One question jumps out at me from the blog entries that Cassini team members have been posting on the probe’s dazzling close pass by Enceladus. It’s from deputy project scientist Linda Spilker, who says: “I am thinking about the two Voyager flybys of the Saturn system that took place over 25 years ago. How in the world did we miss the Enceladus plumes back then???” Indeed, but that’s the nature of exploration, to learn something new each time you revisit a place, especially one that’s fully 10 AU out. The process is addictive, and breathtaking.
Do be aware of the flyby blog, offering an inside view of one of the most interesting of Cassini’s encounters thus far (also be aware that the entries are oddly out of order, a problem apparently being fixed). With the data downlink now started (as of about 0201 UTC today) via the Deep Space Network’s Goldstone station, we can ponder the chutzpah of taking a spacecraft so close to the huge geysers erupting out of the south pole of Enceladus. Cassini came within 200 kilometers of the surface as it flew through the outer edge of the plumes, and closed to a mere fifty kilometers at closest approach.
Image: Not from the latest flyby (images from that later), this false-color view of Enceladus has been enhanced to identify individual jets making up the plume. The images combined to create this view were acquired with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005 at a distance of approximately 148,000 kilometers (92,000 miles) from Enceladus and at a sun-Enceladus-spacecraft, or phase, angle of 161 degrees. Credit: NASA/JPL/Space Science Institute.
In terms of danger to the spacecraft, the threat is smaller than it at first appears, especially when you consider that Cassini deals with dust-size particles routinely as it orbits around Saturn. And the science return should be rich, with information on the density, size, composition and speed of the gas and particles collected. Sascha Kempf (Max Planck Institute, Heidelberg), deputy principal investigator for Cassini’s Cosmic Dust Analyzer, notes that various theories rise or fall on the outcome:
“There are two types of particles coming from Enceladus, one pure water-ice, the other water-ice mixed with other stuff. We think the clean water-ice particles are being bounced off the surface and the dirty water-ice particles are coming from inside the moon. This flyby will show us whether this concept is right or wrong.”
The halo of ice dust around Enceladus supplies Saturn’s E-ring with material from the apparently continuous eruptions. It’s known to be composed of water vapor, carbon dioxide, methane, possibly ammonia and other trace gases, but Cassini will now find out whether the gases coming out of the plume are identical with those surrounding the moon. The result may give us insights into the processes driving the eruptions. With four more Enceladus flybys planned for this year, the success of this encounter may determine whether the next one is even closer.