Ice particles could hit Cassini near Saturn moon

LONDON, Aug 23 (Reuters) Tiny grains of ice or particles of space dust could significantly damage the Cassini spacecraft when it passes close by Saturn's moon Enceladus next March, scientists said today.

Images from Cassini beamed back to earth in 2005 showed multiple jets emanating from the moon's south polar region that scientists suspect arise from warm fractures, known as tiger stripes.

The upcoming flyby in March 2008 will offer a unique view of the little-understood plumes shooting out from these fissures but has also raised concern that this could stir up particles that damage the spacecraft.

Cassini will pass by Enceladus at an altitude of less than 100 km, perhaps moving through some of the spray, which some scientists believe may be made up of water spurting out from the moon under pressure.

''These plumes were only discovered two years ago and we are just beginning to understand the mechanisms that cause them,'' said Larry Esposito, a researcher at the University of Colorado, who presented his findings at the European Planetary Science Congress in Potsdam, Germany.

''A grain of ice or dust less than two millimetres across could cause significant damage to the Cassini spacecraft if it impacted with a sensitive area.'' To try to estimate potential damage, Esposito said a team used measurements from the 2005 flyby to understand better the shape and density of the plumes and what causes them.

This data helped researchers calculate the amount of water vapour present in the column and develop simulations of the speeds and densities of potentially damaging particles in the plumes.

They found that the joint NASA, Italian and European Space Agency Cassini spacecraft would probably fly by the moon in March unharmed, albeit with about a one in five hundred chance of being hit by a dangerously large particle.

''Our results indicated that the average sized particle in the plume was less than a thousandth of the sized that would cause damage but we still needed to find out if high pressure vents could send larger particles into the mix.'' REUTERS JT PM0910