Comets that crash into the sun may be more common than anyone has suspected.
A year ago, the Naval Research Laboratory reported the first ''positive evidence'' of such a collision, which had occurred Aug. 30, 1979. The event was photographed by NRL's SOLWIND coronagraph - an instrument for studying the sun's outer atmosphere - on board the US Department of Defense P78-1 experimental satellite.
More recently, study of SOLWIND data has uncovered two more sun-comet collisions - Jan. 26 and July 20 of this year.
Such comets appear to follow normal orbits through the solar system. However, their perihelions - points of closest approach to the sun - are so low the comets plunge into the solar atmosphere.
Donald J. Michels and colleagues on the SOLWIND project estimated that the impact velocity for the 1979 collision was around 284 kilometers a second (about 640,000 miles an hour.) A giant cloud of debris could be seen in the coronagraph pictures. The energy released was estimated to be about 95,000 quads (quadrillion Btus), which is about a thousand times the annual energy consumption of the United States. Equivalently, it is the energy of 25 million one-megaton nuclear explosions.
Powerful though this may be, it is a tiny fraction of the sun's own energy output. So such a cometary impact would have little effect on that body. But what if it were the Earth?
When a relatively small (100-meter diameter) cometary fragment exploded in the atmosphere near Tunguska in Siberia in 1908, it devastated some 50 square kilometers of forest. Victor Clube and Bill Napier of the Royal Observatory in Edinburgh, Scotland, who are concerned about Earth-comet collisions, note that a major cometary impact would be widely devastating. It could raise tidal waves several kilometers high, create blast waves that would incinerate as much as a hemisphere, and blanket Earth in clouds of hot ash.
Most comet specialists have considered such collisions rare events. However, in recent years, a few scientists have begun to suggest otherwise.
Dr. J. C. Hills of the Los Alamos National Laboratory in the US has suggested that close passage of another star, as may happen from time to time, could cause a massive fallout of comets into the inner solar system from the cloud of billions of comets beyond the planets.
Clube and Napier, taking a novel view of the origin of comets, theorize that the solar system acquires comets when it moves through dense interstellar clouds in one of the spiral arms of our galaxy every 100 million years or so. Again, they suggest the inner solar system, and Earth, may come under cometary attack. They correlate such events with major changes in Earth's geological and biological development.
All of this, of course, is speculative and scientifically controversial. However, with more comets hitting the sun than were suspected, it would behoove scientists to keep an eye on wayward comets.