Cassini probe 

Cassini's principal objectives are to:

1. determine the three-dimensional structure and dynamical behavior of the rings
2. determine the composition of the satellite surfaces and the geological history of each object
3. determine the nature and origin of the dark material on Iapetus' leading hemisphere
4. measure the three-dimensional structure and dynamical behavior of the magnetosphere
5. study the dynamical behavior of Saturn's atmosphere at cloud level
6. study the time variability of Titan's clouds and hazes
7. characterize Titan's surface on a regional scale.

The spacecraft was originally planned to be the second three-axis stabilized, RTG-powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. However, various budget cuts and rescopings of the project have forced a more special design, postponing indefinitely any implementation of the Mariner Mark II series.

The Cassini spacecraft was launched on October 15, 1997 from Cape Canaveral Air Force Station using a U.S. Air Force Titan IVB/Centaur launch vehicle. The launch vehicle was made up of a two-stage Titan IV booster rocket, two strap-on solid rocket motors, the Centaur upper stage, and a payload enclosure or fairing. The complete Cassini flight system was composed of the launch vehicle and the spacecraft. The spacecraft, in turn, is composed of the Cassini orbiter and the Huygens probe.

The Cassini spacecraft, including the orbiter and the Huygens probe, is one of the largest, heaviest, and most complex interplanetary spacecraft built to date. The orbiter alone weighs 2,150 kilograms (4,750 pounds). When the 350-kilogram Huygens probe, launch vehicle adapter, and 3,132 kilograms (6,905 pounds) of propellants were loaded, the spacecraft weighed about 5,600 kilograms (12,346 pounds) at launch. Only the two Phobos[?] spacecraft sent to Mars by the former Soviet Union were heavier. The Cassini spacecraft stood more than 6.8 meters (22.3 feet) high and was more than 4 meters (13.1 feet) wide. The complexity of the spacecraft is necessitated both by its trajectory or flight path to Saturn and by the ambitious program of scientific observations to be undertaken once the spacecraft reaches its destination. It functions with 1,630 interconnect circuits, 22,000 wire connections, and over 14 kilometers (8.7 miles) of cabling.

Because of Saturn's distance from the Sun, solar arrays were not feasible power sources for the spacecraft. To generate enough power, such arrays would have been too large and heavy. Thus, the Cassini orbiter gets its power from three radioisotope thermoelectric generators[?] or RTGs, which use heat from the natural decay of plutonium to generate direct current electricity. These RTGs are of the same design as those flying on the Galileo and Ulysses[?] spacecraft and are designed to have a long operational lifetime. At the end of the 11-year Cassini mission, they will still be capable of producing at least 628 watts of power. These RTGs proved to be a source of some controversy; Cassini's trajectory included a gravitational slingshot maneuver past Earth on its way to Saturn, and had it suffered a malfunction that caused it to impact the contents of the RTGs would have been dispersed into Earth's atmosphere. However, the danger that this actually posed was not significant, and in any event Cassini passed Earth successfully.

The distance of Saturn from Earth is especially important, because it affects communication with the spacecraft. Specifically, when Cassini is at Saturn it will be between 8.2 and 10.2 astronomical units from Earth. Because of this, it will take 68 to 84 minutes for signals to travel from Earth to the spacecraft, or vice versa. In practical terms this means that ground controllers will not be able to give "real-time" instructions to the spacecraft either for day-to-day operations or in cases of unexpected in-flight events. By the time the controllers become aware of a problem and respond, nearly three hours will have passed.

Cassini's instrumentation consists of: a radar mapper, a CCD imaging system, a visible/infrared mapping spectrometer, a composite infrared spectrometer, a cosmic dust[?] analyzer, a radio and plasma wave experiment, a plasma spectrometer, an ultraviolet imaging spectrograph, a magnetospheric imaging instrument, a magnetometer, an ion/neutral mass spectrometer. Telemetry from the communications antenna as well as other special transmitters (an S-band[?] transmitter and a dual frequency Ka-band system) will also be used to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.

The Huygens probe

The Huygens probe, supplied by the European Space Agency (ESA) and named after the Dutch 17th century astronomer Christiaan Huygens, will scrutinize the clouds, atmosphere, and surface of Saturn's moon Titan. It is designed to enter and brake in Titan's atmosphere and parachute a fully instrumented robotic laboratory down to the surface. The Huygens probe system consists of the probe itself, which will descend to Titan, and the probe support equipment (PSE), which will remain attached to the orbiting spacecraft. The PSE includes the electronics necessary to track the probe, to recover the data gathered during its descent, and to process and deliver the data to the orbiter, from which it will be transmitted or "downlinked" to the ground.

The probe will remain dormant throughout the 6.7-year interplanetary cruise, except for bi-annual health checks. These checkouts follow preprogrammed descent scenario sequences as closely as possible, and the results are relayed to Earth for examination by system and payload experts.

Prior to the probe's separation from the orbiter, a final health check will be performed. The "coast" timer will be loaded with the precise time necessary to turn on the probe systems (15 minutes before the encounter with Titan's atmosphere), and then the probe will separate from the orbiter and coast to Titan for 22 days with no systems active except for its wake-up timer.

The main mission phase will be the parachute descent through Titan's atmosphere. The batteries and all other resources are sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or more) on Titan's surface. The probe's radio link will be activated early in the descent phase, and the orbiter will "listen" to the probe for the next 3 hours, which includes the descent plus 30 minutes after impact. Not long after the end of this three-hour communication window, Cassini's high-gain antenna (HGA) will be turned away from Titan and toward Earth.

External links

• Cassini Mission Homepage (http://saturn.jpl.nasa.gov/) by the Jet Propulsion Laboratory
• Cassini information (http://nssdc.gsfc.nasa.gov/planetary/cassini.html) by the National Space Science Data Center[?] (NSSDC).
• ESA Huygens Homepage (http://sci.esa.int/home/huygens/index.cfm) about the probe that will study Titan's atmosphere and possibly the surface