Status: Operational 1997. First Launch: 1997-10-15. Last Launch: 1997-10-15. Number: 1 . Gross mass: 5,712 kg (12,592 lb). Height: 6.70 m (21.90 ft).
After insertion into its orbit about Saturn in 2004, Cassini performed detailed studies of Saturn's atmosphere, magnetosphere, rings and several of its moons. Chief among the goals was a thorough characterization of the large moon, Titan. Titan's surface was mapped using synthetic aperture radar. Its atmosphere and surface was investigated by the ESA built Huygens probe that was released from Cassini and descended to Titan's surface via parachute. Scientists and engineers from the US and 16 European countries participated in the mission.
The spacecraft was 3-axis stabilized; power was supplied by 3 RTG (628W at EOM); a 4 meter high gain antenna supported S-, X-,Ku-, Ka-band signals, and X-band telemetry at 249 kbps. Two backup low gain antennas were provided for emergency commanding. The spacecraft had dual redundant flight computers; dual redundant 445N gimbaled main engines (burning N2O4, MMH), 16 hydrazine ACS thrusters; redundant star sensors, sun sensors, HRG IRUs, reaction wheels; active thermal control using RHUs, heaters, louvers; redundant solid-state recorders; and a high level of spacecraft autonomy and fault protection
The experiments carried aboard the Cassini orbiter included:
The 2.7 meter diameter Huygens probe was deployed by three pyrotechnic release bolts. The heat shield protected the probe during reentry and used AQ60 material developed by Aerospatiale. Three parachutes were used to control final descent. Huygens was equipped with a redundant S-band link (8 kbps max), lithium sulfur-dioxide (LiSO2) primary batteries, and a radar altimeter. The Huygens probe experiments included:
Cassini had a $ 3.4 billion total program costs, including the US Orbiter at $ 1.4 billion; the ESA/Aerospatiale Huygens probe at $ 422 million. and the Titan IVB K32 launch at $ 450 million.
NASA NSSDC Master Catalog Description
The Cassini Orbiter's mission consists of delivering a probe (called Huygens, provided by ESA) to Titan, and then remaining in orbit around Saturn for detailed studies of the planet and its rings and satellites. The 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; and, (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 forced a more specialized design, postponing indefinitely any implementation of the Mariner Mark II series.
Cassini is the largest interplanetary spacecraft ever constructed by NASA. It measures 6.8 m in length with a 4 m high gain antenna. At launch the spacecraft had a mass of 5,655 kg, of which 3,132 kg were propellant.
Cassini is three-axis stabilized. Orientation is maintained through the use of either three reaction wheel assemblies mounted along orthogonal axes on the spacecraft (for fine pointing control) or via 16 0.5 N thrusters (for coarse pointing control). The thrusters are arranged in four groups of four and use hydrazine. Orientation is determined through the use of either three inertial reference units (using solid-state gyroscopes) or a star tracker (which detects stars in its field of view and compares them with an on-board catalog of 5,000 stars). The thrusters are also used in the alteration of the spacecraft trajectory of <5 m/s. For trajectory corrections >5 m/s, one of two identical main engines (one serves as a backup) is used. The engines are gimbaled so that thrust can be maintained through the spacecraft center of mass and burn the bipropellants nitrogen tetroxide and monomethyl hydrazine.
Power is provided to the spacecraft through the use of three radioisotope thermoelectric generators (RTGs). Each RTG uses the heat generated by the decay of 10.9 kg of plutonium dioxide (PuO2) to generate electrical power to be used throughout the spacecraft. At the beginning of the mission, each RTG was capable of producing 300 W of electrical power. By the end of the nominal 11 year mission, the output is expected to degrade to around 210 W per RTG. The resultant electricity, a regulated 30 V DC, is used not only to provide power to the various science instruments and spacecraft subsystems, but also to one-time pyrotechnic devices used in the course of the mission, such as to separate the the spacecraft from the Centaur launch vehicle or to separate the Huygens probe from Cassini.
Although some data will be transmitted in real time, much science data and spacecraft health and status information are recorded on a solid-state data recorder. Although such systems had been used previously on other missions, Cassini is the first to use one in deep space. The spacecraft is equipped with two recorders, each of which has a capacity of 2 Gb (gigabits) in the form of dynamic random access memory (DRAM). Because such memory is vulnerable to radiation effects, the recorders are encased in half-inch thick aluminum. Nonetheless, degradation of the recorders (due to solar and cosmic ray activity) is expected to reduce their capacity by about 10% by the end of the mission. Recorded data are then periodically transmitted to Earth via the HGA and erased.
Unable to be launched directly to Saturn with the propulsion systems available at the time, Cassini took a roundabout route to reach the ringed planet. Referred to as a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist) trajectory, Cassini made two flybys of Venus (April 1998 and June 1999), one of the Earth (August 1999), and one of Jupiter (December 2000). Various observations were made at each of these encounters in order to verify instrument and spacecraft systems as well as to perform calibration observations. At Jupiter, numerous simultaneous observations were made using Cassini, Galileo, and the Hubble Space Telescope, among other missions.
The first object in the Saturnian system to receive close examination was its satellite Phoebe. This will be the only opportunity the Cassini will have to examine Phoebe at close range because of its distance from the planet, but will be much closer than any previous mission has come to the satellite. The trajectory of Cassini during this initial orbit will be the closest of the entire mission, 1.3 Rs (1 Rs = 60,330 km) from the center of the planet. The only other orbits which will come nearly as close will occur late in the mission, around 2.7 Rs. Although this close initial pass caused Cassini to pass through the rings, a region known to be relatively free of particles, the spacecraft was designed to withstand such exposure. A burn of the engine slowed the spacecraft sufficiently to place it into a highly elongated orbit.
The first two orbits around Saturn were designed to set up the necessary trajectory for deployment of the Huygens probe on the third orbit. A maneuver will place the paired spacecraft on an intersect course with Titan and the probe was released on 25 December 2004. The two spacecraft separated with a relative velocity of 0.3-0.4 m/s but remained in the same orbit for about three weeks. At that time, Cassini executed a deflection maneuver to enable it to fly by Titan at an altitude of 60,000 km and positioning it to receive transmissions from Huygens as it entered Titan's atmosphere, some 2.1 hours prior to Cassini's closest approach. Huygens landed on Titan on 14 January 2005.
Cassini is expected to make at least 76 loose elliptical orbits of the planet, each optimized for a different set of observations of the rings, various satellites, and the magnetosphere.
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.
Credit: Manufacturer Image
The Cassini probe to Saturn made a course correction burn at 2000 UTC, changing its velocity by 0.12m/s. At the time Cassini was 1284 million km from the Sun, in an orbit of 1.4462 AU x 9.2769 AU with an inclination of 0.81 deg to the ecliptic plane and the correction altered the orbit by about ten parts per million. On September 14 Cassini was 137 million km from Saturn; Saturn's nominal gravitational sphere of influence is 55 million km in radius and Cassini was to reach that point on 2004 March 10, 2004. The probe was to make a 2000 km flyby of the Saturnian moon Phoebe on June 11, 2004.