Making New Friends

Dione
This mosaic combines five Cassini images of Saturn's moon Dione. The moon's heavily cratered surface and grooved terrain are evident. The scale is 0.9 kilometer per pixel.
Courtesy NASA / JPL / Space Science Institute.
Scientists are beginning to digest a plethora of new images and data following Cassini's successful flybys of two Saturnian moons. On Monday, the NASA craft passed Titan at an altitude of only 1,200 kilometers (750 miles). The next day, it raced by the icy satellite Dione at a range of 72,500 kilometers.

Surprising changes occurred in Titan's weather patterns between this flyby and the previous encounter on October 26th. In the earlier meeting, Titan only had clouds that hovered over the south pole. But in this week's flyby, Cassini's cameras picked up several clouds at mid-latitudes. "This means we see direct evidence of weather, and we can get wind speeds and atmospheric circulation over a region we haven't been able to measure before," says Cassini science team member Kevin Baines (NASA/Jet Propulsion Laboratory).

Titan
About a dozen upper-atmosphere haze layers are distinct in this nighttime ultraviolet image taken by Cassini as it approached Titan in December 2004. The scale is 0.7 kilometer per pixel.
Courtesy NASA / JPL / Space Science Institute.

The Cassini team has learned from both flybys that Titan's upper atmosphere at an altitude of 950 kilometers is twice as dense as previously thought. The flight control team had planned to bring the spacecraft within 950 km of the moon's surface during 22 upcoming flybys. But flying through moderately dense atmospheric gases can cause Cassini to lose attitude control. To compensate, flight controllers will raise the spacecraft's altitude to 1,025 km for the next two flybys, in April and September 2005. "This will allow us to assess the atmospheric effects on the spacecraft at an intermediate altitude so we can be sure how low we can fly," says Cassini Mission Planning Manager David Seal (NASA/Jet Propulsion Laboratory). "This change has not caused any significant science or mission impacts."

Seal adds, "Note that the atmospheric density peaks rather quickly during a flyby, so if we do fly a bit too low and lose attitude control, it will be very brief (seconds to a minute) and no damage to the spacecraft is expected."

The Dione rendezvous — the closest ever of this medium-sized moon — afforded the most detailed images of the so-called "wispy terrain" that covers some of Dione's surface. Scientists were expecting to see thick ice deposits in the wispy areas, but instead found bright ice cliffs sculpted by tectonic forces.

On Thursday, University of Iowa space physicist Don Gurnett announced that Saturn's lightning is much more powerful than previously thought. As Cassini approached Saturn for its July 1st orbital insertion burn, its Radio and Plasma Wave Science instrument began detecting radio signals from lightning at a distance of 161 million kilometers. In contrast, during two earlier Earth flybys, it had to come within 90,000 kilometers to pick up radio signals from lightning. "This means that radio signals from Saturn's lightning are on the order of 1 million times stronger than Earth's lightning," says Gurnett, who reported the results on Thursday at the American Geophysical Union conference in San Francisco.

Dione
Cassini was 603,000 kilometers from Dione when it captured the moon transiting in front of Saturn on December 14, 2004. The visible-light image shows Dione's natural colors and surface brightness variations. Oval-shaped storm systems can be seen on Saturn.
Courtesy NASA /JPL / Space Science Institute.

Cassini's next major event will occur at 2:00 Universal Time on Christmas day, when the European Space Agency's Huygens probe will separate from its mothership for a three-week cruise to Titan. Huygens will slam into the enigmatic moon's upper atmosphere on January 14, 2005, when it will begin a 2- to 2.5-hour parachuted descent. If the craft survives the landing, it could return images and data from the surface of a truly alien world.