Near-Earth Objects: Finding Them before They Find Us

By Donald K. Yeomans | Go to book overview

CHAPTER 5
Discovering and Tracking
Near-Earth Objects

No meteorite large enough to cause catastrophe would
ever again be allowed to breach the defences of Earth.
So began Project Spaceguard.
—Arthur C. Clarke, Rendezvous with Rama, 1973

Periodic active comets are considered near-Earth objects if they pass within 1.3 AU of the Sun and some of them, including comets Halley, Tempel-Tuttle, and Swift-Tuttle, have been recorded in ancient Chinese documents. Near-Earth comets have been known for a long time. For example, the 164 B.C. return of comet Halley was recorded on Babylonian clay tablets that are now housed in the British museum.1 But comets are show-offs. When they enter into the inner solar system, their ices begin to vaporize and the resulting gases and the entrained dust particles stream away in an anti-Sunward direction, sometimes causing a visually spectacular display. Active comets may be show-offs but they make up only about 1 percent of the total near-Earth object population. While near-Earth asteroids don't belch gas and dust, it is these stealth asteroids that dominate the near-Earth object population and only recently have astronomers realized the immensity and importance of this population.

1 See D. K. Yeomans, Comets: A Chronological History of Observations, Science, Myth and Folklore (New York: John Wiley and Sons, 1991), 265.

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