Polar scientists have been rubbing their hands in expectation of the impending "collision of the century" between the 120-kilometer-long (75-mile) B-15A iceberg and a huge floating glacier, the Drygalski Ice Tongue, that juts into McMurdo Sound.

The meeting between these two behemoths was to have taken place by last Saturday at the latest, according to images taken by the NASA satellites Aqua and Terra.

Collisions on this scale are extraordinarily rare, powerfully affecting the area's wildlife, as well as shoreline topography and sea currents, and it is exceptional for scientists to be able to witness such an event by satellite.

However, "B-15A's drift appears to have slowed markedly in recent days," said ESA, whose Envisat satellite also has a ringside seat.

"The iceberg may have run aground just before colliding," a press release quoted Mark Drinkwater of ESA's Ice/Oceans Unit as saying.

"This supports the hypothesis that the seabed around the Drygalski Ice Tongue is shallow, and surrounded by deposits of glacial material that may have helped preserve it from past collisions, despite its apparent fragility.

"What may be needed to release it from its present stalled location is for the surface currents to turn it into the wind, combined with help from a mixture of wind, tides and bottom melting to float it off its perch."

The bottle-shaped B-15A is the size of Luxembourg. It split off from the B-15 iceberg, five times larger, which broke away from the Ross Ice Shelf in March 2000.

B-15A has since drifted to McMurdo Sound, where it has blocked ocean currents and led to a buildup of sea-ice, causing starvation among penguins unable to forage in the open sea and disrupting the resupply of the nearby US and New Zealand Antarctic stations.

The Drygalski Ice Tongue, about 20 kilometers (12 miles) wide, is a projection into the sea from a glacier. Ice tongues are known to rapidly change their size and shape and waves and storms weaken their ends and sides, breaking off pieces to float as icebergs.

The ESA website has images of B-15's progress (http://www.esa.int/export/esaEO/SEMIWT71Y3E_index_0.html).

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