Hale-Bopp was the great comet of 1997 - intrinsically so bright that it was discovered out beyond Jupiter, nearly two years before perihelion. Powered by an exceptionally large nucleus, perhaps 60 km across, it stayed visible to the naked eye for a record 18 months, even from light-polluted cities.
It stayed visible to the naked eye for a record ~18 months through 1996-97.
It was found 7 AU from the Sun - farther out than any previous amateur comet discovery.
Besides its two classic tails it sported a rare third tail of glowing sodium atoms.
One comet, two tails
Hale-Bopp showed the classic double tail beautifully. The blue ion tail is gas ionised by sunlight and snapped straight anti-sunward by the magnetised solar wind; the white-yellow dust tail is heavier grains pushed more gently by radiation pressure, so they lag behind and curve along the orbit. Two tails, two different forces - and neither points back along the comet’s path.
Physical characteristics
Diameter
60 km
Rotation period
11.35 h
Atmosphere and composition
Atmosphere
In 1997 it wrapped itself in an enormous coma of water, CO and dust, millions of km across.
Composition
Ice-rich and unusually active: water, CO, CO₂ and a rich suite of organic gases were detected in its coma.
Orbit and activity
Perihelion distance
0.91 AU
Aphelion distance
370 AU
Origin
Long-period comet from the Oort cloud
Last perihelion
1 Apr 1997
Next perihelion
~4385 AD
Discovery
Discovered by
Alan Hale & Thomas Bopp
Year
1995
How
independent same-night discoveries on 23 Jul 1995, while the comet was still beyond Jupiter
What is still uncertain
Nucleus diameter estimates range from about 40 to 80 km; 60 km is a mid-range value. The orbital period is likewise approximate.