18 May 2000
In the spring and early summer of 1997, stargazers were treated to a beautiful sky show with the passing of wispy-tailed Comet Hale-Bopp. But scientists now say that while all eyes were on this dazzling sight, another near-Earth comet slipped quietly through the sky unnoticed.
Astronomers analyzing archived data from the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) spacecraft have recently reported the presence of a never-before-detected comet, which flew close to Earth in 1997. The comet, temporarily dubbed C/1997 K2, was apparently brighter than every comet discovered by astronomers in the six months preceding its appearance, adding to the mystery of how such a prominent comet may have zoomed past Earth unseen.
"To say we were surprised would be a bit of an understatement," said Finnish astronomer Teemu Mäkinen, lead author of a paper in this week's Nature, which describes the new comet discovery. "It sounded quite unlikely [to us] that a comet of such magnitude could elude both professionals and amateurs alike."
Despite its brightness, Comet K2 would not have been visible to the naked eye. Yet even "inexpensive amateur equipment would have sufficed" for stargazers to see the comet, said Mäkinen. "I believe that many amateurs were lured by the spectacular display of the concurrent Comet Hale-Bopp," he explained.
"Our records are patchy, especially at high latitudes," said Mäkinen. "It was probably missed by dedicated surveys because of its trajectory, which went through the southern ecliptic pole."
Though it slipped by unnoticed by human eyes, evidence for the K2 comet was captured by SOHO's Solar Wind Anisotropies (SWAN) instrument from May to July of 1997.
The SWAN instrument is not dedicated to comet discovery. Rather, it was designed to observe emissions of hydrogen around the sun at ultraviolet wavelengths. But these emissions, known as Lyman-a emissions, are also given off in high quantities by comets. As a result, though its resolution is relatively poor, the instrument is also sensitive to comets.
A great deal of work has taken place to locate and categorize near-Earth objects like comets and asteroids. Although it may sound like science fiction, scientists say that the threat of comet impacts on Earth is very real.
To highlight the threat of such an impact, Massachusetts Institute of Technology professor Richard P. Binzel even created a scale to measure the probability that a given object will hit Earth and cause it harm. Called the Torino scale, this method of measurement highlights the fact that Earth's current environmental balance is precarious, and it and the life that depends on it could be devastated in one swift blow.
So what are the chances of a comet hitting Earth, or of astronomers missing a potentially dangerous on-coming object. No one really knows. What astronomers do know, however, is that better funding for continual sky scans would help them map all comets that streak through the solar system.
"It is just a matter of prioritization. Do we want to use limited resources on preventing something that is likely to happen every day and cause moderate casualties -- like traffic accidents -- or something that will happen once in 30 million years but has potential for wiping out all of humankind?" asked Mäkinen. "In such situations, unfortunately, myopia is usually bliss."