LPL Scientists Take New Pictures of Saturn with Hubble And Cassini

As the Cassini spacecraft hurtles toward a rendezvous with Saturn on June 30 (July 1, Universal Time), both Cassini and the Earth-orbiting Hubble Space Telescope snapped spectacular pictures of the planet and its magnificent rings.

Cassini is approaching Saturn at an oblique angle to the sun and from below the ecliptic plane. Cassini has a very different view of Saturn than Hubble’s Earth-centered view. For the first time, astronomers can compare equally sharp views of Saturn from two very different perspectives.

Erich Karkoschka of the University of Arizona’s Lunar and Planetary Laboratory took Hubble’s latest view of Saturn on March 22, 2004. The picture is so sharp that many individual ringlets can be seen in the planet’s ring plane. Electronic image files and additional information are online at http://hubblesite.org/news/2004/18.

When Cassini returned its picture of Saturn on May 16, it was so close to the planet that the Imaging Science Subsystem (ISS) narrow-angle camera could not fit the whole planet in its field-of-view. Cassini is still about 20 million kilometers (about 12.4 million miles) away and only 36 days from reaching Saturn.

Starting this week, the Cassini Imaging Team will be releasing more new views of the ringed planet on the Cassini Imaging Team website at http://ciclops.org, said ISS team leader Carolyn Porco of the Space Science Institute in Boulder. Porco is an adjunct professor at UA. UA Professor Alfred McEwen is a member of the ISS team.

Hubble’s exquisite optics, coupled with the high resolution of its Advanced Camera for Surveys (ACS), allow it to take pictures of Saturn which are nearly as sharp as Cassini’s, even though Hubble is nearly a billion miles farther from Saturn than Cassini is. Cassini will ultimately far exceed the resolution of Hubble during its close encounter with Saturn. Cassini’s images grew sharper than Hubble’s when the spacecraft came to within 23 million kilometers (14 million miles) of Saturn earlier this month.

Karkoschka combined camera exposures in four filters (blue, blue-green, green, and red) into Hubble’s new Saturn image. “This renders colors similar to what an astronaut would see if it were possible to watch Saturn through the Hubble Space Telescope,” he said.

The subtle pastel colors of ammonia-methane clouds trace a variety of atmospheric dynamics. Saturn displays its familiar banded structure, and haze and clouds of various altitudes. Like Jupiter, all bands are parallel to Saturn’s equator. Even the magnificent rings, at nearly their maximum tilt toward Earth, show subtle hues, which suggest the trace chemical differences in their icy composition.

Cassini has two cameras, a wide angle and a narrow angle. The narrow angle image taken May 16, when the spacecraft was 24.3 million kilometers (15.1 million miles) from Saturn, was made using a combination of red, green and blue filters. The view is from 13 degrees below the equator. Enceladus, one of Saturn’s 31 known moons, appears near the south pole at the bottom of the image.

The differences between the Hubble and Cassini images are mainly due to the different sets of filters used.

The last spacecraft to visit Saturn was NASA’s Voyager-2 space probe that flew by the planet in August 1981. (Former UA planetary sciences Professor Bradford Smith headed the Voyager imaging team.) Since 1990, Hubble has produced high-resolution Saturn images, tracking storms and auroral activity while providing crisp views of the rings over time and from various angles.

Cassini will begin a four-year mission in orbit around Saturn when it arrives on June 30, 2004 PDT (July 1, 2004 UTC). Six months later it will release its piggybacked Huygens probe for descent through Titan’s thick atmosphere.

Martin Tomasko of UA’s Lunar and Planetary Lab leads the Descent Imager Spectral Radiometer (DISR) experiment that will take data during the probe’s two-or-more-hour descent through Titan’s atmosphere. Karkoschka is a member of the DISR science team.

The University of Arizona has more planetary scientists and students involved in the Cassini-Huygens mission than any other university. They are:

• Robert H. Brown, team leader for the Visual and Infrared Mapping Spectrometer (VIMS)
• Caitlin Griffith, member of the VIMS science team
• Jonathan I. Lunine, one of three interdisciplinary scientists for Cassini’s Huygens probe
• Elizabeth Turtle and Doug Dawson, who work with Cassini imaging team scientist Alfred McEwen in planning the observations of Titan
• Martin Tomasko, principal investigator for the Descent Imager/Spectral Radiometer (DISR) that will be deployed to the surface of Titan on the Huygens probe
• Peter Smith, co-investigator on DISR
• Ralph Lorenz, member of the Cassini Radar Team and co-investigator on the Surface Science Package on the Huygens probe
• Donald Hunten, co-investigator on the Gas Chromatograph and Mass Spectrometer on the Huygens probe
• Roger Yelle, team member for the Ion Neutron Mass Spectrometer

The Hubble Space Telescope is an international project between NASA and the European Space Agency (ESA). The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, Md.

The Cassini/Huygens mission is a joint mission of NASA, ESA, and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.