THE comet c/1995 01, more popularly known as Hale-Bopp, has continued to surprise astronomers since its discovery last summer. Discovered at a distance of seven astronomical units from the sun (an astronomical unit is equal to the mean distance between the sun and the earth), the comet is almost one hundred times brighter than Halley's comet at the same distance.

This exceptional brightness is a mystery, because a comet's brightness is due to the reflection of sunlight from dust whic'h is being driven away from the nucleus of the comet due of sublimation of ice. At closer distances to the sun, sublimation of water ice could make the comet very bright but at such large distances, still well outside the orbit of Jupiter, the temperature is not hot enough for this process to be effective.

Now, two groups have independently reported the detection of carbon monoxide (co) from the nucleus which could be a source of the brightness. Nicolas Biver and his colleagues and D Jewitt and his co-workers have reported substantial amount of co outgassing from the Hale-Bopp nucleus. Using extremely sensitive millimetre wave telescopes at Institute de Radio Astronomie Miilimetrique (jRAm), 30-m radio telescope at Spain and the James Clerk Maxwell telescope at Hawaii, both groups have seen a co rotational line.

With these measurements, they have used a simple model by J Crovisier to deduce the co production rates from the comet. They estimate that the comet is shedding co at the rate of 1,000 kg every second. This is remarkable because Halley's comet had this production rate at a distance of about one astronomical unit when the effect of the sun is much stronger.

This result could have a profound effect on our understanding of comets. It.is generally believed that water is the dominant ice in the comet's nucleus. But at these distances, water outgassing cannot be very strong because of the low temperature, about -160'c. Scientists have searched for other molecules like HCN but have not found any. They believe that though co outgassing is the major activity at the moment, other species will become active as the comet approaches the sun, Though the results are independently obtained by two groups, accurate modelling of the cometary brightness is notoriously difficult, as was the case of the famous comet Kohoutek in 1974.

In any case, most astronomers believe that when the comet Hale-Bopp reaches its nearest point to the Sun, the perihelion in March 1997, it will be a spectacular sight. The presence of an extremely bright object with very sophisticated instruments could throw light on many mysteries surrounding the evolution of comets. (Nature, March 14, 1996).