Outer Planets

Overview

The outer solar system is by far the least-studied part of the solar system. The planets that this includes are Jupiter, Saturn, Uranus, Neptune, and Pluto. The first crafts to visit were Pioneer 10 and 11, followed by Voyager 1 and 2, together making a sweep of the four gas giants. The next probe to an outer planet was Galileo, which studied Jupiter in depth for several years. Now, NASA has sent a probe to study the next planet, Saturn, in depth for at least four years. Currently in orbit, Cassini is studying Saturn, and it launched the ESA-sponsored Huygens Titan probe in December of 2004, which landed on Titan in January 2005.

Cassini

Overview

There are two main purposes to this mission. The first is to deliver the Huygens probe to the Saturnian moon Titan. The second is to have the Cassini Orbiter explore Saturn, its rings, and satellites. The main objectives of the orbiter are as follows:

• to determine the three-dimensional structure and dynamical behavior of the rings
• determine the composition of the satellite surfaces and the geological history of each object
• determine the nature and origin of the dark material on Iapetus' leading hemisphere
• measure the three-dimensional structure and dynamical behavior of the magnetosphere
• study the dynamical behavior of Saturn's atmosphere at cloud level
• study the time variability of Titan's clouds and hazes
• characterize Titan's surface on a regional scale

The Cassini project is in orbit of Saturn, having been launched on October 15, 1997, and achieving orbit on June 30, 2004. After leaving Earth, it made two flybys of Venus, one more of Earth, and then of Jupiter (an image it took of Jupiter is to the right, and was taken on December 8, 2000, and also shows Ganymede; the image to the left was the first image Cassini took of Saturn (taken on October 21, 2002), and includes the moon Titan). This trajectory is commonly referred to as a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist).

In December 2004, the Huygens probe separated and headed towards Titan, entering Titan's atmosphere in January 2005. The purpose of this probe was to learn about the characteristics (density, pressure, temperature, etc.) of Titan's atmosphere, measure the chemical make-up of the atmosphere - especially with regard to organic molecules, characterize the weather of Titan - particularly with respect to cloud physics, lightning discharges, and general circulation. Besides these atmospheric inquiries, the Huygens probe will also examine the physical state, topography, and composition of the surface of Titan.

To accomplish this, Huygens entered Titan's atmosphere. For two full minutes after entry, a heat shield protected the probe from the heat of entry. After this, a parachute deployed and the shield was jettisoned. The probe then started to take its data, and it represents the farthest location from Earth that a successful touch-down has occurred. The probe continued to take readings from the surface of Titan after it landed.

Cassini's mission is to orbit Saturn at least 30 times in loose elliptical orbits, each targeted for different purposes. Besides simple visual imaging, Cassini will map in radar, infrared, and ultraviolet frequencies. Cassini will also look for cosmic dust, conduct a radio and plasma wave experiment, and take magnetic measurements. It has a communications antenna to communicate with Earth, but it also has other transmitters to make observations of the atmospheres of Titan and Saturn and to measure the gravity fields of the planet and its satellites.

Experiment Summary

Cassini:

• Imaging Science Subsystem (ISS): This is intended to study Saturn, Titan, and the other satellites of Saturn. The primary science objectives of the ISS are to:
  1. map the three-dimensional structure and motion of the atmospheres of Saturn and Titan
  2. study the composition, distribution, and physical properties of clouds and aerosols
  3. examine scattering, absorption, and solar heating within the atmospheres of Saturn/Titan
  4. search for lightning, aurorae, and airglow
  5. investigate the gravitational interactions among Saturn's rings and satellites
  6. determine the nature and rate of energy and momentum transfer within the rings
  7. determine the thickness of the rings as well as the size, composition, and physical nature of the ring particles
  8. map the surfaces of the satellites and determine the composition of their surface materials
  9. measure the rotation states of the satellites
  To accomplish these goals, the ISS consists of a wide angle camera and a narrow angle camera. The wide angle camera (WAC) telescope is a 20 cm (f/3.5) refractor while the narrow angle camera (NAC) telescope is a 2 m (f/10.5) refractor. The WAC is planned to cover a wavelength range from 380-1100 nm with 18 separate filters while the NAC covers a range of 200-1100 nm with 24 filters. The WAC's field-of-view is 3.5 degrees with an angular resolution of 60 microradians/pixel. The NAC's field-of-view is 0.35 degrees with an angular resolution of 6.0 microradians/pixel. The imaging plane of each will contain a 1024x1024 element CCD array.
• Composite Infrared Spectrometer (CIRS): This is designed to measure infrared emissions from atmospheres, rings, and surfaces over a wavelength range between 7 and 1000 µm. Its objectives for Saturn and Titan are to:
  1. map the global atmospheric temperature structure
  2. map the global atmospheric gas composition
  3. ascertain global atmospheric haze and cloud distributions
  4. gather information on atmospheric energetic processes
  5. search for new atmospheric molecular species
  6. map global surface temperatures at Titan
  7. to determine the thermal characteristics and composition of Saturn's rings and icy satellites
  To accomplish these goals, CIRS consists of a 50.8 cm Cassegrain telescope and three interferometers, a far-infrared, a mid-infrared, and a reference interferometer. The far-infrared interferometer covers a spectral range from 17-1000 micrometers (10-600/cm). The instrument is a polarizing interferometer with substrate-mounted wire grid polarizers to polarize the radiation and subsequently modulate the polarization. The instrument has a 4.3 milliradian field of view. The mid-infrared interferometer is of conventional Michelson design and covers a spectral range of 7-17 micrometers (600-1400/cm) in two bands. The first band (9-17 micrometers or 600-1000/cm) is imaged on a 1 x 10 photoconductive HgCdTe detector array. The second band (7-9 micrometers or 1100-1400/cm) is imaged on a 1 x 10 photovoltaic HgCdTe detector array. Each element in each detector array consists of a 0.273 milliradian square pixel. The reference interferometer provides timing correlation of the science data sampling to the scan mechanism position. Also of a Michelson design, it is used on-axis at the optic center of the mid-infrared interferometer and includes laser diode and LED sources, a quartz beam-splitter/compensator, optics, and a silicon detector. A 70-80 K cold finger serves as a single-stage passive cooler, radiating to space. There are four commandable set points within the range, but the nominal setting is 80 K. The cold finger has heaters for decontamination and detector annealing.
• Radio and Plasma Wave System (RPWS): This is designed to study radio and plasma wave phenomena. These include observations of radio emissions, plasma waves, lightning, and dust impacts as well as measurements of plasma densities and temperatures. These studies will be conducted during the entire mission, including observations of the terrestrial, Jovian, and Saturnian magnetospheres, the ionospheres of Venus and Titan, the solar wind, and the environment near asteroids. Measurements of plasma waves will be made with a pair of orthogonal 10 m antennas similar to those on Voyager. These will enable the measurement of both low- and high-frequency plasma waves (1 Hz-16 MHz) and be capable of providing wave polarization and arrival direction information. A pair of tri-axial search coil magnetic field antennas, covering frequencies between 1 Hz-12.6 kHz, will provide precise measurements of general spectral characteristics, wave amplitudes, and rapid variations in frequency-time characteristics. Use of these two sets of antennas will also permit the simultaneous measurement of electric and magnetic fields. Electron densities and temperatures will be measured utilizing a dedicated Langmuir probe.
• Magnetospheric Imaging Instrument (MIMI): This is designed to:
  1. measure the composition, charge state and energy distribution of energetic ions and electrons
  2. detect fast neutral species
  3. conduct remote imaging of the Saturnian magnetosphere
  To accomplish these goals, MIMI consists of two instruments - the Hot Plasma Detector (HPD) and the Energetic Neutral Analyzer (ENA). The HPD will have two separate solid-state detector telescopes mounted on a stepping platform capable of rotating 180 degrees in discrete steps. A pulse height analyzer will be used to examine data from either of the telescopes. The resulting count rates and pulse heights from the detector systems will permit the analysis of energetic electrons and protons (at energies greater than 15 keV) and heavier ions (at energies greater than 1 MeV/nucleon) as a function of energy, angle, and species. The ENA is to consist of two time-of-flight sensors in series to detect and analyze neutral particles at low densities with an energy threshold of 100 eV. Compositional analysis, within certain energy ranges, resolving H, He, and the CNO group will be performed.
• The Rings and Dust, Satellites and Asteroids, Plasma Circulation and Magnetosphere-Ionosphere Coupling, Plasma Environment in Saturn's Magnetosphere, Origin and Evolution of the Saturn System, Aeronomy of Titan and Saturn, and Atmospheres of Titan and Saturn experiments are lumped together into the Interdisciplinary Science (IDS) Investigation:
  • Plasma Circulation and Magnetosphere-Ionosphere Coupling: This IDS investigation will use data from several of the fields and particles experiments as well as some of the remote-sensing instruments. Some of the objectives of this investigation are to:
    1. map the flow of plasma in the equatorial plane of Saturn's magnetosphere
    2. evaluate the relative importance of large-scale circulation in the radial transport of plasma
    3. investigate the role of thermospheric circulation in the transport of angular momentum
    4. develop a model of the dynamics of the Saturnian thermosphere
  • Plasma Environment in Saturn's Magnetosphere: This IDS investigation will use utilize data from the various fields and particles experiments on Cassini. Among the objectives are to develop models of:
    1. global MHD and generalized transport
    2. field-aligned plasma outflows for the high-latitude ionosphere of Saturn and topside ionosphere of Titan
    3. plasma flows between hemispheres along closed field lines that are continuously depleted by interactions between Saturn's rings and magnetosphere

Huygens:

• Decent Imager / Spectral Radiometer (DISR): This will be used to take images and make spectral measurements using several sensors covering a wide spectral range from the ultraviolet to the infrared (0.35-1.70 micrometers) ranges. It will measure both the upward and downward heat fluxes on Titan. A solar aureole sensor will measure the solar intensity in the "halo" around the Sun, indicating the amount of sunlight scattering caused by aerosols in the atmosphere along the sensor's line of sight. This, in turn, will allow the physical properties of the aerosol particles to be determined. The DISR will also be equipped with a side-looking horizon-viewing imager to take pictures of the clouds.
• Doppler Wind Experiment (DWE): This will use two ultrastable oscillators (USO's), one on the probe and one on the Cassini orbiter. As the probe drifts due to winds in Titan's atmosphere, a measurable Doppler shift will be induced in the carrier signal. The Doppler signature in the probe radio signal will be extracted on-board the orbiter and merged into the probe data stream recorded on the orbiter's solid-state recorders. It is anticipated that the measurements obtained will be so sensitive that, by having the probe transmitting antennae offset from the spin axis by just a few centimeters, it will be possible to determine the spin rate and phase of the probe. It may also be that the swinging motion of the probe beneath its parachute and other perturbations of the radio signal (e.g., atmospheric attenuation) will be detectable.
• Gas Chromatograph and Mass Spectrometer (GCMS): This is designed to identify and quantify the abundances of the various constituents of Titan's atmosphere. It will analyze argon and other noble gasses and make isotopic measurements. The inlet for the GCMS is located at the apex of the probe near the stagnation point where dynamic pressure will drive the gas into the instrument. The instrument can work either in a direct mass spectrometer mode or in a more powerful mode in which the gas sample is passed through gas chromatograph columns. This latter mode helps to separate components of similar mass before analysis. The instrument is also equipped with gas samplers, which will be filled at high altitude for analysis later in the descent when there will be more time available. A separate ionization chamber for analysis of the aerosol pyrolysats fed from the ACP instrument is also a part of the GCMS. The GCMS will also be able to measure the composition of a surface sample providing the probe makes a safe impact that allows it to gather and transmit surface data for several minutes.
• Surface Science Package (SSP): This is designed to determine the composition and physical properties of Titan's surface at the impact site. To achieve this, the SSP consists of a suite of sensors including an accelerometer, tilt sensors, a thermal properties assembly, acoustic properties sensors, and instrumentation to measure fluid permittivity, density, and refractive index. An aperture in the foredome of the probe, with a vent extending upward along the probe axis and up to the experiment platform, will admit fluid, providing direct access to surface materials and mounting for several of the sensors. Although the SSP is primarily designed to investigate Titan's surface, several of its sensors are expected to contribute significantly to the study of atmospheric properties during the descent phase of the mission.
  • Accelerometer subsystem: The accelerometer subsystem consists of two piezoelectric sensors, one (ACC-I) mounted on the SSP electronics on the probe's experiment platform, the other (ACC-E) on a spear which extends below the probe's foredome and is located next to the top hat aperture. ACC-E will provide information only on impact, but ACC-I will also measure atmospheric and surface accelerations.
  • Tilt sensors: The tilt sensors work on an electrolytic principle. They are comprised of sealed glass tubes which contain a methanol-based liquid and platinum electrodes. Each sensor is small and measures the local vertical about a single axis. Both sensors will be used to give the tilt angle in any plane and both are mounted on the SSP electronics box (on the top of the probe experiment platform). One axis of the tilt sensors is aligned (to within one degree) along the radius extending from the platform/probe central axis. The tilt sensors will measure any pendulum motion during descent as well as determining the probe attitude after landing (including any motion due to waves).
  • Thermal properties sensor: The thermal properties sensor assembly consists of platinum wires 5 cm in length and 10 and 25 micrometers in diameter mounted in the top hat. A current is passed through the wires to heat them and the surrounding medium. A series of resistance measurements (taken approximately every 0.1 s) measures the rate of heating of the wires and detects the onset of convection. In this manner, the temperature and thermal conductivity of the surface and lower atmosphere as well as the heat capacity of the surface can be determined.
  • Acoustic properties sensors: The acoustic properties sensors are small piezoelectric ceramic devices similar to those used in marine applications. Two of the transducers (API-V) are mounted facing each other across the top hat, alternating between transmitting and receiving a 1.0 MHz acoustic signal. The third transducer (API-S) points vertically downward emitting a 15 kHz signal and will be used for sounding the depth of the ocean (after landing) as well as to Titan's surface (during descent). API-S may also be capable of atmospheric sounding.
  • Fluid permittivity sensor: The fluid permittivity sensor consists of electrodes placed within the top hat. The capacitance between the electrodes will vary with the permittivity of the substance between them. A measurement of the resistance between the electrodes will also yield the conductivity of the material and may provide information on the presence of polar molecules.
  • Fluid density sensor: The density of any fluid entering the top hat will be measured using an Archimedes buoyancy sensor. Fluid entering the aperture will displace the float and be measured by four strain gauges in a bridge arrangement.
  • Refractive index sensor: Finally, the refractive index sensor is a prism with a curved surface, two LED light sources, and a linear photodiode detector array. The LEDs provide internal or external illumination to the curved surface of the prism via light guides. Light passes through the prism surface onto the photodiode array and the refractive index is determined by the position of the transition from light to dark on the array.
• Aeronomy of Titan, Titan Atmosphere-Surface Interactions, and Chemistry and Exobiology of Titan are lumped into the Interdisciplinary Science (IDS) Investigation.
  • Aeronomy of Titan: This IDS will study Titan's aerosols, photochemistry, and general circulation. The primary science goals are to investigate the temperature structure of Titan's atmosphere; examine the spatial distribution of Titan's minor atmospheric components, especially organic molecules; determine the spatial distribution of aerosols and clouds in Titan's atmosphere; establish noble gas abundances and isotopic rations in the atmosphere, especially the ratio of deuterium to hydrogen; and construct a three-dimensional atmospheric circulation model for Titan.
  • Titan Atmosphere-Surface Interactions: This IDS will study the interactions between Titan's surface and atmosphere on scales from meters to global. The primary science goals are to characterize the chemical and physical nature of Titan's surface; determine the vertical structure of the atmosphere (e.g. gas-phase composition, cloud composition and structure, and wind velocity, all as a function of altitude); quantify the coupled surface-atmosphere interactions, chemical and physical, on time-scales ranging from seasonal to geologic; and understand the long-term evolution of Titan, including the processes by which Titan's current volatile inventory was acquired and to provide constraints on the initial composition.
  • Chemistry and Exobiology of Titan: This IDS will study the organic chemistry in Titan's environment and its implications for exobiology and the origin of life. The primary science objectives are to examine the origin, nature, distribution, and possible evolution of organic compounds in Titan's atmosphere, surface, and/or ocean; and determine the nature and role of chemical couplings between these three components.
• Aerosol Collector and Pyrolyzer (ACP): This is designed to collect aerosols for chemical composition analysis. The instrument is equipped with one deployable sampling device which will be operated twice in order to collect aerosols in two layers. The first sample will be taken from the top of the atmosphere down to about 40 km. The second sample will be taken from about 23 km down to 18 km. After extension of the sampling device, a pump will draw the atmosphere and its aerosols through filters, thus capturing the aerosols. Each sampling device can collect about 30 µg of material. After each sampling, the filter will be retracted into an oven where the aerosols will be heated in three different temperature steps in order to conduct a step pyrolysis. The volatiles will be vaporized first, then the more complex, less volatile organic material, and finally the cores of the particles. The pyrolysis products will then be flushed to the GCMS instrument which will perform the analysis and provide spectra for each temperature step.
• Huygens Atmospheric Structure Instrument (HASI): This is designed to measure the physical properties of Titan's atmosphere, including its electrical properties. As a result, it is comprised of accelerometers, a temperature sensor, a pressure sensor and an electric field sensor array. The accelerometers will be optimized to measure the probe's entry deceleration so as to infer the atmospheric density in the upper stratosphere. The electric field sensor consists of a relaxation probe to measure the ion conductivity of the atmosphere and a quadrupolar array of electrodes for measuring the permittivity of the atmosphere and that of the surface material after and possibly prior to impact. Two electrodes of the quadrupolar array are also to be used as an electrical antenna to detect electromagnetic waves in Titan's atmosphere, such as those produced by lightning. The ability to process the surface-reflected signal of the radar altimeter, which is the altitude sensor provided as part of the probe system, is also a part of HASI. This will allow important information to be relayed to the orbiter about the surface topography and its radar properties below the probe along its descent track.

Craft Data