The European Southern Observatory (ESO) has released a picture of Comet Hale-Bopp showing that the "dirty snowball" is still sloughing off material despite having sped nearly 1.2 billion miles (2 billion kilometers) from the Sun since its close approach in 1997.
The image, obtained with a telescope at the ESO's La Silla Observatory in Chile, shows that Hale-Bopp's 31-mile (50-kilometer) wide nucleus of ice and dust continues to be active, despite the very low temperature in its present location. This is unusual for a comet.
Hale-Bopp is now located at a distance that corresponds to nearly midway between the orbits of Saturn and Uranus. However, as the comet's orbit is highly inclined to the main plane in which the major planets move, Hale-Bopp is now far below that plane. It is seen deep in the southern sky, south of the Large Magellanic Cloud in the constellation Dorado (the Goldfish). It can therefore only be observed with telescopes located in the Southern Hemisphere.
As it moves away, observations are made from time to time to document the comet's behavior.
A famous comet
When Comet Hale-Bopp passed through the inner solar system in early 1997, it was admired in the sky by observers worldwide.
It was the true image of a "classical" comet, with a bright head and an enormous, multicolored tail. Due to its fortuitous orbit, it remained visible in the evening sky during several months with all the associated positive effects. Professional observers at large telescopes around the world gathered the richest data ever obtained from a single comet; amateurs at star parties in different countries made large numbers of beautiful images, and hardly a day passed without media reports about the latest developments of this spectacular celestial phenomenon.
Persistent activity
Hale-Bopp still has the prominent, curved jet-like structure in the coma that has been observed earlier. No changes in this structure were observed during the three nights of observation by astronomers at La Silla.
The jet consists of dust and gas escaping from the nucleus. It shines in reflected sunlight, as does the rest of the coma and also the very broad, fan-shaped "tail." The total size of the comet is still a staggering 1.2 million miles (2 million kilometers), or about five times the distance between Earth and the Moon.
Another famous comet, Halley, was found in 1991 to have a significant coma at about the same distance from the Sun. However, while Halley apparently underwent a major, short-lived outburst, possibly because of a collision with a piece of rock or ice, Hale-Bopp has been steadily emitting dust and gas all the time since the perihelion passage four years ago. Most astronomers believe that this unusual state must in some way be connected to the exceptionally large size of its nucleus, but the details are not known.
Hale-Bopp observations continue
Astronomers at the ESO and elsewhere will continue to follow Hale-Bopp as long as possible, perhaps during the next several decades. It is still relatively bright (magnitude 14.5, or about 2,500 times fainter than what can be seen with the unaided eye in a dark sky). It is now moving outwards at a speed of about 6.8 miles (11 kilometers) per second, or 621,400 miles (1 million kilometers) per day.
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