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Astronomers baffled by biggest comet's lack of impact: Collision with the planet Jupiter causes burst of light and heat, but soon recedes compared to previous impacts, writes Susan Watts

ASTRONOMERS are baffled by the modest impact of what they had identified as being the biggest chunk of comet Shoemaker-Levy 9, which dive-bombed into Jupiter's frozen atmosphere on Wednesday evening.

Observers said yesterday that they had expected the crash of fragment Q1 to be the most dramatic yet. Instead, after a promising initial burst of light and heat lasting about 10 minutes, it soon paled beside earlier impacts. After a few hours, the dark scar it left on the planet's surface had all but disappeared.

Fragment Q1 was the first of a clump of three large chunks in the series of 21 expected to hit Jupiter in quick succession at almost the same spot. Yesterday, fragment R appeared to have landed on target - very close to the still-glowing impact zone of Q1. Astronomers in South Africa reported seeing the impact site of fragment S, twice as bright as Q1 and visible for about 15 minutes.

A ring of blotches, some formed by collisions several days ago, is still clearly visible from ground-based telescopes, although it is now beginning to dim. Fragments G and L, which hit on Monday and Tuesday evenings, are thought to have caused the biggest wounds so far.

Astronomers at the Royal Greenwich Observatory in Cambridge said Q1 may have been smaller than they thought, or more 'floppy' so it disintegrated as it approached Jupiter.

Alternatively, Q1 may have been far more dense than anticipated, and plunged deeper into the Jovian atmosphere before exploding - causing a less visible plume than explosions closer to the surface. However, astronomers said they would have expected the scar to have been bigger and longer-lasting if this were the case.

Four fragments crashed into the planet yesterday - the remaining two of the close-packed trio and, in the evening, the next two in the string.

A team of staff and pupils at Taunton School in Somerset claimed to have picked up radio emissions from Jupiter from three of this week's impacts. Trevor Hill, head of physics at the independent school, said that all three followed a similar pattern - a burst of activity about an hour before impact (possibly caused as the fragment passed through the orbit of Jupiter's moon, Io), then a multiple burst on impact followed by two hours of constant emissions.

The amateur team picked up its strongest signals after Q1's impact. It is monitoring Jupiter at lower radio frequencies than professional observers, who use higher frequencies to receive more detailed information.

Micheal Garrett, an astronomer at Britain's Jodrell Bank radio observatory in Cheshire, said the Somerset findings appeared to correlate with those from the Beijing Astronomical Observatory in China monitoring at similar frequencies from Xinxiang.

By this morning, only two large chunks of the 3-million-mile-long comet train remained. These were due to hit Jupiter at about 5.15am and just after 9am. But the show does not quite stop then. For several weeks, astronomers expect small pieces of comet, from just a few centimetres to a few hundred metres across, to carry on raining into Jupiter.

Astronomers were yesterday trying to explain why they had not seen evidence of water vapour in Jupiter's atmosphere after Shoemaker-Levy 9 forced material from inner levels to the surface. It is possible that the comet chunks have not penetrated deep enough.

Robin Scagell, vice-president of the Society for Popular Astronomy, said amateur astronomers saw two big spots on Jupiter on Wednesday evening - the remains of impacts by fragments G and L: 'Like a huge pair of eyes looking out at us.'

(Photograph omitted)