Cassini's Cosmic Dust Analyser. (Courtesy of Ralf Srama of the Max Planck Institute for Astrophysics in Heidelberg.)
with the solar wind. There is a torus coincident with Titan's orbit which is mainly hydrogen, but there may be more complex species leaking from the moon. CAPS will characterise the ion flow around Titan to investigate the processes by which the material leaking from the atmosphere enters the torus.19 The principal investigator is D.T. Young of the Southwest Research Institute in San Antonio, Texas.
The Radio and Plasma Wave Spectrometer (RPWS) is an improved version of an instrument carried on the Galileo spacecraft.20 It has a Langmuir probe to measure the temperature and electron density of plasma in the immediate vicinity, and separate electric and magnetic field sensors with high-, medium- and wide-band receivers. The electric field sensor has three antennas composed of interlocking beryllium-copper sections, and a preamplifier to boost the antenna signals. Individual electric motors will deploy the antennas to a length of 10 metres. The magnetic search coil sensor assembly comprises a tri-axial sensor assembly and preamplifier. The tri-axial sensor consists of three mutually orthogonal metallic alloy cores with two sets of windings each, one to produce flux in the core and another to detect the flux. The Langmuir probe has a sensor, preamplifier and associated control electronics.21 The sensor is a 5-centimetre-diameter sphere located at the end of a 1-metre-long rod which is folded in a stowed state for launch and then deployed in space. On the interplanetary cruise RPWS will monitor the solar wind. Once within Saturn's magnetosphere, it will investigate the configuration of the magnetic field to determine the planet's rotation rate; diurnal variations in the ionosphere; electrical discharges between storms in the atmosphere, and perhaps to the ionosphere; the kilometric radio emissions; and the long-term variability of the planet's interaction with the solar wind. During the Titan fly-bys, RPWS will listen for evidence of discharges to suggest lightning in the troposphere and investigate the processes by which the material leaking from the atmosphere enters the torus.22,23,24,25 The principal investigator is D.A. Gurnett of the University of Iowa.
The Magnetospheric Imaging Instrument (MIMI) used three sensors. The Low-Energy Magnetospheric Measurement System (LEMMS) measures the angular and spectral distributions of low- and high-energy protons, ions and electrons in the
energy range 20 keV to 20 MeV. It is an improved version of part of the Energetic Particle Detector on the Galileo spacecraft The Charge-Energy Mass Spectrometer (CHEMS) is similar to an instrument provided for the Geotail spacecraft as part of the International Solar-Terrestrial Physics Programme. It will measure the charge and composition of the ions in the most energetically important portion of a magnetospheric plasma. The Ion and Neutral Camera (INCA) can undertake two different types of measurements: firstly, the camera has a two-dimensional field of view covering 90 x 120 degrees and it can remotely image the global distribution of the energetic neutral emission of hot plasmas in a magnetosphere and determine the composition and velocities of those energetic neutrals for each image pixel; secondly, it can measure the three-dimensional distribution, velocities and composition of ions in the regions of interplanetary space and planetary magnetospheres in which the energetic ion fluxes are very low.26'27
In addition to remotely imaging magnetospheres, the MIMI will directly measure the composition, charge state and energy distribution of the fast neutral species and the energetic ions and electrons in order to investigate how magnetospheres interact with the solar wind. When at the apoapsis of its Saturn orbit, Cassini will be outside the magnetosphere, and the MIMI will be able to image Titan's torus as a whole, and measure the composition, charge and energy distribution of fast neutral species and energetic ions and electrons in order to determine how this interacts with the magnetosphere, or, when it is exposed to it, with the solar wind. The principal investigator is S.M. Krimigis of the Applied Physics Laboratory, part of Johns Hopkins University in Maryland. "Every new spacecraft has instruments which expand our ability to see things,'' he pointed out, "and with MIMI, we are able to visualise the invisible.'' The CHEMS team is headed by D.C. Hamilton of the University of Maryland, the INCA team by D.G Mitchell and the LEMMS team by Stefano Livi, both of the Applied Physics Laboratory.
The Dual-Technique Magnetometer (MAG), the latest in a series of ever more sophisticated magnetometers, will measure the strength of the magnetic field in the vicinity of the spacecraft. It consists of a vector/scalar helium magnetometer sensor, a fluxgate magnetometer sensor and a Data Processing Unit (DPU). The Vector/ Scalar Helium Magnetometer (V/SHM) sensor, supplied by JPL, will make both vector (magnitude and direction) and scalar (magnitude only) measurements of magnetic fields. The Fluxgate Magnetometer (FGM) sensor that will make vector field measurements was provided by the University of London's Imperial College. The DPU provided by the Technical University of Braunschweig will interface with Cassini's computer. Because magnetometers are sensitive to electric currents and ferrous components on the spacecraft, they are mounted on an extended boom away from the vehicle. The FGM sensor is halfway along the magnetometer boom and the V/SHM sensor is at its end.28 The boom comprises thin non-metallic rods that are compressed compactly prior to launch and then deployed en route. In addition to monitoring the solar environment during the interplanetary cruise, MAG will map the magnetic fields in the Jovian and Saturnian magnetospheres in order to study how they interact with the solar wind. During the Saturnian tour, it will also investigate how that planet's magnetosphere interacts with the ring system, the icy satellites and Titan's atmosphere.29,30 D.J. Southwood of the University of London's Imperial College is the principal investigator.
In addition to the instruments carried for specific experiments, the spacecraft's communications link to Earth doubles as the Radio Science Subsystem (RSS). When Cassini is undergoing superior conjunction, the way in which its signal is degraded by passing close to the Sun provides information concerning the solar corona. In an occulation, the signal's refraction can profile an atmosphere to reveal its physical state and chemical composition. When in interplanetary space, 'anomalous' Doppler shifts could indicate the passage of a gravitational wave through the Solar System. During fly-bys, tracking of how the spacecraft is deflected by a body's gravity will measure its moment of inertia, the key parameter from which internal structure can be inferred. A.J. Kliore of JPL is the principal investigator for the radio science investigations.
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