Cassini Closes In on Saturn

False color image, Saturn rings showing detail of A through F rings. Image Credit: JPL/Space Science Institute

After a seven-year journey through interplanetary space, Cassini-Huygens is about to reach its destination. Wednesday night (early Thursday morning in Europe) the $3 billion spacecraft will arrive at Saturn; and if a 96-minute engine burn comes off as planned, become the first artificial satellite ever to go into orbit around the ringed planet.

Cassini mission planners say that everything looks good for Saturn orbital insertion (SOI), the engine burn that will slow the spacecraft down enough to allow it to be captured by Saturn’s gravitational field. Mission engineers have verified that Cassini’s systems are working as expected; they have double-checked the spacecraft’s position and trajectory; and they have sent the command to initiate the SOI sequence. All the team can do now, says Cassini Program Manager Robert Mitchell, is "chew our nails."

The SOI will proceed under the automatic control of the spacecraft’s onboard computers. Saturn is too far away for mission managers to direct the maneuver in real-time. Saturn and Earth presently are about 1.5 billion kilometers (934 million miles) apart. It takes light (and therefore radio-control signals) about 84 minutes to travel from one planet to the other. By the time engineers in the mission’s control room at the Jet Propulsion Laboratory (JPL) in Pasadena, California, get back a signal from the spacecraft confirming that the SOI has begun, the spacecraft’s engine will be nearing the end of its 96-minute burn.

Cassini won’t be collecting much in the way of scientific data during the SOI, but scientists are looking forward eagerly to the data that will be gathered just before the spacecraft’s engines are turned on and just after they are turned off. Cassini will be closer to Saturn during the SOI than at any other time during the mission, and the science team is anxious to get an extreme-close-up look at the planet’s enigmatic rings. Cassini will cross Saturn’s ring plane just before the SOI, passing up through the gap between the F and G rings; after the spacecraft’s engines reduce its speed, it will dip back down through the rings.

Cassini Science Targets. Image Credit: JPL/NASA

Once the SOI is completed, Cassini will begin its four-year orbital mission, circling Saturn 77 times and cruising by more than 50 close encounters (and another dozen or so more-distant encounters) with the planet’s moons. In all, Cassini will aim its instruments at 8 of Saturn’s 31 known moons. It may also discover a few that are, as yet, unknown.

Titan will get the lion’s share of attention: 45 close flybys are planned for the giant moon. Titan will also be the target of the Huygens probe, which will be released by Cassini on Christmas day (Christmas eve in the U.S.) for descent through Titan’s atmosphere 3 weeks later. Titan is of particular interest to scientists because, like Earth, it has an atmosphere that contains nitrogen and organic molecules such as methane. Some scientists speculate that Titan’s chemistry may offer a snapshot of what Earth’s chemistry was like before life took hold.

Titan will not only be an object of study by Cassini, it will also help the spacecraft achieve its other scientific goals. Throughout the mission, the moon’s gravity will be used to induce changes in the spacecraft’s orbital changes. Before each flyby of Titan, mission engineers will point Cassini in the direction required to get the proper gravity boost. And, of course, each time a flyby occurs, Cassini will get another chance to unlock Titan’s secrets.

After Cassini releases the Huygens probe, it will spend about 10 exploring several of Saturn’s icy satellites. Cassini will make close flybys of Enceladus, Hyperion, Dione and Rhea during this grand lunar tour. Its next task will be to explore Saturn’s magneotail, the portion of the planet’s magnetosphere that points away from the sun. During this 7-month-long investigation, Cassini will study the structure of the magnetosphere and its interactions with the solar wind and with Saturn’s rings and moons.

Titan descent by Huygens probe around Christmas 2004. Image Credit: JPL/Space Science Institute

Cassini will spend most of the following year tweaking its orbit to get in position for the final phase of the mission. During this extended orbital adjustment period, the spacecraft will log 19 additional close encounters with Titan. In the final phase, mission planners will elongate Cassini’s orbit and tilt it up to an angle of 75 degrees above the plane in which Saturn’s moons and rings orbit the planet. From this vantage point, scientists hope to get some spectacular views of the ring system and to study its dynamics in detail.

Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass