Checking on an old and active cometary friend
Posted: March 7, 2001

When Comet Hale-Bopp passed through the inner solar system in early 1997, it was admired in the sky by a substantial fraction of the world's population. It was the true image of a "classical" comet, with a bright head and an enormous, multi-coloured tail. Due to its fortuitous orbit, it remained visible in the evening sky during several months, with all the associated positive effects.

Comet Hale-Bopp, still active at a distance of nearly 2,000 million kilometres from the Sun. The photo is a colour composite of several exposures in different wavebands, obtained with the Wide-Field Imager (WFI) camera at the MPG/ESO 2.2-m telescope at the La Silla Observatory. Despite the very large distance from the Sun, the comet is still "active" - it continues to lose material, as demonstrated by the curved jet, and also possesses an enormous coma. The broad, fan-shaped extension in the tail direction (to the upper left) measures at least 2 million kilometres. Photo: ESO
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. It is no wonder that, as an extra bonus, the general interest in astronomy received a major boost on this occasion.

Since the passage four years ago, the comet has been moving away from the Sun and 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 behaviour. The large 'dirty snowball' nucleus of ice and dust (probably about 50 km diameter) continues to be active, despite the very low temperature where it is now. This is quite unusual for a comet and is clearly confirmed on the present photo (above) from the WFI camera on the MPG/ESO 2.2-m telescope at La Silla, obtained a few days ago. The comet was about 1950 million kilometres (13.0 AU) from the Sun (and about 1965 milion km from the Earth).

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. 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' (towards the upper left of below picture). The total size of the comet is still a staggering 2 million kilometres, or about five times the distance between the Earth and the Moon.

This image of the comet has been enhanced to show these faintest parts better. The frames on which this photo is based were obtained during three nights (February 27 - March 2, 2001). As the comet moved in the meantime, there are several images of each star in the field - they appear multi-coloured, according to the optical filters that were used. Photo: ESO
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.

Astronomers at 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 2500 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 11 km/sec, or 1 million km per day. It will be interesting to see how long the present, highly unusual activity continues.