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On October 9th, during its third and penultimate Enceladus encounter in 2008, Cassini will fly closer to the moon's surface than ever before and deeper into the icy plumes emanating from the south pole. Unlike the previous close flyby, on August 11, when ISS and the rest of the optical remote sensing (ORS) instruments were aimed at the surface during closest approach, this time Cassini will be oriented so that the fields and particles instruments can make direct measurements of the environment surrounding the spacecraft and thus analyze samples of plume particles. Cassini will follow essentially the same trajectory as in both the March and August flybys, approaching from the north, skimming past the nightside of Enceladus, and receding with a view of the south polar terrain while the moon slips into Saturn's shadow for about 2.5 hours. Closest approach occurs at 19:06:40 UTC on October 9, 2008, approximately 25 km (15.6 miles) above latitude 30 S, longitude 98 W. Analyses of previous flybys have shown that the spacecraft does indeed “twist in the wind” as it flies through the plumes, so for this flyby Cassini will use its thrusters, rather than its reaction wheels, to maintain attitude control when the spacecraft plunges deeper into the plumes than it ever has before. Cassini routinely conducts close flybys of Titan on thrusters because the upper atmosphere of Saturn's largest moon exerts similar torques on the spacecraft. The detailed schedule of flyby activities is described below.
Flyby encounter observations of Enceladus begin on October 9 at 10:30:40 UTC, 8.5 hours prior to closest approach with a three-hour-long RADAR observation, during which the instrument will obtain simultaneous scatterometry and radiometry. Cassini will measure the albedo, or reflectivity, of Enceladus at radio wavelengths and compare this value to the moon's unusually high visible albedo.
After a twenty-three-minute-long turn, at 14:29:40 UTC, Cassini's ORS instruments will be aimed at Enceladus and the Composite and Infrared Spectrometer (CIRS) will be "prime" for the next three hours , meaning that spacecraft pointing will be optimized to meet the scientific objectives of that instrument. The other ORS instruments (ISS, UVIS, and VIMS) are all aligned so even though they do not control spacecraft pointing at this time, for the most part they will be pointed at Enceladus and making measurements simultaneously as "ride along" observations. At the beginning of this observation, Enceladus is about 265,000 km away, and Cassini is directly above latitude 63 N, longitude 52 W, staring at northern portions of the sunlit trailing hemisphere. VIMS will be obtaining spectra of these northern terrains at a phase angle of 109 degrees. Although Enceladus is the most reflective body in the Solar System at visible wavelengths, its nearly pure water-ice-covered surface is quite dark between 2.5 and 5 microns. Therefore, the duration of this observation is particularly useful for VIMS, which will use the extended period to acquire long exposures (and thus high "signal-to-noise") in these dark regions of Enceladus' near-infrared spectrum where non-water-ice surface components have absorption features and may be detected. During previous flybys, VIMS detected carbon dioxide, hydrogen peroxide, and light organics near the south pole. This observation will characterize the surface composition of the northern, cratered terrains. In addition VIMS can also measure the crystallinity, temperature, and size of particles on Enceladus' surface. At the end of this three-hour-long observation, when Enceladus nearly fills the entire field of view of the narrow angle camera (NAC), ISS will acquire a total of 13 images in the clear (CL1/CL2), ultraviolet (UV3), green (GRN), and near-infrared (IR3) filters and at polarization angles of 0, 60, and 120 degrees in the ultraviolet (UV3), green (GRN), and methane (MT2) filters. The NAC “Voyager class” clear filter image (900 meters/pixel) of the cratered terrains on the trailing side of the north pole will be useful for limb topography and satellite shape measurements and can also be used with similar views acquired during previous flybys to create stereo images and digital terrain maps.
At 17:06:40 UTC, 2 hours prior to closest approach, UVIS will be prime for the following hour, during which it will scan across Enceladus, starting well beyond its sunlit limb. These observations of the space around Enceladus will search for the signature of oxygen, a product of the dissociation of the water molecules coming from Enceladus' plume. Once the scan brings the sunlit surface of Enceladus into view, UVIS will measure the ultraviolet albedo of the surface and ISS will obtain five images, the last two of which are a clear (CL1/CL2) filter 'BOTSIM,' or BOTh SIMultaneous, observation in which the NAC and WAC (Wide Angle Camera) take exposures at the same time. The preceding three NAC images are in the GRN, RED, and clear filters. The resolution in the NAC will be about 400 meters/pixel and the phase angle is 107 degrees. VIMS will ride along here as well.
Following the UVIS observation, at 18:06:40 UTC, one hour before closest approach, spacecraft attitude control will switch from reaction wheels to thrusters. The transition takes only one minute, and at 18:07:40 UTC the Ion and Neutral Mass Spectrometer (INMS) will be the prime instrument for the next hour and 14 minutes, which includes closest approach at 19:06:40 UTC. Cassini's Cosmic Dust Analyzer (CDA) and Plasma Spectrometer (CAPS) will also be riding along as the spacecraft travels through a denser region of the plume than it has previously dared to pass through. At the time of closest approach Cassini will be 25 km directly above latitude 30 S longitude 97 W. Moving at 17.7 km/second, the spacecraft will enter Enceladus' plumes 30 seconds later, 200 km above the surface, directly above latitude 75 S, longitude 136 W. INMS expects to obtain its best results to date of its measurements of the composition of the plumes. Although hydrazine from the thruster firings may contaminate some of the measurements, INMS does not anticipate that it will preclude their acquisition of high signal-to-noise data for two reasons: 1) the turn from ORS to INMS pointing occurs well before entry into the plume and 2) INMS routinely sees an increase in the products of the hydrazine burn during close Titan flybys on thrusters. CDA will distinguish between the composition, number density, and size distribution of surface ejecta and plume particles, as well as identify individual particle jets. Cassini Radio and Plasma Wave Science (RPWS) will measure plasma waves associated with the interaction between Saturn's magnetosphere and Enceladus. Specifically, RPWS will look for evidence of local ionization in the plumes and measure the flux and size distribution of dust in the plumes. Cassini's Magnetometer (MAG) will compare its measurements with those obtained during the March and August flybys in an effort to understand temporal variations in plume activity. At 19:21:40 UTC, 15 minutes after closest approach, ISS will be the prime instrument for the next 28 minutes. Had Cassini maintained attitude using its reaction wheels rather than thrusters for this flyby, most of those 28 minutes allocated to ISS for their prime observation would have been spent turning the spacecraft to point the ORS instruments at Enceladus' surface, leaving sufficient time to acquire perhaps only one image. However, turning the spacecraft using thrusters takes only 7 minutes, leaving 21 minutes for imaging at resolutions as high as 140 meters/pixel. ISS will aim the Narrow Angle Camera (NAC) so that CIRS mid-infrared focal plane FP4 instrument is centered on a known local hot spot and site of previously observed eruptions (labeled “III” on the accompanying graphics) in the tiger stripe known as Damascus Sulcus for about 4 minutes, then move slightly so that CIRS other mid-infrared focal plane FP3 instrument is centered on the same spot. ISS will then follow these observations with a 5-image mosaic, dwelling at each position for at least 2 minutes so that VIMS can integrate long enough to get high signal-to-noise. At 19:49:40 UTC, 43 minutes after closest approach, CIRS will be the prime instrument for the next 46 minutes. Enceladus goes into eclipse about 2 minutes later at 19:51:51 UTC. CIRS will attempt to acquire data at “super resolution” by dithering its focal planes over Damascus and Baghdad sulci. Then, CIRS will scan the entire south polar terrain to fill in gaps in the spatial coverage acquired on previous flybys and to look for any temporal variability in thermal emission from the region. ISS, UVIS, and VIMS will all ride along on this observation.
At 20:35:40 UTC, about 1.5 hours after closest approach, Cassini will switch back to reaction wheel control after being on thrusters for the past 2.5 hours. All ORS instruments will be riding along during this transition, staring at latitude 88 S longitude 0 W for just over 21 minutes. Once Cassini is back under reaction wheel control, CIRS continues its prime observations at 20:57:40 UTC with a 30-minute stare at latitude 88 S, longitude 0 W using its mid-infrared focal plane FP3, sensitive to wavelengths between 9 and 17 microns. ISS will be riding along, obtaining observations of the south polar terrain while Enceladus is in eclipse, some of which support CIRS science objectives by providing valuable pointing information. At this time the resolution in the NAC is 700 m/pixel and the phase angle is 73 degrees. CIRS follows this stare with a global scan using FP3, then another stare with the far-infrared focal plane FP1, sensitive to wavelengths between 17 and 1000 microns, to constrain total heat flow from the region. As on previous flybys, in an effort to study the physical properties of Enceladus' surface, CIRS will observe the warming of the surface with FP1 as the moon egresses from Saturn's shadow. By the end of this three-hour-long observation, the resolution will have decreased to 1.4 km/pixel. ISS will obtain multi-spectral NAC images as well as polarization in the UV3 and GRN filters once the moon is no longer in eclipse.
UVIS will be the prime instrument for the final ORS observation of the flyby beginning at 00:06:40 UTC on October 10, five hours after closest approach. For the next two hours, UVIS will measure differences in the ultraviolet albedo of the surface in order to characterize the grain sizes of particles near the tiger stripes and those far from them. ISS, CIRS, and VIMS will ride along. ISS will obtain multi-spectral and polarimetric NAC images with resolution 2.4 km/pixel at a phase angle of 83 degrees, centered at 77 S, 20 W.
Cassini's next close flyby of Enceladus occurs on October 31 (only 22 days later, in Rev. 91) and follows a trajectory very similar to the one described here in which the spacecraft approaches from the north, passes 200 km above the surface, and recedes with a view of the eclipsed and sunlit South Polar Terrain. ISS and the rest of the ORS instruments will once again be “prime” at closest approach, and the flyby will see a reprise of the spectacularly successful “Skeet Shoot” maneuver used during the Rev 80 August flyby (images here; flyby description here).