IF COMETS are harbingers of great events, there should be momentous times ahead. Take the dramatic fireworks in July 1994 as Comet Shoemaker-Levy plunged into Jupiter, leaving vast bruises on the giant planet. Or Comet Hyakutake, which in March this year came so close to the Earth that it was visible even through the glare of city lights.
But the best may still be to come. Next spring, if predictions hold true, the northern skies will be lit by a brilliant new comet, which will arc through the sky trailing a rich tail and outshining every star. Comet Hale-Bopp could be the astronomical sight of the century.
Let's keep our fingers crossed. Back in 1973 astronomers predicted great things from Comet Kohoutek, only to find that it fizzled out under the eyes of an expectant public. Their chagrin was such that they underplayed their expectations for Comet West in 1976, with the result that many people missed what turned out to be a splendid sight.
So why are astronomers so confident now about Hale-Bopp? One hopeful sign is that it is already astonishingly bright, even while it is still far from the Sun. Comets are balls of dust, rocks and ice. They "switch on" when they are close enough to the Sun for its warming rays to vaporise the ice and form a cloud of gas and dust, or "coma", around the frozen nucleus. This coma scatters sunlight back to us-the more dust and gas, the brighter the comet is.
Most comets are detected when they are close enough to the Sun for their water ice to vaporise-between two and three times as far away from the Sun as the Earth. But Hale-Bopp was spotted in July 1995 when it was still seven times as far away from the Sun as the Earth. "Out beyond the orbit of Jupiter, this thing was already a tremendous producer of gas and dust," says Hal Weaver, an astronomer at Johns Hopkins University in Baltimore. "It just made our eyes bulge thinking about the prospects."
It's not just the prospect of a spectacular display that excites Weaver. Comets like Hale-Bopp could hold the key to the continuing mystery of how the Solar System formed. Astronomers believe that the Sun and its planets originated from a giant, cold, dense cloud of gas and dust. Something-perhaps the explosion of a nearby star-destabilised the cloud, and it began to collapse. Material falling into the centre eventually became hot and dense enough for nuclear fusion to begin, and the Sun was born. Farther out, the cloud collapsed into a disc, where lumps of rock collided, stuck and began forming planets.
Between and beyond the newly forming planets, small lumps of ice and dust began to agglomerate to form comets. Closer to the Sun than Jupiter it was too warm for the ice to stay frozen. In the vicinity of Jupiter and its giant neighbour Saturn, the orbits of newly forming comets were unstable: most would have been sucked in by the planets' gravity, or flung out of the Solar System. And way out, beyond the influence of any of the planets, the comets formed a disc-the Kuiper Belt-in the same plane as the planets.
But most comets that formed not quite so far out, in the region around the planets Uranus and Neptune, were neither sucked into the planets nor lost to the Solar System altogether. Instead, they were slung into distant orbits, which were quickly randomised by the influence of passing stars, leaving them in a spherical cloud, dubbed the Oort Cloud. Hale-Bopp, with its orbit almost at right angles to the plane of the Solar System, is almost certainly a visitor from the Oort Cloud.
This means that Hale-Bopp is probably one of the most ancient, pristine bodies in the Solar System. The surfaces of the Moon and the planets have been changed beyond recognition by billions of years of bombardment by meteorites. And relentless chemical activity has made its mark on the warm inner planets, and dense gaseous interiors of the giant gas planets farther out. Even comets from the Kuiper Belt may not be totally protected from such changes. In August this year, Paolo Farinella of the University of Pisa and Donald Davis of the Planetary Science Institute (PSI) in Tucson, Arizona, published a paper in
Hale-Bopp has already brought some surprises. For most comets, the spectra show that their icy parts are dominated by water. Hyakutake's ices, for instance, are around 80 per cent water. But out where Hale-Bopp is now, it is far too cold for water ice to vaporise, so something else must be making it so extraordinarily bright. Dave Jewitt and his colleagues from the University of Hawaii discovered in September last year that Hale-Bopp was throwing out vast amounts of carbon monoxide-around a tonne of gas per second. And more recently, using the European Space Agency's orbiting Infrared Space Observatory, Jacques Crovisier from the Paris Observatory at Meudon and his colleagues have spotted carbon dioxide coming from the comet at about a third the rate of the carbon monoxide.
Water could even be a minor constituent in the comet's ices, according to Mike Mumma of NASA's Goddard Space Flight Center in Maryland. There have been a couple of other examples of comets that do not seem to be dominated by water, but there has never been a chance to study one in detail as it travels into the heart of the Solar System.
"It's a clue to the chemistry that actually occurred either in the early solar nebula or in the dense interstellar cloud core before the solar nebula formed," says Mumma. The fact that some comets seem to contain much more water than others, he says, suggests that they formed under different chemical conditions. Perhaps these comets were born in parts of the solar nebula where there was a higher concentration of hydrogen atoms, making it easier for water to form. The interstellar medium, from which the original gas cloud must have formed, contains oxygen, acetylene and carbon monoxide as well as hydrogen. If there was less hydrogen around when Hale-Bopp formed, that should mean less water, but also smaller concentrations of complex hydrocarbons, and of formaldehyde and methanol, too. If this reasoning is correct, then comets containing less water should also contain relatively small amounts of these substances, compared with the amount of other frozen gases.
But this reasoning assumes that comets formed from a mix of molecules that was similar to that of the interstellar medium, and things may well have been more complicated than that. Most theories assume that the dense cloud from which the Sun was formed contained grains of dust coated with ice of various kinds on their surfaces. As these grains fell into the disc surrounding the young Sun, they grew hot, the ices evaporated, and the resulting gases then reacted with one another to create a brand-new soup of chemicals from which the comets formed.
And even this may be too simplistic. It is hard to understand how the large amounts of carbon dioxide that Hale-Bopp seems to contain could have formed in the chemical conditions of the early Solar System. So perhaps some of the ices from the initial gas cloud survived longer than the accepted theories predict, and the composition of comets could reflect the make-up of the interstellar medium as much as the conditions in the early Solar System.
With so many possibilities, and so much uncertainty, measurements from a handful of comets cannot solve the problem. What astronomers are hoping for from Hale-Bopp, however, is more data to feed into their models. This should help them pin down the origin of the ices from which comets form. Mumma and his team are especially interested in the hydrocarbon ethane, which they recently identified in Comet Hyakutake, and they already have a tentative sighting in Hale-Bopp. They would also like to identify and measure related families of chemicals-methane, ethane and propane, say, and methanol, ethanol and propanol-many of which have never been seen before in comets.
Hale-Bopp's spectacular brightness is a big plus for observations of this kind. Most comets are so faint that you need to collect data for hours simply to measure one chemical species. This leaves little or no time to hunt for anything unexpected or even unusual. "Having a lot of photons helps," says Mike A'Hearn of the University of Maryland in College Park. "You can search for all the things you predicted in an hour, and then you've got hours left over to look through other parts of the spectrum to see what's there. The most interesting results will be the ones you can't predict."
Studying the composition of Hale-Bopp could also give a handle on the temperature at which it formed. One key molecule A'Hearn hopes to spot is diatomic sulphur. If it is there, it will mean that the comet formed at a temperature of 40 kelvin or less. Mumma is hoping to get a similar insight from the abundances of argon and neon in the comet. Neon condenses at around 20 K and argon at 40 K, and mixtures of the two gases condense at temperatures in between. Knowing the ratio of the two could pin the formation temperature down even more precisely.
Because Hale-Bopp is so bright, it was detected nearly two years before it is due to reach perihelion-its closest approach to the Sun. This is unheard-of luxury. When Hyakutake appeared on the scene, astronomers had just a few months to scramble for their telescopes before it reached perihelion. With Hale-Bopp, they have had time to plan, and to mobilise the really big resources. "We have time to think," says Jewitt. "The science will be better because of that."
The Hubble Space Telescope has already spent many hours trained on Hale-Bopp. It established, for instance, that the nucleus is huge-somewhere around 30 to 40 kilometres across. But this week it has had to quit. The reason is that the comet is about to move within 50° of the Sun-too close for Hubble and most other optical observatories, which would be swamped by the Sun's light. Hubble will be unable to study Hale-Bopp again until September 1997, five months after it has reached perihelion. "We're missing the most active period," says Weaver ruefully. "It's very disappointing".
Fortunately, radio and infrared observatories are not hampered in the same way, as the Sun is relatively faint at these wavelengths. And these parts of the spectrum can tell us what we want to know about the most interesting chemicals making up the comet. "The radio and infrared observatories will clean up," says Weaver. "They are already having a field day."
While most astronomers expect a scientific bonanza from Hale-Bopp, they are quick to point out that there is only so much to be learnt from one comet. "There are so many things that we are still ignorant about-drawing definite conclusions will be very difficult," says Weaver. "Hale-Bopp is going to be a giant step forwards, but I don't think we're going to see all the answers in this crystal ball."
But Hale-Bopp should still be a thrilling sight as it approaches perihelion, just inside the Earth's orbit, on 1 April. Unlike the disappointing Comet Kohoutek, Hale-Bopp is not making its first foray to the inner regions of the Solar System. Hale-Bopp's orbit is well established, and astronomers calculate that it last visited some 3000 years ago.
Before Kohoutek arrived in 1973 it had spent 4.5 billion years in the Oort Cloud being bombarded by galactic cosmic rays. These would have broken almost every chemical bond in the comet's outermost layer, creating large numbers of unstable molecules. When the comet headed inwards for the first time, these molecules were blasted out from the surface as soon as the temperature warmed to about 40 K, somewhere around Pluto's orbit. The explosion produced a bright halo of gas and dust. The problem in 1973 was that astronomers mistook the leftovers from this explosion for a bright, active nucleus. In fact, the explosion was short-lived, leaving the relatively inactive nucleus to head inwards as a celestial damp squib.
Hale-Bopp's present brightness is certainly not from the same cause as Kohoutek's. But there is still a niggling doubt. When the spacecraft Giotto visited Comet Halley in 1986, it produced images showing that the comet's activity seemed to be coming from just 10 per cent of its surface. Something similar seems to be happening with Hale-Bopp, according to Jewitt and his colleagues, who have been inspecting the new comet through the James Clerk Maxwell telescope in Hawaii. They report that the comet's gases are being released from just a handful of places on the surface of the nucleus. "You see five or six jets in the plane of the sky-it's like a potato with spotty eyes that are the active areas," he says.
One possible explanation is that cometary nuclei could be made up of several different snowballs with different chemical compositions. Stuart Weidenschilling at the PSI has suggested that small comets, formed just outside the plane of the Solar System, might stick together before falling into the disc. In that case, different parts of the comet could contain different amounts of volatile ices. Alternatively, there could be a rocky mantle covering some parts of the nucleus, protecting the ices beneath it from being vaporised. This could happen if dust and gas were gradually drawn out from the surface over time, leaving behind larger pebbles that are too heavy to dislodge. The dark patches that Giotto spotted in Halley's inactive areas support this idea.
Either way, there is a chance that as Hale-Bopp approaches perihelion, it will present a different, less active face to the Sun, switching off its dramatic brightness. But there are good grounds for believing this will not happen. "So far we've had an almost continuous stream of activity," says Weaver. "It's shown no sign of wimping out on us. It's hard to believe that this thing is not going to be an incredible monster comet in the spring." Most astronomers are fervently hoping he is right.
Where and when to see Hale-Bopp
Comet Hale-Bopp's orbit around the Sun is almost perpendicular to the plane of the Solar System. At the moment, the comet is coming up from the south and is about to disappear into the glare of the Sun. It will re-emerge in January in the morning sky near the constellation Aquila. It should then get brighter as it moves across Cygnus and Pegasus.
On March 22 it reaches its closest point to the Earth, still more than 190 million kilometres away. In late March, the comet will move into the evening sky. Then on April 1 it will reach perihelion-the point of its orbit that is closest to the Sun. If all goes well, Hale-Bopp should now be at its brightest-perhaps even brighter than Sirius, which is the most brilliant star in the sky.
During March and April, Hale-Bopp will be difficult to see from the Southern Hemisphere. For those at high northern latitudes, above around 45°, it will be visible throughout the night.
Because it is much farther from the Earth than this year's comet, Hyakutake, Hale-Bopp should look more like a traditional comet, and less like a large fuzzy ball. It should have a condensed head, like a bright star, and a long tail.
Hale-Bopp should still be clearly visible throughout May as it passes through Taurus and Orion. Eventually, by June it will probably begin to fade as it passes away from both Sun and Earth. It will not return for more than 3000 years.
Hale-Bopp's orbit is almost at right angles to the plane of the Solar System. Coming up from the south, it will be closest to the Sun on 1 April 1997
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20:00 03 July 2008
