The Cassini mission to Saturn is a joint undertaking of the National Aeronautics and Space Administration, the European Space Agency (ESA), the Agenzia Spaziale Italiana, and numerous other European academic and industrial participants. The Cassini mission will provide a close-up investigation of the Saturn system, including Saturn's atmosphere and magnetosphere, its rings, and several of its moons. Saturn's largest moon Titan is of particular interest. ESA is developing the Huygens probe that will descend through Titan's atmosphere, directly sampling the atmosphere and determining its composition. To accomplish its ambitious scientific objectives, the orbiter and the probe carry 18 scientific instruments to conduct a total of 27 scientific investigations. The Cassini Spacecraft is scheduled for launch on a Titan IV/Centaur in October of 1997. Cassini will reach the Saturn system in 2004. The tour of the Saturn system is scheduled for 4 years and includes 63 orbits of Saturn and more than 36 flybys of Titan. During the first Saturn orbit, the Huygens probe will separate from the Cassini orbiter and descend through the atmosphere of Titan. This paper summarizes the current status of the Cassini program.

The Cassini mission is a joint venture between NASA, European Space Agency (ESA), and Agenzia Spaziale Italiana (ASI). A major interface is between the NASA/Jet Propulsion Laboratory (JPL) provided Orbiter and the ESA provided Atmospheric Probe. The Orbiter is designed to orbit the planet Saturn, while the Probe will descend into the atmosphere and impact the surface of Titan. After years of study and scientific meetings between NASA, JPL, and ESA, a Memorandum of Understanding was signed between NASA and ESA on 17 December 1990. Project teams were formed at JPL and ESA and work began on the interface definition and integration of the Probe onto the Orbiter. Unique challenges were presented due to the fact that this was the first such atmospheric probe designed and developed in Europe, the complicated mechanical and electrical interface between the Probe and the Orbiter, and the nine hour time difference between JPL and ESA. As with the Galileo mission to Jupiter, the Orbiter provides an environmentally acceptable vehicle for the Probe during the long cruise phase, a communication link to Earth for periodic Probe checkouts during this cruise phase, a stable and accurately pointed platform for Probe separation, and a relay link to Earth for the actual Probe mission descent into the Titan atmosphere. Management and technical interchange tools were developed to address these challenges. The Probe design and its interface with the Orbiter are the result of this effort.

Cassini is the name given to an interplanetary spacecraft and also to the mission which it is designed to fly. The Cassini spacecraft carries a smaller spacecraft, the Huygens Titan atmospheric probe, which will go to Titan, Saturn's largest moon. Scheduled for launch in October 1997, Cassini will take seven years to reach Saturn. In November of 2004, about six months after the Cassini spacecraft begins its first orbit about Saturn, Huygens will be dropped into Titan's atmosphere. The Huygens probe will descend slowly, by parachute, through Titan's thick, obscuring atmosphere. On the way down it will observe and measure the atmosphere and finally the surface. With the completion of Huygens' mission, the Cassini spacecraft will then embark upon an orbital tour of the Saturnian system, including an extended series of flybys of Titan, and close encounters with several of the smaller, icy satellites. Some instruments will observe by remote sensing; others will make in situ measurements. All of the elements of the systems (rings, magnetosphere, satellites, and Saturn itself) will be studied.

The Cassini mission to Saturn employs a Saturn orbiter and a Titan probe to conduct an intensive investigation of the Saturnian system. The Cassini orbiter flies a series of obits, incorporating flybys of the Saturnian satellites, called the 'satellite tour'. During the tour, the gravitational fields of the satellites are used to modify and control the orbit, targeting from one satellite flyby to the next. The tour trajectory must also be designed to maximize opportunities for science observations, subject to mission-imposed constraints. Tour design studies have been conducted for Cassini to identify trades and strategies for achieving these sometimes conflicting goals. Concepts, strategies, and techniques previously developed for the Galileo mission to Jupiter have been modified, and new ones have been developed, to meet the requirements of the Cassini mission.

Titan is Saturn's largest moon, and the second-largest natural satellite in the solar system. The surface is obscured from view by suspended haze layers, which are the products of methane photochemistry, within an atmosphere composed largely of molecular nitrogen and with a density exceeding that of Earth's. Titan thus affords a remarkable opportunity to understand the evolution of an organic-rich, planet-sized world with chemical cycles powered over geologic time by sunlight. Because of the difficulties of viewing the surface remotely, a full understanding of the nature of this complex world requires a campaign of in-situ and close flyby observations. The Cassini/Huygens payload is uniquely designed to conduct such an exploration from Saturn orbit and within Titan's atmosphere. Direct sampling of the atmospheric chemistry by gas-chromatography and mass spectroscopy will be complemented by global remote spectra collected in the UV through the infrared. Probe images in the optical and near-infrared right up to the point of impact will be complemented by Orbiter imagery in the near- infrared and through active radar sounding. The synergy between Orbiter and Probe observations required to do a first comprehensive exploration of Titan is a uniquely powerful capability of this mission.

A system has been built at the University of Kent at Canterbury to calibrate devices that will be carried on the Huygens probe to the surface of Titan as part of the Cassini mission to the Saturnian system. This system can simulate Titan's atmosphere to an effective altitude of around 30 km, and it can condense, store, and sample quantities of liquid hydrocarbons. The tested devices form the Huygen's surface science package (SSP) which measures a wide variety of parameters, making it a critical tool for exploring Titan's troposphere and surface. Preliminary results from the calibration of SSP instruments are presented and an assessment is made of the accuracy with which a medium's composition can be determined by sensors acting alone, and in concert with other SSP instruments.

The Huygen's probe of the atmosphere of Saturn's moon Titan includes one optical instrument sensitive to the wavelengths of solar radiation. The goals of this investigation fall into four broad areas: 1) the measurement of the profile of solar heating to support an improved understanding of the thermal balance of Titan and the role of the greenhouse effect in maintaining Titan's temperature structure; 2) the measurement of the size, vertical distribution, and optical properties of the aerosol and cloud particles in Titan's atmosphere to support studies of the origin, chemistry, life cycles, and role in the radiation balance of Titan played by these particles; 3) the composition of the atmosphere, particularly the vertical profile of the mixing ratio of methane, a condensable constituent in Titan's atmosphere; and 4) the physical state, composition, topography, and physical processes at work in determining the nature of the surface of Titan and its interaction with Titan's atmosphere. In order to accomplish these objectives, the Descent Imager/Spectral Radiometer (DISR) instrument makes extensive use of fiber optics to bring the light from several different sets of foreoptics to a silicon CCD detector, to a pair of InGaAs linear array detectors, and to three silicon photometers. Together these detectors permit DISR to make panoramic images of the clouds and surface of Titan, to measure the spectrum of upward and downward streaming sunlight from 350 to 1700 nm at a resolving power of about 200, to measure the reflection spectrum of >= 3000 locations on the surface, to measure the brightness and polarization of the solar aureole between 4 and 30 degrees from the sun at 500 and 935 nm, to separate the direct and diffuse downward solar flux at each wavelength measured, and to measure the continuous reflection spectrum of the ground between 850 and 1600 nm using an onboard lamp in the last 100 m of the descent.

First results about the performance of the temperature sensors (TEM) of Huygens Atmospheric Structure Instrument (HASI) obtained during Flight Model test campaign are presented and discussed. TEM belongs to the STUB subsystem of HASI, which is a multidisciplinary experiment package dedicated to the investigation of Titan atmosphere during the descent of the Huygens probe. TEM sensors are described, their characteristics and performances discussed and the data of thermal tests carried out at subsystem level and at probe level evaluated. From this preliminary analysis it seems that the sensors are suited to achieve the scientific objective of HASI experiment if post flight data correction is appropriately done.

The Cassini spacecraft will carry eighteen scientific instruments to Saturn. After it is inserted into Saturn's orbit, it will separate into a Saturn Orbiter and an atmospheric probe, called Huygens, which will descend to the surface of Titan. The Orbiter will orbit the planet for four years, making close flybys of five satellites, including multiple flybys of Titan. Orbiter instruments are body- mounted; the spacecraft must be turned to point some of them toward objects of interest. Optical instruments provide imagery and spectrometry. Radar supplied imaging, altimetry, and radiometry. Radio links contribute information about intervening material and gravity fields. Other instruments measure electromagnetic fields and the properties of plasma, energetic particles, and dust particles. The Probe is spin- stabilized. It returns data via S-band link to the Orbiter. The Probe's six instruments include sensors to determine atmospheric physical properties and composition. Radiometric and optical sensors will provide data on thermal balance and obtain images of Titan's atmosphere and surface. Doppler measurements between Probe and Orbiter will provide wind profiles. Surface sensors will measure impact acceleration, thermal and electrical properties, and, if the surface is liquid, density and refractive index.

The cosmic dust analyzer (CDA) is designed to characterize the dust environment in interplanetary space, in the Jovian and in the Saturnian systems. The instrument consists of two major components, the dust analyzer (DA) and the high rate detector (HRD). The DA has a large aperture to provide a large cross section for detection in low flux environments. The DA has the capability of determining dust particle mass, velocity, flight direction, charge, and chemical composition. The chemical composition is determined by the chemical analyzer system based on a time-of-flight mass spectrometer. The DA is capable of making full measurements up to one impact/second. The HRD contains two smaller PVDF detectors and electronics designed to characterize dust particle masses at impact rates up to 10⁴ impacts/second. These high impact rates are expected during Saturn ring plane crossings.

Cassini/Huygens is a joint project of NASA and the European Space Agency designed to explore the Saturnian system in depth during its four-year mission. Cassini, the orbiter spacecraft, will carry twelve hardware investigations while Huygens, the Titan atmospheric probe, will carry an additional six. The Cassini Plasma Spectrometer (CAPS), one of 12 orbiter investigations, includes 3 plasma sensors designed to cover the broadest possible range of plasma energy, composition, and temporal variation. It is conservatively estimated that CAPS will provide a factor of ten or more improvement in measurement capabilities over those of the Voyager spacecraft at Saturn.

The Cassini Orbiter Ion and Neutral Mass Spectrometer (INMS) is designed to measure the composition and density variations of low energy ions and neutral species in the upper atmosphere of Titan, in the vicinity of the icy satellites and in the inner magnetosphere of Saturn where densities are sufficiently high for measurement. The sensor utilizes a dual radio frequency quadrupole mass analyzer with a mass range of 1-99 amu, two electron multipliers operated in pulse-counting mode to cover the dynamic range required and two separate ion sources. A closed ion source measures non-surface reactive neutral species which have thermally accommodated to the inlet walls such as N₂ and CH₄. An open ion source allows direct beaming ions or chemically active neutral species such as N and HCN to be measured without surface interaction. The instrument can alternate between these three different modes. Characterization and calibration of each of these three modes is done using a low energy ion beam, a neutral molecular beam and a neutral thermal gas source. An onboard flight computer is used to control instrument operating parameters in accordance with pre-programmed sequences and to package the telemetry data. The sensor is sealed and maintained in a vacuum prior to launch to provide a clean environment for measurement of neutral species when it is opened to the ambient atmosphere after orbit insertion. The instrument is provided by NASA/Goddard Spaceflight Center, Code 915. Operation of the instrument and data analysis will be carried out by a Science Team.

This paper describes the magnetometer instrument (MAG) to be flown on the Cassini spacecraft. The instrument consists of two magnetometers and an on-board data processing unit. One magnetometer is a Vector Helium device of a type previously flown on Ulysses and several other missions which has been modified to operate in a Scalar mode providing measurements of the magnitude of the local magnetic field with very small absolute error. This is the first flight of such an instrument. The other magnetometer is a Fluxgate device of similar design to that flown on Ulysses and on many previous missions but with newly developed electronics. Both magnetometers can provide vector measurements of the three components of the magnetic field with a sensitivity of about 10pT. The unique combination of Fluxgate/Scalar or Fluxgate/Vector Helium operation offers increased possibilities for scientific investigation of the magnetic field environment in the Saturnian system. The data processing unit contains dual redundant systems based on the 80C86 microprocessor. It features sophisticated on-board data processing, large internal data storage capability and internal failure detection and recovery, giving the instrument the capability to operate autonomously for extended periods. The MAG instrument development team is drawn from institutes in four countries reflecting the multi-national flavor of the Cassini/Huygens mission.

The INCA sensor is the first energetic neutral atom (ENA) imager funded for flight by NASA. It is a part of the Magnetrospheric Imaging Instrument (MIMI) on the Cassini mission to Saturn, where it will be well suited to monitoring the global dynamics of the Saturn-Titan magnetospheric system throughout the orbital tour. INCA will perform remote sensing of the magnetospheric energetic ion plasmas by detecting and imaging charge exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. The escaping charge exchange neutrals were detected by the Voyager-1 spacecraft outside Saturn's magnetosphere, and can be used like photons to form images of the emitting regions, as has been done at Earth. Since Cassini is 3-axis oriented, INCA is designed as a 2D imager with a field of view of 90 by 120 degrees. The technique involves sensing the position of the ENA as it penetrates an entrance foil and again ont he back-plane microchannel plate, thereby establishing the ENA's trajectory and time- of-flight. Along with rough composition determined by pulse- height analysis, the sensor produces images of the hot plasma interaction with the cold ambient neutral gas as a function of species and energy, from approximately 20 keV to several MeV. A large geometric factor allows sufficient sensitivity to obtain statistically significant images in approximately 1 to 30 minutes, depending on conditions and location. We will discuss several of the design details unique to this instrument, as well as recent calibration results.

The composite infrared spectrometer (CIRS) is a remote sensing instrument to be flown on the Cassini orbiter. CIRS will retrieve vertical profiles of temperature and gas composition for the atmospheres of Titan and Saturn, from deep in their tropospheres to high in their stratospheres. CIRS will also retrieve information on the thermal properties and composition of Saturn's rings and Saturnian satellites. CIRS consists of a pair of Fourier Transform Spectrometers (FTSs) which together cover the spectral range from 10-1400 cm^-1 with a spectral resolution up to 0.5 cm^-1. The two interferometers share a 50 cm beryllium Cassegrain telescope. The far-infrared FTS is a polarizing interferometer covering the 10-600 cm^-1 range with a pair of thermopile detectors, and a 3.9 mrad field of view. The mid-infrared FTS is a conventional Michelson interferometer covering 200-1400 cm^-1 in two spectral bandpasses: 600-1100 cm^{- 1100-1400 cm(superscript -1} with a 1 by 10 photovoltaic HgCdTe array. Each pixel of the arrays has an approximate 0.3 mrad field of view. The HgCdTe arrays are cooled to approximately 80K with a passive radiative cooler.

The composite infrared spectrometer (CIRS) instrument, an important component of the Cassini mission, consists of 3 focal plane arrays for sensing IR radiation of the Saturnian planetary system. Goddard Space Flight Center has fabricated, tested, and delivered high performance, 10- element HgCdTe photoconductive (PC) arrays for use on CIRS FP3, the focal plane responsible for detection of radiation in the 9.1 to 16.7 micrometers spectral band. The delivered flight array has peak responsivity 100 percent above CIRS specification, detectivity 30 percent or more above specification, and a cutoff wavelength of 17.3 micrometers at the operating temperature of 80 K. In order to achieve high performance at low frequency while maintaining limited power dissipation, we adopted a split-geometry detector structure. This design also ensured the buttability of the PC arrays to photovoltaic arrays supplied by CE-Saclay-France for detection of radiation in the 7.1 to 9.1 micrometers range. The detector structure is also noteworthy for its use of 0.05 micrometers Alumina powder-loaded epoxy to minimize reflection at the epoxy/HgCdTe interface, thus spoiling undesired optical resonance. This was done in order to meet the CIRS spectral uniformity requirement, which would have been difficult at these long wavelengths without this feature.

The Cassini imaging science subsystem uses two separate camera designs to satisfy the scientific objectives of the Cassini mission. The first is a narrow angle camera (NAC) design which obtains high resolution images of the target of interest. The second is a wide angle camera (WAC) design which provides a different scale of image resolution and more complete coverage spatially. Each camera is a framing charge coupled device (CCD) imager. They differ primarily in the design of the optics: the NAC has a focal length of 2000 mm and the WAC has a focal length of 200 mm. Both cameras have a focal plane shutter of the Voyager/Galileo type, and a two-wheel filter changing mechanism derived from the Hubble Space Telescope Wide-Field/Planetary Camera. The detector is cooled to suppress dark current and is shielded from space radiation. The electronics for each camera are identical and contain the signal chain and CCD drivers, the Engineering Flight Computer, command and control compressor, and a lossy compressor. The CCD detector design is a square array of 1024 X 1024 pixels. Each pixel is 12 micrometers on a side. The detector uses three phases, front side illuminated architecture, with a coating of lumogen phosphor to enhance ultraviolet response. This paper will describe the ISS in detail, review the technologies involved, and the design challenges with these cameras.

The newest generation of JPL imaging experiments, the Cassini Imaging Science Subsystem, required calibration analysis effort beyond that of its predecessor instruments. This called for streamlining the data reduction process with automation and flexibility while using software inherited from support of the Galileo Solid State Imaging instrument. A set of enhancements was implemented which automated many tasks, tracked their inputs and outputs and generated easily distributable products.

The visual and infrared mapping spectrometer (VIMS) is a remote sensing instrument developed for the Cassini mission to Saturn by an international team representing the national space agencies of the United States, Italy, and France. A dual imaging spectrometer, VIMS' unique design consists of two optical systems boresighted and operating in tandem, coordinated by a common electronics unit. The combined optical system generates 352 2D images simultaneously, each in a separate, contiguous waveband. These are combined by the electronics to produce 'image cubes' in which each image pixel represents a spectrum spanning 0.3 to 5.1 microns in 352 steps. VIMS images will be used to produce detailed spatial maps of the distribution of mineral and chemical species of Saturn's atmosphere, rings, and moons, and the atmosphere of Titan. At some wavelengths VIMS will penetrate Titan's atmosphere to map its surface, and image the night side of many Saturnian objects.

The Cassini Science Operations and Planning Computers (SOPC) are meant to give Cassini investigators and the Huygens Probe Operations Center (HPOC) more direct control of and responsibility for their instruments and their data and to reduce mission operations cost. SOPCs give the investigators the ability to command their instrument directly, but also the responsibility to do so correctly. The SOPCs help reduce mission operations cost by allowing the instrument designers also to be the operators. The SOPCs allow the operators to participate directly even though they are at a distance from JPL. The SOPCs are direct extensions of the Cassini Ground System to the investigator's home institution. They provide links to the rest of the Cassini Ground System for both uplink and downlink functions. The SOPCs use the same software as the rest of the ground data system. They also provide a platform for project and investigator provided planning and analysis tools. The current SOPC design is targeted toward Cassini Assembly Test and Launch Operations and cruise. Improvements planned during cruise include hardware upgrades, changes in software architecture, changes to the data communication systems and enhanced security features. The SOPCs, without proper precautions, could pose a security liability. The philosophy of the security requirements and some particular measures will be discussed.

The Cassini Resource Exchange was developed to assist the Cassini Science Instrument Manager with the management of the spacecraft's science payload. This system, unlike previous development approaches, allocated the entire mass, power, data rate, and budget resources for the science instruments to the principal investigators. The result removed the Cassini project from solving instrument development issues. Problems that did occur were resolved by the principal investigators themselves thought the use of a 'resource exchange'. A resource exchange allowed principal investigators to submit 'bids' to a database. Any other principal investigators with their own resource issue could swap resources with investigators in the database. The resulting trade could mitigate both instrument problems.

The Cassini spacecraft will explore the planet Saturn and its rings and moons with an orbiter and atmospheric entry probe, both of which have a sophisticated set of science instruments. The spacecraft design is responsive to mission and science objectives, and is influenced by technical and programmatic constraints e.g., a cost cap, fixed schedule, space environment, and interfaces to other fixed systems like the launch vehicle and ground system. The spacecraft design must also consider the limited post-launch resources allocated to do the flight operations. This paper presents an overview of the spacecraft system design with emphasis given to the orbiter and only a high level summary of the probe.

A high-capacity, high-performance solid-state recorder (SSR) design was chosen for the main software and data-storage system of the Cassini spacecraft. The SSR design was selected rather than candidate moving-tape data-transport designs, which have a demonstrated lifetime. In contrast, the SSR design offers unlimited read and write opportunities. Additionally, the design of the SSR permits definition of up to 16 individual partitions, each controllable and accessible independently of the others. The Cassini SSR is designed to operate during intense radiation events, including passage through Saturn's trapped-proton belt, with indiscernible impact on data. Custom application- specific integrated circuits are used throughout the SSR. This paper presents the basic failure-tolerant SSR design and compares it with the design of earlier mechanical data- storage devices. The paper discusses why the move to solid- state recording techniques has brought with it optimal data storage.

The pointing control functions of the Cassini spacecraft attitude and articulation control subsystem have been designed to enhance operability by establishing a behavioral model at the command interface that raises pointing operations to a more intuitive level. The control system tracks this model to closely achieve the commanded behavior. Versatility is achieved by composing the behavioral model of independently commandable, interacting modules. Each directs activities directly related to a particular pointing issue, such as observation goals, instrument characteristics, attitude constraints, and navigation. A key feature of this design is the use of propagated vectors that precisely describe the motion of targets. Our design has enabled a new, more streamlined approach to mission operations whereby the many science and engineering activities sharing this system can be given direct control over pointing activities. This is possible because the behavioral model is easy to replicate in distributed ground software, it includes enforcement of constraints, and the maintenance of its components can be performed independently.

The Cassini Stellar Reference Unit (SRU) is the prime attitude determination sensor on the Cassini spacecraft. It must operate continuously and reliably during both the cruise and the Saturnian tour phases of the mission. In fact, accuracy requirements are most critical toward the end of the mission, during the four years of scientific observations at Saturn. To ensure that the SRU will operate within specification for the entire mission, an extensive test program has been undertaken to characterize the SRU performance prior to launch and to quantify any expected performance degradation. Results from several complimentary test programs are presented and compared with pre-test performance predictions. Additionally, a unique approach is described for enabling closed-loop testing of the SRU with the other elements of the Cassini Attitude and Articulation Control Subsystem when no optical stimulation is available.

The JPL Inertial Reference Unit (IRU) is the single most sophisticated assembly on the Cassini spacecraft. At the core of the IRU is the state-of-the-art, Litton Hemispherical Resonator Gyro (HRG). Launched in October 1997, Cassini's trajectory utilizes gravity assist maneuvers around Venus (two), Earth, and Jupiter over a seven year period, arriving at Saturn in June 2004. Its tour of the Saturnian system will last an additional four years. Although the Stellar Reference Unit (SRU) provides the ultimate reference for the spacecraft Attitude and Articulation Control System (AACS) and can be used to control the spacecraft under benign conditions, the Cassini IRU is essential during maneuvers and fault recovery operations, and for precision attitude stabilization during science data acquisition. Therefore, IRU reliability over the long Cassini mission is a critical concern. Following an extensive evaluation of possible alternatives, the Hemispherical Resonator Gyro (HRG) based IRU developed by Litton Guidance and Control Systems, was chosen for the Cassini mission. The HRG is an attitude rate sensor that has no physical wear-out mechanisms. Based on a principle first described by G. H. Bryan (1890) in his paper, 'On Beats in the Vibrations of a Revolving Cylinder or Bell', the HRG is created by vibrating a quartz resonator. This paper discusses the theory and modifications required to the design of the standard Space Inertial Reference Unit to adapt it to meet the requirements of the Cassini mission and the AACS interface. The Cassini mission is the first use of an IRU for a deep space planetary mission that does not use a spun-mass sensor.

The Cassini spacecraft uses a CCD-based star tracker, the Stellar Reference Unit (SRU), for attitude identification in the Attitude and Articulation Control Subsystem (AACS). SOftware to process SRU data resides in the Flight Computer (AFC) and is integrated with all other AACS functions. The Cassini mission will use autonomous star identification for initial attitude determination,and a star tracking function for maintaining attitude, both performed by processing pixel data produced by the SRU and sent to the AFC via a dma interface. Because of the complexity of the StarID software, special software simulation tools were created to simulate the SRU output as a function of commands, spacecraft attitude, and star screen, and allow the introduction of fault conditions.