Lord Of The Rings

By Jeffrey Kluger

Saturn has been ready for its close-up for a long time. On June 30 the planet is going to get it as the $1.4 billion Cassini-Huygens spacecraft, after a journey of 2.2 billion miles, fires its retrorockets and puts itself into Saturnian orbit.

It's the start of a four-year tour, during which the ship will make at least 76 loops of the planet and engage a dozen cameras and instruments. NASA will be able to tweak the trajectory of the orbiter so it can slalom among nine of Saturn's 31 moons. The grandest of the satellites is Titan, which has long frustrated scientists because its dense atmosphere, laced with organic gases, obscures its surface.

The spacecraft will penetrate that atmosphere with radar and a free-flying 705-lb. conical probe (the Huygens half of the Cassini-Huygens tandem), scheduled to plunge into the Titanian skies on Jan. 14, 2005. Huygens will make a 2 1/2-hr. parachute descent, transmitting data all the way. When it touches down, having made the most distant landing ever by an earthly probe, it is expected to survive for no more than half an hour. Cassini, designed to live much longer, should be beaming back data until 2008.

THE CASSINI ORBITER

Twenty-two feet long and weighing in at 4,700 lbs. unfueled, Cassini is an extraterrestrial truck bristling with a dozen instruments. It is protected by Kapton blankets that shield it from micrometeorite hits and warm it against the deep freeze of space. Inside, it operates at room temperature

Exploring the rings

Saturn has seven rings, ranging in width from just 30 miles to 188,000 miles. Each is a shallow river of ancient ice and rocky rubble. Gravitational eddies-not to mention small moons circling within the rings and at their edges-twist the rubble into braids, ridges and gaps

Encke division The tiny moon Pan sweeps this space clear

G ring (faint) 6,000 miles wide

F ring 30 miles wide

A ring 9,080 miles wide

E ring (farther out) 188,000 miles wide

Cassini division 5,000-mile gap discovered by astronomer Gian Cassini

B ring 15,890 miles wide

C ring 10,870 miles wide

D ring 4,690 miles wide

...the planet Saturn

One of the solar system's gas giants, Saturn is made up mostly of hydrogen and helium and has a volume 764 times as great as Earth's. That much gas concentrated in one place ought to be dynamic, and Saturn is. Winds blow at more than 1,100 m.p.h. at the equator-the strongest gusts on any planet in the solar system-and helium rain is thought to fall out of the clouds. Temperatures at the cloud tops are a frigid -218-oF

Diameters

SATURN 74,897 miles

EARTH 7,925 miles

TITAN 3,200 miles

EARTH'S MOON 2,160

...and the moon Titan

Larger than Mercury and Pluto, Titan would be a perfectly respectable planet if it were orbiting the sun instead of Saturn. Its dense atmosphere is made of organic materials like methane and ethane, strikingly similar to our own atmosphere before life emerged. Since Titan's surface temperature approaches -300-oF, the moon almost certainly does not sustain life, but studying it can give scientists a peek at a sort of cryopreserved version of Earth long ago

THE HUYGENS PROBE

The 705-lb. Huygens probe will be released by Cassini on Christmas Eve 2004 and will reach Titan three weeks later. It carries a camera and five sensors, all of which must work fast. Three hours after entering Titan's airspace, the probe will run out of power

1 The probe pops free of the ship and spins at seven revolutions a minute for stability

2 Striking the atmosphere at 12,400 m.p.h., Huygens generates heat reaching 21600-oF

3 Two minutes after entry, the probe slows to 895 m.p.h., and the first of two parachutes deploys

4 The probe's cameras and instruments operate both during descent and on the ground

Saturn's moons The Saturnian system has 31 known moons. During its 76 orbits, Cassini will fly by nine of them. Planet-size Titan, with its organic atmosphere, will be visited at least 45 times

Diameters of moons* *Moon photos not to scale

Miles from center of Saturn

Mimas 244 miles

Enceladus 310 miles

Tethys 659 miles

Dione 696 miles

Rhea 949 miles

TITAN 3,200 miles

Hyperion 176 miles

Iapetus 892 miles

Phoebe 137 mile

Sources: NASA, J.P.L., European Space Agency