July 5, 2016


CASSINI orbiter

Credits CNES/Illus. David Ducros, 1997

The Cassini orbiter was built by NASA's Jet Propulsion Laboratory, with the Italian Space Agency (ASI) contributing its high-gain antenna which also serves as radar. The spacecraft is one of the largest, heaviest and most complex interplanetary spacecraft ever built.

The Cassini orbiter alone weighs 2,125 kg, to which is added the Huygens probe and 3,267 kg of propellant, to reach a total mass of 5,712 kg. More than half of this mass is constituted by the liquid propellant needed for the travel toward Saturn and above all its insertion in orbit. The Cassini spacecraft stands more than 6.7 metres high and is more than 4 metres wide.

Three Radioisotope Thermoelectric Generators (RTGs) provide power for the CASSINI spacecraft. The RTGs contain plutonium dioxide which produces radioactivity and thus heat. This heat is converted in electricity. The RTGs are a very reliable and durable source of energy - without moving parts that can break during the long travel to Saturn.

Cassini travelling through areas with very high or very temperatures, has needed to be equipped with systems to protect it from these extreme temperatures among which thermal blankets against the cold, or reflecting paint to decrease the solar energy.

Cassini communicates with the Earth thanks to three different antennas: one high-gain antenna (of 4 m diameter) and two low-gain antennas. The orbiter receives the data as a radio signal at the speed of the light, data that will then take 68 to 84 minutes to reach the Earth (function of the position of Saturn on its orbit; between 8.6 and 10.6 Astronomical Units from the Earth).

The following instruments are on board CASSINI orbiter:

  • Cassini Plasma Spectrometer (CAPS): explores plasma (highly ionized gas) within and near Saturn's magnetic field.

  • Cosmic Dust Analyzer (CDA): studies ice and dust grains in and near the Saturn system.

  • Composite Infrared Spectrometer (CIRS): measures infrared energy from the surfaces, atmospheres and rings of Saturn and its moons to study their temperature and compositions.

  • Ion and Neutral Mass Spectrometer (INMS): examines neutral and charged particles near Titan, Saturn and its moons to learn more about their extended atmospheres and ionospheres.

  • Imaging Science Subsystem (ISS): imagery in visible, near-ultraviolet and near-infrared light.

  • Dual-Technique Magnetometer (MAG): studies Saturn's magnetic field and its interactions with the solar wind, the rings and the moons of Saturn.

  • Magnetospheric Imaging Instrument (MIMI): images Saturn's magnetosphere and measures interactions between the magnetosphere and the solar wind, a flow of ionized gases streaming out from the Sun.

  • Cassini Radar (RADAR): maps surface of Titan using radar imager to pierce veil of haze. Also used to measure heights of surface features.

  • Radio and Plasma Wave Spectrometer (RPWS): investigates plasma waves (generated by ionized gases flowing out from the Sun or orbiting Saturn), natural emissions of radio energy and dust.

  • Radio Science Subsystem (RSS): studies gravity fields of Saturn and its moons by measuring telltale changes in radio waves (Doppler Effect) sent from the spacecraft.

  • Ultraviolet Imaging Spectrograph (UVIS): measures ultraviolet energy from atmospheres and rings to study their structure, chemistry and composition.

  • Visible and Infrared Mapping Spectrometer (VIMS): identifies the chemical compositions of the surfaces, atmospheres and rings of Saturn and its moons by measuring colours of visible light and infrared energy emitted or reflected.

HUYGENS descent module

The Huygens probe is 350 kg of concentrated technology designed to study the chemical composition of Titan's atmosphere and surface.

On 14 January 2005, Huygens penetrated Titan's atmosphere (the pressure on ground is 1.5 times denser than Earth's. With the lower temperature, the atmospheric density is 4.5 times the Earth's one) at a speed of 20,000 kph. The probe's heat shield was subjected to temperatures up to 12,000° C. From an altitude of 180 km above the surface, a series of braking parachutes were then deployed to take it down to a soft landing on Titan.

Huygens' instruments were able to collect a wealth of data on Titan's atmosphere during a descent lasting more than two hours, as well as on the moon's surface. The probe's batteries kept it going for another two hours after landing, largely exceeding the expected two-and-a half-hour nominal mission duration.

The instruments on board HUYGENS module are as follow:

  • Aerosol Collector and Pyrolyser (ACP): instrument collecting aerosols at different altitudes to analyze their chemical composition. After extension of the sampling device, a pump will draw the atmosphere through filters which capture aerosols. Each sampling device can collect about 30 micrograms of material.

  • Descent Imager/Spectral Radiometer (DISR): imager/spectral radiometer covering a wide spectral range. A few hundred metres before impact, the instrument will switch on its lamp in order to acquire spectra of the surface material.

  • Doppler Wind Experiment (DWE): instrument uses radio signals to deduce atmospheric properties. The probe drift caused by winds in Titan's atmosphere induces a measurable Doppler shift in the carrier signal. The swinging motion of the probe beneath its parachute and other radio-signal-perturbing effects, such as atmospheric attenuation, may also be detectable from the signal.

  • Gas Chromatograph and Mass Spectrometer (GCMS): chemical analyzer of gases designed to identify and quantify atmospheric molecular constituents.

  • Huygens Atmosphere Structure Instrument (HASI): sensors for measuring the physical and electrical properties of the atmosphere and an on-board microphone that will send back sounds from Titan.

  • Surface Science Package (SSP): suite of sensors to determine the physical properties of the surface at the impact site and to provide unique information about its composition. The package includes an accelerometer to measure the impact deceleration, and other sensors to measure the index of refraction, temperature, thermal conductivity, heat capacity, speed of sound, and dielectric constant of the (liquid) material at the impact site.