I Didn’t Know That

Q: Astronomers say that the last time Comet Hale-Bopp visited Earth was 4,200 years ago. How do they know this, since there are no written records?

A: They know how often it visits Earth by calculating its orbital characteristics, according to astronomer Donald Yeomans of the Jet Propulsion Laboratory. In essence, they determine precisely where the comet is on at least two occasions, then use Newton’s gravitational laws to calculate its orbit. Subsequent measurements--there have now been 2,200 observations--allow more accurate calculations. They can then extrapolate the orbit backward to get a good guess of its last approach to Earth, accurate to within a few years for a comet with an orbit as large as Hale-Bopp’s. Its next visit will be sooner. Because a close approach to Jupiter perturbed its orbit, Hale-Bopp will return in 2,380 years, Yeomans said.