By Paul Rincon
BBC News Online science staff
The Cassini spacecraft has caught two huge, swirling storms in the act of merging on Saturn.
The storms approached each other at different speeds...
It is just the second time this has been seen, occurring as the probe nears Saturn to begin a four-year mission of exploration in orbit around the planet.
Scientists observed events for about a month as two 1,000km-wide storms approached on a collision course.
The storms twisted around each other in an anti-clockwise direction as they merged over 19 and 20 March.
The only other time this phenomenon has been witnessed was in August 1981 when Voyager captured images of storms partially merging.
Cassini will enter orbit around Saturn when it arrives on 1 July 2004.
It will release its piggybacked Huygens probe about six months later for descent through the thick atmosphere of Saturn's moon Titan.
The two southern hemisphere storms were observed moving west, relative to the rotation of Saturn's interior, between 22 February and 22 March.
The more northerly storm was calculated to be moving at about 11m/s (25mph), about twice the speed of the southerly one which was estimated to be travelling at around 6m/s (13mph).
As they merged, they circled around each other in an anti-clockwise direction, the opposite of how hurricanes spin in Earth's Southern Hemisphere.
"These are long-lived vortices, and the vortices seem to cannibalise each other and grow," Dr Carolyn Porco, leader of Cassini's Imaging Science team, told BBC News Online.
"We don't know how rare they are, so it was really rather thrilling to capture it."
They merged to form an enormous storm which was elongated in a north-south direction and had bright clouds at either end.
...and merged to form a bigger one
By 22 March, the storm had settled into a more circular shape and the bright clouds had spread around its circumference in a halo.
Although these storms travelled west, storms at Saturn's equator move east at speeds of up to 450m/s (1,000mph), about 10 times the speed of Earth's jet streams.
The data from Cassini is certain to enrich our knowledge of the inner workings of Saturn, which are currently poorly understood.
"Many storms on the giant planets end their lives by merging. How they form, however, is still uncertain," said Professor Andrew Ingersoll, of the California Institute of Technology in Pasadena, US, and a Cassini imaging team member.
"On Earth, storms last for a week or so and usually fade away when they enter the mature phase and can no longer extract energy from their surroundings.
"On Saturn and the other giant planets, storms last for months, years or even centuries."
The Cassini-Huygens mission is a collaboration between the US and European space agencies (Nasa and Esa) and the Italian Space Agency.