The French-Finnish SWAN instrument on the SOHO spacecraft had already been observing Comet LINEAR by ultraviolet light for two months, and continued to watch it until the remnants faded from view in mid-August.

Today, the SWAN team reports in the journal Science that their observations showed four major outbursts in June and July.

The fragmentation seen by SWAN began on 21 July, 2000, almost a week before observers on the ground noticed it. Between 25 May and 12 August, the dying comet released altogether 3.3 million tons of water vapor into space, as its ice evaporated in the warmth of the Sun. The data also suggest that the density of Comet LINEAR was extremely low.

"Only SWAN on SOHO saw the entire drama of this self-destroying object," comments Teemu Mäkinen of the Finnish Meteorological Institute, lead author of the report in Science. "The ice on the surface of the comet's nucleus did not simply vaporize as in a normal comet, but came away in large chunks. We saw 90 per cent of the ice falling off before the complete fragmentation of the remainder began."

Comet LINEAR, known more formally as Comet 1999 S4, was discovered by the LINEAR asteroid-hunting telescope in the USA, and may have been making its first visit to the Sun. It disappointed amateur astronomers by not becoming bright enough to see with the naked eye.

The break-up occurred near the time of the comet's closest approach to the Sun on 26 July, when it was moving across the sky from Ursa Major towards Leo.

In early August, the NASA-ESA Hubble Space Telescope and the European Southern Observatory's Very Large Telescope in Chile both saw about 16 fragments in the form of mini-comets, which faded away by the middle of the month. These observations by visible light indicated that the pieces were about 100 meters in diameter. A prominent dust tail still visible in early August corresponded with the onset of fragmentation seen by SWAN on 21 July.

SWAN's unique capability in observing comets comes from its continuous scanning of the whole sky, at just the right ultraviolet wavelength to see the cloud of hydrogen atoms that surrounds every moderately active comet.

The hydrogen comes from the breakup of water molecules released from the comet by the Sun's warmth. SWAN also benefits from its location on the ESA-NASA SOHO spacecraft 1.5 million kilometers from the Earth, well clear of a hydrogen cloud that surrounds the Earth itself.

"Our primary aim is to study the interaction of the solar wind with interstellar hydrogen," explains Jean-Loup Bertaux of France's Service d'Aéronomie, the principal investigator for SWAN. "But we always knew that we'd have an excellent view of comets too. They are quite often traceable in our records even before their formal discovery by others."

Complete fragmentation provides a rare opportunity for scientists to learn about the internal makeup of a comet. Members of the SWAN team believe that their newly published results compel them and their fellow scientists to think afresh about Comet LINEAR's construction, and to consider that different parts of the young Solar System may have produced comets of different sorts.

"Comets do not usually blow themselves to smithereens," says lead author Mäkinen. "So we should not be surprised if Comet LINEAR was peculiar in composition and structure compared with other comets."

The character of the comet did not change throughout the months of observation by SWAN, even when deep layers inside the nucleus were being laid bare. Comet scientists usually have to consider the possibility that the surface of the nucleus is different in composition from the interior. One lesson from the "SWAN song" of Comet LINEAR seems to be that, in this case at least, the surface exposed at the outset was representative of the whole nucleus.

The SWAN team also suspects that Comet LINEAR was as flimsy and light as the expanded polystyrene used for packing fragile equipment. The density of its water ice may have been as low as 15 kilograms per cubic meter, compared with 917 kg/m^3 for familiar non-porous ice on the Earth.

Even allowing for a possibly equal mass of dust grains within the comet, a total density of 30 kg/m^3 would be far less than the 500 kg/m^3 often assumed by comet scientists. By this reckoning, the initial diameter of Comet LINEAR on its approach to the Sun was about 750 meters.

"Our opinion about the low density is tentative and controversial," says Jean-Loup Bertaux. "We expect plenty of arguments with our colleagues when we put all the observations of Comet LINEAR together. But we start with the advantage of having seen the whole course of events, which no one else did."

The breakup of Comet LINEAR gave a small-scale impression of the disintegration, many centuries ago, of a far larger comet into an enormous swarm of mini-comets. LASCO, another instrument on SOHO, has observed hundreds of the fragments from that event falling into the Sun.

[Contact: Dr. Paal Brekke, Dr. Teemu Mäkinen, Dr. Jean-Loup Bertaux ]

18-May-2001