In this artist's concept, the Cassini spacecraft makes a close pass by Saturn's inner moon Enceladus to study plumes from geysers that erupt from giant fissures in the moon's southern polar region.
Credit: Karl Kofoed
A NASA spacecraft is set to make the closest flyby yet of an icy moon of Saturn on Thursday.
The Cassini probe is poised for a return plunge through the cold shower plume of the Saturnian moon Enceladus on Oct. 9, and can expect another icy reception for a following flyby on Oct. 31 in an attempt to uncover possible changes inside the frigid satellite.
"The October doubleheader gives Cassini two more opportunities to hit the ball out of the park," said Bob Pappalardo, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "With high scores in geology, surface heat, watery plumes and magnetospheric effects, Enceladus could win the 'world championship' title this year!"
Scientists expect Cassini to pass just 16 miles (25 km) above the moon's surface during the closest approach at 4:30 p.m. EDT (2030 GMT). The probe's cruising speed should hit 39,594 mph (63,720 kph).
Cassini is currently on a $160-million, two-year extended tour of Saturn and its moons following the completion of its main four-year mission on June 30. Scientists took the opportunity to schedule seven flybys of Enceladus, where they have debated the existence of an ocean of water beneath the moon's frozen shell.
Previous visits to Enceladus detected comet-like organic molecules within the moon's icy, geyser-like jets. A more recent Aug. 11 flyby pinpointed the locations of surface fractures, nicknamed "tiger stripes," where those jets erupt out into space.
The upcoming October flybys may support findings from the most recent Enceladus flyby that suggest changes within the moon. Temperatures over a surface fracture dubbed ?Damascus Sulcus? checked out at a chilly minus 171 to minus 159 degrees Fahrenheit (160 to 167 Kelvin), well below the minus 136 degrees Fahrenheit reported during a March flyby this year (180 Kelvin).
"We don't know yet if this is due to a real cooling of this tiger stripe, or to the fact that we were looking much closer, at a relatively small area, and might have missed the warmest spot," said John Spencer, a Cassini scientist at the Southwest Research Institute in Boulder, Colo.
Scientists should have another shot beyond this year with a second Enceladus doubleheader on Nov. 2 and Nov. 21 in 2009.
- Video - Enceladus' Cold Faithful
- Cassini's Greatest Hits: Images of Saturn
- Special Report: Cassini's Journey's