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Following a nearly three-year hiatus since its last encounter with Enceladus in 2005, the Cassini spacecraft will make its fourth and closest-yet flyby of this icy world on March 12, 2008 at 19:06:12 UTC. Previously, on Revs. 3, 4, and 11, Cassini flew within 1,267 km, 500 km, and 168 km, respectively, of Enceladus’ surface. The closest approach distance this time will be only about 50 km over latitude 21 S, longitude 135 W. However, the spacecraft will be moving too quickly (14.4 km/sec) to acquire images from that remarkably close vantage point. Thirty seconds after closest approach, the spacecraft will fly through Enceladus’ south-polar plume at an altitude of 200 km. Following the plume passage, and three minutes after closest approach, Enceladus slips into Saturn’s shadow at 19:09:06 UTC where it will remain in darkness during an eclipse which lasts just over two hours. The detailed schedule of events for the flyby is described below. The planned imaging coverage is shown on the map at left, and here.
* Figures in the images seen here represent the ISS narrow-angle camera (NAC) field of view, unless otherwise noted.
Flyby encounter observations of Enceladus begin on March 12 at 3:31:12 UTC, 15 hours prior to closest approach, with a 9-hour-long observation by the optical remote sensing (ORS) instruments. Cassini's Visual and Infrared Mapping Spectrometer (VIMS) will be “prime” here, meaning that the spacecraft pointing will be optimized to meet the scientific objectives of that instrument. The other ORS instruments (Imaging Science Subsystem [ISS], Ultraviolet Imaging Spectrograph [UVIS], and Composite Infrared Spectrometer [CIRS]) are all “boresighted,” or aligned, so that even though they do not control spacecraft pointing at this time, they will for the most part be pointed at Enceladus and making measurements of its surface simultaneously as “ride along” observations. At the beginning of this long observation, Enceladus is still over 600,000 km away, and Cassini is directly above latitude 54 deg N, longitude 61 deg W. The long observation affords a continuous stare at the northern portions of Enceladus’ anti-Saturn and trailing hemispheres while the satellite traverses more than one quarter of its 33-hour-long orbit around Saturn. The VIMS instrument will be obtaining spectra of these northern terrains at phase angles ranging from 111 to 124 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, just as water ice is dark in these spectral regions. Therefore, the duration of this observation is particularly useful for VIMS, which will use this 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 other 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 crystalinity, temperature, and size of particles on Enceladus’ surface. ISS will acquire a total of eight images with its narrow-angle camera (NAC) in the clear (CL1 CL2), ultraviolet (CL1 UV3), green (CL1 GRN), and near-infrared (CL1 IR3) filters, four at the beginning and four at the end of this extended observation period. The resolution of the first four will be 3.7 km/pixel at a phase angle of 113 degrees. By the end of this 9-hour-long period, the resolution will have increased to 2 km/pixel and the phase angle will be 122 degrees. At 12:26:12 UTC, still more than six hours prior to closest approach, ISS will be prime for a 2-hour-long observation in which NAC GRN and UV3 images will be obtained at polarization angles of 0, 60, and 120 degrees. Eight more images will be acquired: two each with the clear, UV3, GRN, and IR3 filters. During this two-hour-long period, the resolution in the NAC will improve from 1.8 km/pixel to 1.3 km/pixel. [The other three ORS instruments, CIRS, UVIS, and VIMS, all plan to ride along here as well. In the graphic left, the NAC field of view has been offset slightly in order to place the center of the UVIS slit on the center of Enceladus.] The phase angle remains 122 degrees.
At 14:36:12 UTC, 4.5 hours prior to closest approach, the spacecraft will turn so that RADAR can take over as the prime instrument for a 2-hour-long observation in which simultaneous scatterometry and radiometry are performed. RADAR will measure the albedo of Enceladus while Cassini is directly above latitude 68 N, longitude 105 W--far from the region viewed during previous tracks on Revs. 3 ( 0 N, 0 W), 4 (13 S, 70 W), 28 (61 N, 186 W), and 32 (29 S, 243 W). This will help to constrain the distribution of radar albedo across Enceladus, which is due to near-surface compositional and/or structural variations. The ORS instruments are not pointed at Enceladus at this time, so there will be no “ride along” observations here. At 16:36:12 UTC, 2.5 hours before closest approach, Cassini will turn back to the orientation which points the ORS instruments at Enceladus. CIRS will be the prime instrument for a 37-minute-long observation in which it will scan the northern hemisphere of Enceladus with its mid-infrared focal planes (FP3 and FP4) to measure the temperature of the surface and search for any possible hot spots. Since these focal planes are sensitive to light at infrared wavelengths between 7 and 17 microns (far beyond the range in which the human eye can see), the fact that more than half of Enceladus is in darkness (at a phase angle of 116 deg) does not matter. CIRS will be able to "see" any warm regions near the north pole of Enceladus even if they are dark at visible wavelengths. ISS, UVIS, and VIMS are all riding along on this observation. ISS will obtain NAC clear filter, UV3, GRN, and IR3 “Voyager class” (resolution ~640 m/pixel) images of the cratered terrains on the trailing side of the north pole which will be useful for limb topography and satellite shape measurements. At 17:13:12 UTC, just under two hours prior to closest approach, VIMS takes over as the prime instrument for a 27-minute-long observation. Once again, ISS, CIRS, and UVIS will ride along. Enceladus now fills the field of view of the ISS NAC, which will acquire 11-multi-spectral and polarization images of the northern cratered terrains as the resolution continues to increase, now to 540 m/pixel. The phase angle is 115 degrees.
At 17:40:12 UTC, about 1.5 hours before closest approach, it’s UVIS’ turn to be prime, with ISS, CIRS, and VIMS riding along on this 27-minute-long observation. UVIS will scan across the disk of Enceladus to measure the ultraviolet albedo of the surface and look for oxygen in the environment around Enceladus which may have dissociated from water molecules in the plumes. ISS will acquire four NAC images: one each in the clear, UV3, GRN, and IR3 filters; and one WAC clear filter image to study limb topography and analyze the moon's shape. The resolution in the ISS NAC is 400 m/pixel and the phase angle is still 115 deg.
At 18:07:12 UTC, one hour before closest approach, CIRS takes over as the prime instrument once again, this time using its far-infrared focal plane, FP1. There is an offset between the CIRS FP1 field of view and that of the ISS NAC, so UVIS and VIMS will be the only riders here. Again, CIRS will be characterizing the thermal emission from the surface of Enceladus’ northern hemisphere while the moon is at a distance of 50,000 km and a phase angle of 115 degrees, and Cassini closes in on Enceladus at a speed of 14 km/sec. Following CIRS, ISS will be prime at 18:25:12 UTC, 41 minutes before closest approach, when it will make a three-panel clear filter NAC mosaic at the highest resolution of this encounter, 200 m/pixel. One clear filter WAC image will also be obtained. CIRS, UVIS, and VIMS will also ride along, and toward the end of this 20-minute-long observation period, Cassini's Plasma Spectrometer (CAPS) and Magnetospheric Imaging Instrument (MIMI) will begin making measurements as closest approach is imminent.
At 18:44:42 UTC, Cassini will commence a 19-minute-long turn so that CAPS can be the prime instrument for the remainder of a 26-minute-long observation. Closest approach occurs at 19:06:12, five minutes prior to the end of the CAPS observation. At this time Cassini will be 52 km directly above latitude 21 S longitude 135 W. Moving at 14.4 km/second, the spacecraft will enter Enceladus’ plumes 30 seconds later, 200 km above the surface, directly above latitude 75 S, longitude 136 W. The Ion and Neutral Mass Spectrometer (INMS), also observing at closest approach and during the plume passage, will make significant improvements in determining the composition of the gas around Enceladus over what was obtained during Cassini’s second close flyby in 2005. Cassini's Cosmic Dust Analyzer (CDA) and INMS can separate the plume and global sources of dust around Enceladus. Cassini Radio and Plasma Wave Science (RPWS) will measure plasma waves associated with the interaction between Saturn’s magnetosphere and Enceladus. In addition, RPWS will measure the flux of micron-sized particles in the vicinity of Enceladus and within its plumes. Cassini's Magnetometer (MAG) will attempt to determine whether Enceladus generates an induced magnetic field from a subsurface ocean. Three minutes after closest approach at 19:06:12 UTC, Enceladus goes into eclipse and the spacecraft begins a turn to set up observations of the south polar terrain. CIRS is the prime instrument at 19:11:12; however, Cassini does not complete its turn until 19:21:38. At that time, CIRS will conduct detailed mapping of the warm "tiger stripe" fractures with FP3 at the south pole on departure, in Saturn eclipse, providing the first contiguous regional temperature map with spatial resolution sufficient to resolve the tiger stripes and locate all the major hot spots. CIRS will also conduct long infrared wavelength mapping of the south polar region with FP1 to constrain total heat flow. Much of the heat radiation may be emitted at long wavelengths at which south polar emission was not mapped on the previous Enceladus flybys. ISS will be riding along at this time and will obtain multi-spectral WAC and NAC images in an attempt to detect any non-thermal emission which may be generated by the plumes. The resolution in the ISS NAC will be as high as 200 m/pixel, though surface features will likely not be visible while the satellite is in eclipse. UVIS will ride along and search for airglow. VIMS will also ride along on this 59-minute-long observation, during which the phase angle will be 67 degrees.
At 20:10:12 UTC, an hour after closest approach, Cassini turns once again for a 44-minute-long observation in which RADAR will be the prime instrument while Enceladus is still in eclipse. Cassini will be directly above latitude 69 S, longitude 333W, also far from previous tracks along which Enceladus’ radar albedo was measured, further contributing to an understanding of the variation in near-surface composition and/or structure.
At 20:51:12 UTC, nearly two hours after closest approach, Cassini begins a 17-minute-long turn after which CIRS will be prime instrument for the next 50 minutes, as Enceladus emerges from Saturn's shadow. CIRS will observe the warming of the surface with the far-infrared focal plane (FP1) in order to understand its physical properties. ISS will ride along and obtain multi-spectral WAC and NAC images of the sunlit surface after the eclipse ends. The phase angle at this time is 64 degrees and the resolution is 780 m/pixel. UVIS will be the prime instrument for the final ORS observation of the flyby beginning at 22:01:12 UTC, almost two hours after closest approach. For just over 2 hours, UVIS will measure differences in the ultraviolet albedo of the surface at high resolution 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 18 multi-spectral and polarimetric NAC images with resolutions around 860 m/pixel, at a phase angle of 64 degrees, and centered at 68 S, 350 W.
As Cassini turns its high-gain antenna to Earth to play back the treasure trove of data stored on its solid-state recorder from this fourth and final Enceladus flyby of the primary mission, the Radio Science Subsystem (RSS) will make gravity measurements of the icy moon using its Ka-band transmitter. In addition to collecting the data downlink, the Deep Space Network (DSN) will measure the Doppler shift of Cassini’s signal as the spacecraft decelerates in response to Enceladus’ gravitational pull. This information can be used to characterize Enceladus’ gravity field.
Cassini's next close flyby of Enceladus occurs only five months later, in Rev. 80 on August 11, and follows a trajectory very similar to the one described here, in which the spacecraft approaches from the north, passes about 50 km above the surface, and recedes with a view of the eclipsed and sunlit south polar terrain. The Rev. 80 flyby includes an even deeper plume passage than here in Rev. 61, and still deeper plume passages are planned for flybys later during the extended mission, in Revs. 120 and 130.