“…More recently, an extensive radar search has also failed to detect small comets [Knowles et al, 1999a, b]. This controversy has led to a lively and ongoing debate in the scientific literature since 1986 [e.g., Dessler, 1991;Sigwarth, 1993, 1999, and references therein]. Perhaps the least model dependent test of the hypothesis is an optical search for small comets as they approach the Earth but before they break up.…”

Cometary supply of terrestrial organics: Lessons from the K/T and the present epoch

“…Turning attention to meteorite-producing bodies, there is growing evidence contradicting the idea that these all arrive from random orbits (as would be expected for individual objects being ejected from the asteroid belt by chaotic dynamics: Wisdom, 1985), with evidence for meteorite streams accumulating (Oberst and Nakamura, 1987;Dodd, 1989;Halliday et al, 1990). Such streams are not though to be dynamically stable for time-scales longer than a few times 105 yr (Wetherill, 1986), although streams such as the TC are certainly still recognizable, with considerable dispersion, a few times 104 yr after their formation (Steel et at., 1991).…”

“…For example the stability of small comets near 1 AU, if they are composed largely of water and other volatiles, as are large comets, would be limited by the vapor pressure of their constituents unless a strong insulating crust exists (McKay, 1986;Dessler, 1991). Further, Oberst and Nakamura (1991) find that the majority (85%) of meteoroids of mass > 103 g have orbits unlike those of meteorites, and suggest that they are more fragile than meteorite parents, making them unable to survive atmospheric entry; they conjecture that such meteoroids may be derived from SPCs and the more fragile NEAs.…”

Small comets: Naked‐eye visibility

“…The energy balance is sensitive to radius through mass, cross-section and scattering properties and radius is controlled by temperature through the sublimation rate. If the particle is created with an initial temperature of 300 K, the large resulting As pointed out in FS-93, the theoretical vaporization lifetime of ice particles in Appendix A of Dessler [ 1991] is an underestimate for small particles. While Pike et al…”