From: Jet Propulsion Laboratory
Posted: Wednesday, February 9, 2005
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TITAN - 3 MISSION DESCRIPTION
Jet Propulsion Laboratory
California Institute of Technology
PD 699-100, Rev O (supplement) JPL D-5564, Rev O (supplement)
The third targeted flyby of Titan occurs on Tuesday, February 15, 2005 at 6:58 SCET (Ground: 8:06 UTC - 12:06 AM Pacific time). Cassini's closest approach to Saturn's largest satellite is at an altitude of 1577 km (980 miles) above the surface at a speed of 6.1 kilometers per second (14,000 mph). Titan has a diameter of 5150 km (3200 miles), so the spacecraft passes within 1.6 Titan radii.
This encounter is set up with two maneuvers: an apoapsis maneuver scheduled for February 1st, and a Titan approach maneuver, scheduled for February 11th. Titan-3 is an inbound flyby, with Saturn periapsis occurring about two days after closest-approach, on February 17th. The Navigation team expects to deliver the orbiter to within 30 km of the target altitude at a confidence of 99% (three sigma).
Titan-3 is Cassini's fourth targeted satellite encounter. The first was Phoebe, on June 11 th, at an altitude of 2000 km. The second was Titan A, on October 26th, at an altitude of 1200 km and the third was Titan-B, on December 13th at an altitude of 1200 km. 1.2
Titan is one of the primary scientific interests of the Cassini-Huygens mission. Through observations by Earth based telescopes and the Voyager spacecraft, Titan has been revealed to be an intriguing world both similar in nature to Earth and unique among both satellites and terrestrial planets. The largest of Saturn's satellites, Titan is larger than the planets Mercury or Pluto. Titan is the only satellite in the solar system with an appreciable atmosphere. Like Earth's atmosphere, Titan's atmosphere is composed mostly of Nitrogen, yet appears to have few clouds. However, it also contains significant quantities of aerosols and organic compounds (hydrocarbons), including methane and ethane. Although Titan's thick smoggy atmosphere masks its surface, scientists have speculated Titan's surface could contain solid, liquid and muddy material creating features such as lakes, seas, or rivers. Additionally liquid reservoirs may exist beneath the surface forming geysers or volcanoes that feed flowing liquid onto the surface.
Titan's peak surface temperature is about 95 Kelvins, too cold for liquid water, and due to its thick atmosphere, the pressure at the surface is 1.6 times greater than Earth's atmosphere. At this temperature and pressure, chemicals such as methane, ethane, propane, ammonia, water-ice and acetylene may be involved in complex interior-surface-atmosphere chemical cycles resulting in eruptions, condensation and precipitation (or rain). Initial observations obtained by Cassini during the first two passes of Titan provided our first close up views of Titan in wavelengths ranging from visible light to infrared to radar. The Huygens probe successfully returned atmosphereic data and images of the surface, providing groud truth for the Cassini Orbiter measurements. The results show a mysterious world even more complex than previously thought. The and the diversity of surface composition and its connection to Titan's geologic features remains a fundamental question. Huygens results indicate the methane esits as a liquid just below the surface and may rain from the atmosphere periodically. Clouds in Titan's atmosphere were observed in the southern hemisphere, yet no clear explanation has emerged on what the clouds are composed of, or why more clouds do not exist. Observations of Titan's interaction with Saturn's magnetosphere indicates the presence of complex processes complicated by Titan's occasional emergence out of Saturn's magnetosphere into the solar wind.
Observations by the Cassini orbiter during T3 will provide critical information to our understanding of Titan. New infrared and visible measurements will help to understand the relationship between features seen in the visible and radar wavelength images and surface composition. RADAR Synthetic Aperature Imaging will provide detailed swaths across Titan's surface features and imaging from the ISS and VIMS instruments will provide joint imaging of RADAR observations. ISS and UVIS will also examine particle distributions in the atmosphere as well as cloud formation and atmospheric variations and together, these new observations will continue the detailed investigation of the variability on Titan. Are there volcanoes or geysers? Is there evidence for flowing liquids on the surface? What is the extent of weather on Titan? How often does methane fall as rain? How does Titan's upper atmosphere react to the variability of Saturn's magnetosphere? The variations of Titan's surface, atmosphere and magnetosphere are clues to the active processes dictating its evolution and the origin of its intriguing complexity.
TITAN-3 SCIENCE ACTIVITIES
The Cassini/Huygens project is interested in four broad science themes concerning Titan: its interior stucture, surface characteristics, atmospheric properties, and interaction with Saturn's magnetosphere.
At an altitude of 1577 km, Titan-3 will not provide direct measurements of the density of Titan's atmosphere.
CAPS will make measurements of Titan's upper ionosphere and gather science from Cassini's crossing through Titan's plasma wake. They will make both ion and electron measurements during the flyby. CAPS will also closely examine the interaction between Titan and Saturn's Magnetosphere. CIRS will perform two 4-hr limb integrations using their mid-IR detectors to search for new molecules in the stratosphere. They will also continue their campaign of far-IR integrations (begun on T0) to search for species at longer wavelengths, and obtain a thermal map of the stratosphere, lending insight into the dynamics of Titan's atmosphere.
ISS plans to do a full-disk mosaic, during T03, that covers much of the leading and anti-saturnian hemispheres (including Xanadu) at multiple wavelengths to observe the surface and atmosphere, a 5x5 mosaic that covers western Xanadu and the dark area to the west, and two footprints designed to coincide with points along the T03 RADAR altimetry and SAR observations. It will be the first joint coverage of the same site by ISS, and VIMS, who will be riding along, and RADAR-altimetry/SAR. ISS also has a number of ride-along observations, including outbound observations of Titan's night side.
RPWS will study the interaction of the magnetosphere with Titan at intermediate distances for evidence of ion pickup, radio emissions, density profiles, and the general wave environment. Given the approximate similarity of the T3 flyby geometry to that of Ta and Tb, RPWS will concentrate on variations between the three flybys. Already, a significant difference has been observed in the inbound vs. outbound asysmmetry of the plasma density when comparing the Ta and Tb results. RPWS will use a somewhat modified instrument configuration to concentrate on lower-frequency plasma waves associated with the magnetosphere-Titan interaction and to fine-tune the Langmuir Probe thermal plasma measurements.
RADAR will control spacecraft attitude for the two hours around closest approach, beginning 046T06:19. During that time, RADAR will perform SAR Imaging including the first real ´bright terrain´, which includes part of Xanadu to be the subject of joint ORS/RADAR investigation. RADAR Altimetry measurements taken during T3 will establish whether the lack of topography identified during the Titan-A flyby was atypical.
MAG will investigate the large-scale and distant aspects of the Titan interaction by observing during the entire period around an encounter from 10 to 25 RS. (03TI (T3)). T3 will complete the triad of close flybys at almost the same Saturnian local time just before local noon in the magnetospheric wake region of Titan. This will provide a data set Ta,Tb,T3 to describe and understand the formation of Titan's induced magnetic tail in three dimensions.
MIMI will investigate micro-scale and near aspects of the Titan interaction by observing during ~one hour period around the encounter. With -Y pointed toward Titan, within 30 minutes of the targeted flyby, the secondary axis is optimized for corotation flow as close to the S/C -X, +/- Z plane as works with the other constraints on pointing. Also, Titan's exosphere/magnetosphere interaction is measured by imaging in ENA with INCA.
UVIS will perform several slow scans across Titan's visible hemisphere to form spectral images of Titan's upper atmosphere.
VIMS will image the surface of Titan at small solar phase angles, and investigate the formation and evolution of clouds on Titan. They will also search for lightning and hot spots and will attempt airglow characterization.
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