Cassini's principal objectives are to:
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2 The Huygens probe 3 External links |
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, 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.
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.
Because of Saturn's distance from the Sun, solar arrayss 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 (in the form of plutonium dioxide) 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.
Cassini's use of plutonium -- 32.8 kg, at the time the most ever launched into space -- attracted significant protest from environmental groups, physicists, and some former NASA staff. NASA made several statements about the safety of the mission, all of which intended to mean the mission was acceptably safe: the chances of radioactive release during the first 3 1/2 minutes after launch were 1 in 1,400; the chances of a release later in the rocket's climb into orbit were 1 in 476; the chances of the craft falling to earth later into the mission were less than 1 in a million; a worst-case scenario would mean 120 humans could die from Cassini-caused cancer over 50 years. These figures were derided as wild guesses by commentators that included noted physics professor Michio Kaku, who suggested 200,000 would die if Cassini landed in a city.
To gain momentum for the voyage to Saturn, Cassini's trajectory included several gravitational slingshot maneuvers: two passes of Venus, one past the Earth, then one past Jupiter. The Earth fly-by, which occurred successfully on August 18, 1999, was the final point at which Cassini posed any danger to humans. Had it suffered a malfunction that caused it to impact, the plutonium contents of the RTGs would have been dispersed into Earth's atmosphere. A small number of activists continued to protest after the maneuver. Counterdemonstrators from the National Space Society carried signs reading CASSINI IS GO.
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.
Spacecraft design
Plutonium power source and controversy
The Huygens probe
External links