From: Jet Propulsion Laboratory
Posted: Friday, October 22, 2004
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The first targeted flyby of Titan occurs on Tuesday, October 26, 2004 at 15:30 UTC (8:30 am Pacific time). Cassini's closest approach to Saturn's largest satellite is at an altitude of 1200 km (746 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 1.5 Titan radii. This encounter is set up with three maneuvers: the Periapsis Raise Maneuver, and Periapsis Raise Maneuver cleanups, both of which took place successfully on August 23 and September 7, 2004, respectively; and the Titan minus three day targeting maneuver, scheduled for October 23rd. Titan A is an inbound flyby, with Saturn periapsis occurring about two days afterwards, on October 28th.
During approach to Saturn, and since orbit insertion, the navigation team has engaged in near-daily optical navigation of Titan and Saturn's other satellites in order to refine their orbit estimates as much as possible. They expect to deliver the orbiter to within 30 km of the target altitude at a confidence of 99% (three sigma).
Titan A is Cassini's second targeted satellite encounter. The first was Phoebe, on June 11, at an altitude of 2,000 km.
1.1 ABOUT TITAN
Titan is a highly complex world and is closer to a terrestrial planet than a moon typical of the outer planetary systems. Titan was first seen by the dutch astronomer Christiaan Huygens (after whom our Titan probe is named) in 1655. Though Galileo was the first person ever to observe the disk of Saturn forty-five years earlier, Huygens' telescopes were more powerful. Huygens was also the first to identify the rings as a flat disc encircling the planet without touching it.
Not only is Titan the largest of Saturn's satellites, it is also larger than the planets Mercury and Pluto, and is the second largest satellite in the solar system (only eclipsed by Ganymede). It is the only satellite in the solar system with an appreciable atmosphere, composed mostly of Nitrogen, but also contains aerosols and hydrocarbons, including methane and ethane. Titan's atmosphere was first confirmed in 1944 when Gerard Kuiper confirmed the presence of gaseous methane with spectroscopy.
Titan's peak surface temperature is about 95°K, and surface pressure is 1.6 Earth atmospheres. At this temperature and pressure, many simple chemicals that are present in abundance (methane, ethane, water, ammonia) provide materials in solid, liquid and gaseous form which may interact to create exotic features on the surface. Precipitation, flowing liquids, lakes, eruptions are all possible.
Titan orbits Saturn at a distance of just over 20 Saturn radii (1,222,000 km / 759,000 miles) which is far enough to carry the moon in and out of Saturn's magnetosphere. Titan's orbital period is 16 days, and the orbit has a slight inclination of 0.33 degrees and eccentricity of 0.03. Like most of the major satellites of Saturn, and Earth's moon, Titan is tidally locked to the planet, with the same face pointed towards it at all times. Very little is known about Titan's interior structure, including whether it has its own magnetic field. Titan's surface has been difficult to study, as it is veiled by a dense hydrocarbon haze that forms in the dense stratosphere as methane is destroyed by sunlight. From the data collected so far, dark features can be seen crossing the equatorial region of Titan, with a large bright region near longitude 90 degrees now named "Xanadu", and possibly a large crater in the northern hemisphere
1.2 TITAN-A 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.
Titan A will provide the first in-situ sampling of Titan's atmosphere ever. This will contribute significantly to atmospheric model updates necessary to validate the 950 km minimum flyby altitude (and perhaps the Huygens mission profile as well). The sources of this improvement will come primarily from INMS data and AACS attitude control telemetry during the flyby.
CAPS will make its first measurements of Titan's upper ionosphere and gather science from Cassini's first crossing through Titan's plasma wake. They will make both ion and electron measurements during the flyby, except for the period from about closest approach –85 to –30 minutes.
CIRS will measure the stratospheric temperatures versus pressure (and therefore density), in part to contribute to Huygens mission validation at the altitudes of parachute deployment.
ISS will conduct its first medium and high resolution imaging of Titan, including imaging of the Huygens landing site. The cameras will perform distant observations at about 2.7 kilometers per pixel, a full-disk color mosaic at about 2 km/pixel, regional to global mapping of the western bright/dark boundary at 200-600 meters per pixel, and very high resolution imaging of an edge of a bright area at 23-81 meters per pixel. INMS, again, will perform the first ever in situ measurements of Titan's upper atmosphere, to determine the density and composition.
MAG will perform a detailed study of Titan's interaction with Saturn's magnetosphere during the entire flyby, as well as further constrain the possible internal magnetic field of Titan.
MIMI will examine Titan's exosphere with ENA imaging and characterize the ion composition and charge state near Titan.
RADAR will perform its first Synthetic Aperture Radar (SAR) imaging of Titan's surface, as well as scatterometry of the Huygens landing site. Scatterometry should provide roughness and solid/liquid discrimination, and radiometry should contribute to temperature mapping.
RPWS will take measurements while passing through Titan's ionosphere and contribute to the understanding of Titan's interaction with Saturn's magnetosphere. UVIS will perform two high resolution scans across Titan to investigate the composition and distribution of aerosols.
VIMS hopes to perform surface composition and fluid feature mapping (lakes, rivers), as well as see aerosol and cloud structures in the atmosphere, methane fluorescence and look for volcanic activity. They also contribute to mapping of the Huygens landing site at 1 km spatial resolution.
1.3 TITAN-A SEQUENCE OF EVENTS AND SAMPLE SNAPSHOTS
The Titan-A flyby does not require use of the live update capability; however, the encounter does occupy a ground movable block with 15 minutes of dead time on each end. Very little of this dead time is expected to be necessary, as even the latest trajectory (041001) lists a flyby shift (compared to 030201) of only 14 seconds.
Also, no special fault protection is planned for the Titan-A flyby. The nominal tour fault protection strategies should be sufficient to protect the spacecraft from any unexpected events.
1.4 TA DATA RECORDING AND PLAYBACK
The Titan-A data recording and playback strategy is the same as the nominal tour strategy, with one notable exception. INMS and AACS data collected at closest approach will be rerouted to partition 5 and saved until near the end of the downlink pass, so that it may be played back over two complexes. This was done to further ensure successful playback as these data sets contribute to the minimum Titan altitude and Huygens mission.
Goldstone's 70m station is down for nearly the entire latter half of 2004 for preventitive maintenance and upgrades, so the high priority science is played back over Madrid's 70m dish. Goldstone's 34m HEF station comes up near the end of the pass as the redundant station for INMS and AACS data playback.
A detailed time ordered description of the data playback is shown on the following pages. The SSR is nearly filled during the flyby with a total of 3.5 Gbit of data. Playback begins on DOY 301 at 00:16 (spacecraft time) and completes at 09:16. Dual playback to both Madrid and Goldstone takes place during the last hour of the downlink (08:16-09:16 SCET). One-way light time at the time of the encounter is 1 hour and 14 minutes.
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