The great comet of 1997
Comet Hale–Bopp
For anyone who looked up at the sky in the spring of 1997, Hale–Bopp was the comet of a lifetime: two clearly distinct tails, visible even from cities, and a brightness that kept it within reach of the naked eye for a year and a half. The secret was its size: a nucleus about 60 km across, among the largest ever observed.
Perihelion passage of 1 April 1997, from the 3D simulation
- Nucleus
- 60 km
- Orbital period
- 2,400 years
- Perihelion
- 0.919 AU
- Aphelion
- 357 AU
- Last perihelion
- 1 April 1997
- Discovered
- 1995
Nucleus and tail
The nucleus of Hale–Bopp has never been seen up close: estimates, derived from its brightness and from Hubble images, give it a diameter of about 60 km, with an uncertainty of twenty or so. It is about five times wider than that of Halley's Comet. In the model on this page it is shown as a sphere of that diameter, because its true shape is unknown. It rotates in about 11.4 hours.
A nucleus that large releases enormous amounts of gas and dust: at perihelion Hale–Bopp was losing about 300 tonnes of water per second. Besides the blue ion tail and the dust tail, yellowish and very broad, astronomers discovered a third, thin and made of sodium atoms, identified in 1997 by a team led by the Italian Gabriele Cremonese.
Orbit
Hale–Bopp follows a very long orbit almost perpendicular to the plane of the planets, tilted 89°. At perihelion it passed 0.92 AU from the Sun, just inside Earth's orbit; at aphelion it recedes to about 360 AU, twelve times the distance of Neptune. Today one full lap takes about 2,400 years.
It was not so before 1997. On its way in, the comet passed 0.77 AU from Jupiter in April 1996, and the planet's gravity shortened its period, which had previously been about 4,200 years. Its previous passage therefore dates back more than four thousand years; the next is expected around the year 4400.
Discovery and exploration
It was discovered on the same night, 23 July 1995, by two observers who did not know each other: the astronomer Alan Hale, in New Mexico, and the amateur Thomas Bopp, in Arizona, who did not own a telescope and was using a friend's. The comet was still 7.2 AU from the Sun, beyond the orbit of Jupiter: none had ever been discovered so far out by non-professional observers.
No spacecraft has visited it, but it was one of the most observed comets in history, followed from the ground and from space for years. It remained visible to the naked eye for about 18 months, from May 1996 to December 1997, twice the previous record, which had stood since 1811, and in the spring of 1997 it was brighter than almost every star. Ten years later, more than 25 AU from the Sun, it was still faintly active.
Frequently asked questions
When will Hale–Bopp return?
In more than two thousand years: its next perihelion is expected around the year 4400. Its current period, about 2,400 years, is shorter than the one it had before 1997, because on its last passage the comet was slowed by Jupiter's gravity.
Why was Hale–Bopp so bright?
Because of the size of its nucleus, about 60 km: five times that of Halley's Comet. More surface means more ice exposed to the Sun, and so more gas and dust to reflect light. The comet was already active 7 AU from the Sun, where almost all others are still dormant.
How big is Hale–Bopp's nucleus?
About 60 km across, according to the most widely cited estimates, which range from 40 to 80 km. It is one of the largest nuclei among the comets that have entered the inner Solar System. As it has never been visited by a spacecraft, its exact shape and size remain uncertain.
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