Astronomers Friday reported the discovery of a third tail, made up of sodium gas, emanating from Comet Hale-Bopp - a type of tail never found on any other comet.
“I don’t think it’s been either seen or predicted” before, said Brian Marsden, the astronomer who runs the International Astronomical Union’s clearinghouse for astronomical discoveries in Cambridge, Mass.
Comets have long been known to have two kinds of tails, one made of dust and the other of electrically charged gas, called plasma. The new discovery “seems indeed to be a third tail,” Marsden said. Some past comets have had multiple tails - one as many as eight - but always of the conventional two types.
The discovery was made by a team of astronomers at the Isaac Newton Group of telescopes on La Palma, in the Canary Islands. The new tail, the group said, has a completely different shape, structure and position from the dust and plasma tails of Hale-Bopp.
The dust tail is very broad and bright, forming a curved fanlike structure, and the plasma tail consists of a complex tangle of narrow wisps, seen from Earth to the left of the dust tail. The sodium tail, however, is even farther to the left; it is very thin and extends outward, straight as an arrow, apparently featureless and showing no broadening as it goes off into space.
The team used a camera they built specifically to make observations of Comet Hale-Bopp. Unlike most astronomical telescopes, it was designed with a very wide field of view - about 20 degrees, or 40 times the width of the full moon - in order to reveal details of Hale-Bopp’s long tails.
The tails, although they extend as far as 100 million miles through space - a distance as great as the span from Earth to the sun - are composed of very tiny particles, as fine as those in cigarette smoke, and so represent a very small amount of matter.
The team that made the discovery, as well as other astronomers, said they had no theory to explain how the sodium tail - whose gas, unlike the plasma tail, has no electrical charge - is created, or why it has its unique configuration.