HALE-BOPP, the comet heading for its closest approach to the Sun before it swings off into outer space for 2,500 years could be carrying a cargo of hydrogen cyanide, scientists report.
Astronomers have detected a gas, cyanogen, possibly caused by the breakup of hydrogen cyanide trapped in the ice, dust and methane of the huge comet.
The US journal Science reports that other astronomers have discovered grains of crystalline silica, in the ghostly tail of the comet from the edge of deep space: this means that 4.5 billion years ago when the solar system formed from a nebula of star dust, the silica was very hot. And observers using the Hubble space telescope report huge sudden bursts of dust from the comets nucleus as bits of the surface arc heated by the sun.
Hale Bopp was at its closest to the Earth on March 22nd, when it was 120 million miles away. It will be closest to the sun on April 1st and is likely to get brighter.
The frozen, muddy iceberg at the heart of the heavenly display could be up to 25 miles across, three or four times bigger than the more famous Halley's Comet. By the time it leaves the solar system, Hale Bopp will be the most studied comet ever.
Although it was first spotted in July 1995 by two land based astronomers called Alan Hale, of New Mexico, and Thomas Bopp, of Arizona, it was picked up within a month by both the Hubble telescope and an astronomical research satellite called the International Ultraviolet Explorer. Scientists at Mt Palomar in California have also been watching it through an infra red instrument attached to a 200 inch telescope. European scientists have been watching it through their own Infrared Space Observatory, launched in 1995.
Both US and European teams are also using the Ulysses spacecraft, flying round the Sun's equator, to match activity on the Sun with changes in the brightness of the comet. They have been watching with increasing excitement as the comet accelerated across more than 500 million miles of space to its close encounter with a delighted world, growing a "tail" millions of miles along.
Comets are thought to be stardust fragments formed by longdead stars which have been deep frozen for billions of years and which are now the only known relics of the original "bricks" from which the star system was formed.
Collisions with comets are thought to have altered the course of evolution several times. Comets may even have had a role in the making of life in the first place even now, every year 300 tonnes of organic chemicals - left behind by asteroids and comets - drift down through the atmosphere to the surface of the Earth.
Hale Bopp probably last swung around the Sun 4,000 years ago it could be 2,500 years before it appears again.
Astronomers seeking to discover the raws materials from which Earth was made have so far detected more than 50 chemicals on Hale Bopp, including water, carbon monoxide, carbon dioxide, carbon disulfide, methane cyanogen and the mineral olivine.
"This is a unique opportunity," said Dr Harold Weaver, an astrophysicist at Johns Hopkins University. "We have never had a chance to examine a comet in this much detail over this large a range of distance from the Sun."