Planetary securityNew threat to Earth: "Dark" comets

Donald Rumsfeld used to divide things into “known unknowns” and “unknown unknowns.” He explained: “There are two kinds of things we don’t know: things we know we don’t know, and things we don’t know we don’t know.”

We should begin with the known unknowns. There are about 5,000 “near Earth objects” (NEOs) orbiting between Mars and Venus that are wider than 460 feet. Nearly 20 percent of these — 789 to be precise — have been identified as potentially hazardous objects because they might pass within 5 million miles of Earth (20 times the distance to the Moon). Lindley Johnson, the manager of NASA’s asteroid detection program, predicted that future surveys will find at least 66,000 near Earth objects and 18,000 potentially hazardous objects (see 26 November 2008 HS Daily Wire). A collision with one or more of these many objects littering the solar system is inevitable, Johnson said. “Once every hundred years there might be something to worry about, but it could happen tomorrow” (see 22 December 2008 HS Daily Wire).

Now to the unknown unknowns. Swathes of dark comets may be prowling the solar system, posing a deadly threat to Earth. Hazardous comets and asteroids are monitored by various space agencies under an umbrella effort known as Spaceguard. The majority of objects found so far are rocky asteroids. Now, Paul Parson writes, U.K.-based astronomers Bill Napier at Cardiff University and David Asher at Armagh Observatory in Northern Ireland claim that many comets could be going undetected. “There is a case to be made that dark, dormant comets are a significant but largely unseen hazard,” says Napier.

In earlier work, Napier and Janaki Wickramasinghe, also at Cardiff, have suggested that when the solar system periodically passes through the galactic plane, it nudges comets in our direction. These periodic comet showers appear to correlate with the dates of ancient impact craters found on Earth, which would suggest that most impactors in the past were comets, not asteroids.

Now Napier and Asher warn that some of these comets may still be zipping around the solar system. Other observations support their case. The rate that bright comets enter the solar system implies there should be around 3,000 of them buzzing around, and yet only 25 are known.

We may not see them, say the pair, simply because they are too dark. Such dark comets are not unheard of (see “Scientists Probe Mysterious 1908 Siberian Mystery,” 2 July 2008 HS daily Wire; and “Evidence of Acid Rain Supports Meteorite Theory of Tunguska Catastrophe,” 16 july 2008 HS Daily Wire). They occur when an “active” comet’s reflective water ice has evaporated away, leaving behind an organic crust that only reflects a small fraction of light. In 1983 Comet IRAS-Araki-Alcock passed by Earth at a distance of 5 million kilometers, the closest known pass by any known comet for 200 years. It was spotted only two weeks ahead of its closest approach. “It had only 1 percent of its surface active,” says Napier. Comet Borrelly, visited by NASA’s Deep Space 1 probe in 2001, was found to have extremely dark patches over much of its surface.

“There may be merit to this idea,” says Steve Larson of the University of Arizona’s Catalina Sky Survey in Tucson, one of the main contributors to Spaceguard. Clark Chapman at the Southwest Research Institute in Boulder, Colorado, is skeptical, but points out that such dark comets “would absorb sunlight very well” and so could be detected by the heat they would emit.

-read more in Bill Napier and David Asher, “The Tunguska Impact Event and Beyond,” Astronomy & Geophysics 50 (February 2009) (DOI: 10.1111/j.1468-4004.2009.50118.x); and J. T. Wickramasinghe and W. M. Napier, “Impact Cratering and the Oort Cloud,” ArXiv (17 March 2008) (arXiv:0803.2492v1)