Cassini

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.

Spacecraft and Subsystems

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.

Mission Profile

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 will be 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 will cause Cassini to pass through the rings, a region known to be relatively free of particles, the spacecraft has been designed to withstand such exposure. A burn of the engine will slow the spacecraft sufficiently to place it into a highly elongated orbit.

The first two orbits around Saturn are designed to set up the necessary trajectory for deployment of the Huygens probe on the third orbit. Currently planned for December 2004, a maneuver will place the paired spacecraft on an intersect course with Titan. A few days later the probe will be released. The two spacecraft will separate with a relative velocity of 0.3-0.4 m/s but remain in the same orbit for about three weeks. At that time, Cassini will execute 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 enters Titan's atmosphere, some 2.1 hours prior to Cassini's closest approach.

After relaying the Huygens data, 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.