The following is an excerpt from

Cosmic Collisions

by Sally Stephens, Astronomical Society of the Pacific

(c) 1993 Astronomical Society of the Pacific

A primer on asteroid collisions with Earth

What about the comet that's supposed to hit the Earth in 130 years?

Recent news reports have mentioned a possible collision between the Earth and Comet Swift-Tuttle on August 14, 2126. Comet Swift-Tuttle was first seen in July 1862 (when Abraham Lincoln was President) by two American astronomers (Lewis Swift and Horace Tuttle, hence the name Comet Swift-Tuttle). Three months later it was too faint to be seen. Based on observations during those months, astronomers calculated that the comet had a highly elongated orbit with a period of about 120 years. Thus it was expected to return to the area near the Sun in the early 1980s.

Shortly after its discovery appearance, Italian astronomer Giovanni Schiaparelli noticed that the orbit of Comet Swift-Tuttle was remarkably similar to the orbit of the dust particles responsible for the Perseid meteor shower each August. During the peak of meteor showers, upwards of hundreds (and occasionally thousands) of meteors an hour can be seen (as compared to a few per hour visible on a normal night from a dark location). Schiaparelli's connection established comets as the originators of meteor showers -- as comets move close to the Sun, solar heat turns their ice to gas, which explodes away from the surface of the comet in ``jets'' of gas that pull some of the comet's dust out with them. The dust is left behind in the comet's orbit. When the Earth crosses the orbit, at the same time each year, it plows through the dust, unleashing a meteor shower.

During the 1970s, the number of meteors seen each year in the Perseid meteor shower increased. It seemed that Comet Swift-Tuttle was about to reappear. But it failed to show, and soon afterward, Perseid meteor activity dropped sharply. Astronomers wondered if the comet had somehow come and gone unnoticed. After all, its orbit was based on only three months of observations a century ago, and there was plenty of room for error.

In 1973, astronomer Brian Marsden, of the Harvard-Smithsonian Center for Astrophysics, suggested that the comet seen in 1862 might be the same comet reported in 1737 by a Jesuit missionary, Ignatius Kegler, in Beijing, China. The connection was possible if ``jets'' on the comet, caused by ice turning into gas because of the Sun's heat, exploding away from the comet's surface and acting like rocket exhaust, slightly altered its orbit. Marsden predicted that Comet Swift-Tuttle, with a newly calculated period of 130 years, would return at the end of 1992. Increased numbers of Perseid meteors the past few years indicated the comet might be near.

On September 26, 1992, Japanese amateur astronomer Tsuruhiko Kiuchi, using six-inch binoculars, noticed a comet moving through the Big Dipper in an area where scientists had calculated Comet Swift-Tuttle should be, if it was indeed reappearing. Other astronomers confirmed that the lost comet had been found. Marsden's 1973 prediction was confirmed, although the date of the comet's closest approach to the Sun (its perihelion) was off by 17 days from his prediction. On November 7, 1992, the comet passed 177 million kilometers (110 million miles) from Earth (its closest approach) on its way to a December 12th perihelion.

Armed with new observations of the comet's motion, Marsden went to work revising his calculations of its orbit. He predicted the next perihelion would occur on August 14, 2126. But if the actual date of perihelion was off by 15 days from his prediction (as the 1992 perihelion had been off by 17 days), the comet and the Earth might be in the same place in space at the same time. Since Comet Swift-Tuttle is thought to be about six-miles across, about the same size of the asteroid that killed the dinosaurs, a possible collision looked ominous.

Marsden continued to refine his calculations, and discovered that he could trace Comet Swift- Tuttle's orbit back almost two thousand years, to match comets observed in 188 AD and possibly even 69 BC. The orbit turned out to be more stable than he had originally thought, with the effects of the comet's jets less pronounced. Marsden concluded that it is highly unlikely the comet will be 15 days off in 2126, and he called off his warning of a possible collision. His new calculations show Comet Swift-Tuttle will pass a comfortable 15 million miles from Earth on its next trip to the inner solar system. However, when Marsden ran his orbital calculations further into the future, he found that, in 3044, Comet Swift-Tuttle may pass within a million miles of Earth, a true cosmic ``near miss.''

Marsden's prediction, and later retraction, of a possible collision between the Earth and the comet highlight that fact that we will most likely have century-long warnings of any potential collision, based on calculations of orbits of known and newly discovered asteroids and comets. Plenty of time to decide what to do.