On 12th March 2008, Cassini will swing by Saturn's moon Enceladus at an altitude of less than 100 kilometres at the point of closest approach. This will give scientists and unprecedented opportunity to study the plumes of water vapour emanating from the "tiger stripe" fissures near the moon's south pole, but it has also given the Cassini team pause for thought as to whether ice grains lofted by the jets could damage the spacecraft.
Dr Larry Esposito, who is presenting results of a study at the second European Planetary Science Congress in Potsdam on Thursday 23rd August, said, "These plumes were only discovered two years ago and we are just beginning to understand the mechanisms that cause them. 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. We have used measurements taken with Cassini's UVIS instrument during a flyby of Enceladus in 2005 to try and understand the shape and density of the plumes and the processes that are causing them."
Cassini's UVIS instrument was used to measure how much light from a star was absorbed when the star was obscured by the plumes, and this data used to calculate the amount of water vapour present in the column. Dr Esposito, who leads the UVIS instrument team, and colleagues at the University of Colorado developed simulations of the speeds and densities of particles in the plumes, based on emissions from multiple jets along each tiger stripe.
By comparing the results to the UVIS observations, they were able to calculate the average size of particles at the point where the plumes will be most dense during Cassini's March '08 encounter. Dr Esposito said, "Our results indicated that the average sized particle in the plume was less than a thousandth of the size that would cause damage, but we still needed to find out if high-pressure vents could send larger particles into the mix."
For a dangerous particle to be lofted by the plume, the mass of the particle must be equalled by the mass of the supporting column of water vapour. Dr Esposito has used two independent searches of the plumes, which could spot jets just 50 metres across with opacity of 10 percent.
Dr Esposito said, "In both these searches, we have seen no evidence for high-pressure jets. We estimate that the chance of Cassini being hit by a dangerously large particle appears to be no more than one in five hundred. Better measurements of the size distribution and its opacity would improve the model, but we think this is a conservative estimate. The chances are that Cassini should be unharmed by the flyby."
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