NASA’s Cassini spacecraft is set to cap off 2004 with an encounter of Saturn’s ying-yang moon Iapetus (eye-APP-eh-tuss) on New Year’s Eve. 

This is Cassini’s closest pass yet by one of Saturn’s smaller icy satellites since its arrival around the ringed giant on June 30 of this year.  The next close flyby of Iapetus is not until 2007. 

Iapetus is a world of sharp contrasts.  The leading hemisphere is as dark as a freshly-tarred street, and the white, trailing hemisphere resembles freshly-fallen snow. 

Cassini will fly by the two-toned moon at a distance of approximately 123,400 kilometers (76,700 miles) on Friday, Dec. 31. This flyby brings to an end a year of major accomplishments and rings in what promises to be a year filled with new discoveries about Saturn and its moons.

“I can think of no better way than this to wrap up what has been a whirlwind year,” said Robert T. Mitchell, program manager for the Cassini mission at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.  “The new year offers new opportunities, and 2005 will be the year of the icy satellites.” 

In 2005 Cassini will have 13 targeted encounters with five of Saturn’s moons.  “We have 43 close flybys of Titan still ahead of us during the four-year tour.  Next year, eight of our 13 close flybys will be of Titan.  We will also have a number of more distant flybys of the icy satellites, and let’s not forget Saturn and the rings each time we come around,” said Mitchell.

With a diameter of about 1,400 kilometers (890 miles), Iapetus is Saturn’s third largest moon.  It was discovered by Jean-Dominique Cassini in 1672.  It was Cassini, for whom the Cassini-Huygens mission is named, who correctly deduced that one side of Iapetus was dark, while the other was white. 

Scientists still do not agree on whether the dark material originated from an outside source or was created from Iapetus’ own interior. One scenario for the outside deposit of material would involve dark particles being ejected from Saturn’s little moon Phoebe and drifting inward to coat Iapetus.  The major problem with this model is that the dark material on Iapetus is redder than Phoebe, although the material could have undergone chemical changes that made it redder after its expulsion from Phoebe. One observation lending credence to the theory of an internal origin is the concentration of material on crater floors, which implies that something is filling in the craters. In one model proposed by scientists, methane could erupt from the interior and then become darkened by ultraviolet radiation.

Iapetus is odd in other respects. It is the only large Saturn moon in a highly inclined orbit, one that takes it far above and below the plane in which the rings and most of the moons orbit. It is less dense than objects of similar brightness, which implies it has a higher fraction of ice or possibly methane or ammonia in its interior.

The last look at Iapetus was by NASA’s Voyager 1 and 2 spacecraft in 1980 and 1981.  The Cassini images will be the highest resolution images yet of this mysterious moon.

The Iapetus flyby by Cassini follows the successful release of the Huygens probe on December 24.

More information on the Cassini-Huygens mission is available at:  and .

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.  JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA’s Science Mission Directorate, Washington, D.C.  JPL designed, developed and assembled the Cassini orbiter.  The European Space Agency built and managed the development of the Huygens probe and is in charge of the probe operations.  The Italian Space Agency provided the high-gain antenna, much of the radio system and elements of several of Cassini’s science instruments.

Cassini spacecraft targeted satellite encounters for 2005:

  • Titan: January 14, 2005
  • Titan: February 15, 2005
  • Enceladus: March 9, 2005
  • Titan: March 31, 2005
  • Titan: April 16, 2005
  • Enceladus: July 14, 2005
  • Titan: August 22, 2005
  • Titan: September 7, 2005
  • Hyperion: September 26, 2005
  • Dione: October 11, 2005
  • Titan: October 28, 2005
  • Rhea: November 26, 2005
  • Titan: December 26, 2005