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SKEET SHOOTING ENCELADUS
On August 11, with the Games of the XXIX Olympiad underway in Beijing, Cassini will be going for the gold in Skeet as it executes an ambitious plan to 'skeet shoot' the most fascinating locations on Enceladus at closest approach during the first of seven close flybys planned for the Cassini Equinox Mission. During the previous flyby of Enceladus on March 12, the Imaging Science Subsystem (ISS) and the rest of the optical remote sensing (ORS) instruments were pointed away from Enceladus at closest approach so the Ion and Neutral Mass Spectrometer (INMS) could determine the composition of the icy plumes emanating from the moon's south pole (see the flyby preview and follow-up science press release). This time, however, the ORS instruments will take center stage: the eyes of Cassini will be focused on the south polar terrain immediately following closest approach, at an altitude of 50 km (31 miles) above the surface.
Cassini will be flying over the surface of Enceladus during this flyby at enormous speed - a bit under 18 km/sec (about 40,000 mph) - making image-taking very difficult. The challenge is equivalent to trying to capture a sharp, unsmeared picture of a roadside billboard about a mile away with a 2,000 mm telephoto lens held out the window of a car moving at 50 mph. To solve this problem, imaging scientists have devised a technique which, if all goes well, will yield some of the most highly-anticipated images of the entire mission: high-resolution views of active vent regions on the 'tiger stripes' (formally known as 'sulci'), which have been identified as both sources of the jets by the imaging cameras (ISS) and hot spots identified by the Composite Infrared Spectrograph (CIRS). In order to acquire images during its rapid flight, Cassini will be tracking a point in space while it waits for Enceladus to move into the field of view of the ISS (and the rest of the ORS instruments). Once the south polar region is in the line of fire, the cameras will pull the trigger in rapid succession, shooting seven, very high priority surface targets: a suite of images ranging in resolution from 8 to 28 meters/pixel, using exposure times that are long enough to see the surface in the twilight near the terminator yet short enough to avoid smear.
As in the March flyby, Cassini will follow a trajectory that approaches Enceladus from the north, staring at latitude 63 N at a phase angle near 110 degrees. As the spacecraft closes in on Enceladus, it will pass over the equator while the surface below is in darkness. After closest approach, at an altitude of 50 km above latitude 30 S, longitude 98 W, Cassini recedes from the moon with a view of the South Pole, staring directly at latitude 62 S. Also as in March, shortly after closest approach, Enceladus slips into Saturn's shadow and remains in eclipse for almost 2.5 hours. The detailed schedule of flyby activities is described below.
Flyby encounter observations of Enceladus begin on August 11 at 12:06:19 UTC, nine hours prior to closest approach. The first action is a 24-minute-long turn to point Cassini's X-band antenna at Earth so the Radio Science Subsystem (RSS) can then make a 3.5-hour-long measurement of Enceladus' gravity field in an effort to detect a diapir beneath the surface. RSS will make another such 'gravity pass' following closest approach on this flyby. Following the gravity pass and downlink, Cassini begins a 25-minute turn which will aim the ORS instruments at Enceladus' northern hemisphere.
Five hours prior to closest approach (at 16:06:19 UTC), the Visual and Infrared Mapping Spectrometer (VIMS) will become "prime" for the next 3.5 hours, meaning that spacecraft pointing will be optimized to meet the scientific objectives of that instrument. The ORS instruments (ISS, VIMS, Ultraviolet Imaging Spectrograph [UVIS], and CIRS) are all aligned, so even though VIMS controls spacecraft pointing at this time, for the most part the others will be pointed at Enceladus and making measurements simultaneously as "ride along" observations. At the beginning of this observation, Enceladus is over 280,000 km away, and Cassini is directly above latitude 63 N, longitude 49 W, staring at northern portions of the trailing hemisphere. VIMS will be obtaining spectra of these northern terrains at phase angles ranging from 110 to 107 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 3.5-hour-long observation, when Enceladus nearly fills the entire field of view of the narrow-angle camera (NAC), ISS will acquire a total of 18 images in the clear (CL1 CL2), ultraviolet (UV3), green (GRN), and near-infrared (IR1, IR3) filters and at polarization angles of 0, 60, and 120 degrees in the ultraviolet (UV3), green (GRN), and methane (MT2) filters. The final NAC 'Voyager class' clear filter image (600 meters/pixel) of the cratered terrains on the trailing side of the north pole will be useful for limb topography and satellite shape measurements. At 19:36:19 UTC, only 1.5 hours prior to closest approach, UVIS will become prime for 50 minutes, during which it will scan across Enceladus, starting on the sky 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 its ultraviolet albedo and ISS will obtain six images, the first two of which are a clear filter (CL1/CL2) "BOTSIM," in which the NAC and WAC (Wide-Angle Camera) BOTh obtain SIMultaneous exposures. The following four NAC images are in the UV3, GRN, IR3, and clear filters. The resolution in the NAC will be 325 meters/pixel and the phase angle is 107 degrees. The other ORS instruments, CIRS and VIMS plan to ride along here as well. Beginning at 20:07:00 UTC, Cassini's Cosmic Dust Analyzer (CDA) will ride along for two hours, characterizing properties of E-ring particles as the spacecraft crosses the ring plane. At 20:26:19 UTC, 40 minutes before closest approach, ISS will be prime and will obtain two two-tile mosaics of the northern cratered plains at a resolution of 250 meters/pixel and a phase angle of 107 degrees in the NAC clear filters, and 500 meters/pixel in the UV3, GRN, IR1, and IR3 filters. This mosaic, combined with a similar one obtained during the March flyby, will provide stereoscopic views that can be combined to produce new 3-D digital terrain models of heavily cratered terrain near the North Pole. Next, Cassini will turn to a staging attitude to set up the 'skeet shoot' imaging run. Beginning at 20:36:25 UTC, the Cassini Plasma Spectrometer (CAPS) will ride along for one hour during which it will observe the interaction between Enceladus and Saturn's magnetosphere. During the initial phase of the high-resolution imaging sequence (the skeet shoot), the spacecraft will be so close to Enceladus and moving so quickly that it is not physically possible for it to target and track with stability any specific geologic feature on the moon's surface. A special spacecraft maneuver implemented for this flyby first points Cassini's ORS platform ahead of Enceladus while the spacecraft is spun at its fastest speed in the direction that Enceladus moves across the sky. When Enceladus' apparent motion eventually overtakes the spacecraft spin and passes in front of the ISS cameras, the relative motion of the camera boresight across the surface of Enceladus will be briefly matched well enough so that the ISS NAC will be able to obtain seven very high resolution snapshots of selected geological features. The ground track of the camera's pointing has been selected so that it will cut a swath across three tiger stripes, or sulci -- the prominent rifts through which jets of water vapor and ice particles are known to be erupting. The swath was chosen to pass over three particular segments of the tiger stripes that are known to be local hot spots and which are sites of previously observed eruptions. It begins as the spacecraft view passes across the terminator from the night side of Enceladus to the illuminated dayside. Closest approach occurs at 21:06:19 UTC.
At 21:07:19 UTC, one minute after closest approach, ISS will acquire the first of the skeet-shoot targets, a WAC image centered on a point just over the night side of the terminator at a resolution of 52 meters/pixel (and not shown on the graphic above). The first fully daylit footprint, with a spatial resolution of 8 meters/pixel in the NAC, will be placed 33 seconds later next to the tiger stripe known as Cairo Sulcus. The subsequent two footprints (with spatial resolutions of 11 and 14 meters/pixel, respectively) will be placed directly on Cairo Sulcus -- the first of which will be adjacent to a hot spot identified by CIRS, and the next directly on the hot spot. As the ground track continues to traverse the surface, ISS will obtain a 17 meter/pixel image of the bright, grooved terrain that lies between Cairo Sulcus and the neighboring tiger stripe, Baghdad Sulcus. Next, ISS will acquire an image of a known eruption site on Baghdad Sulcus at 21 meters/pixel. The following image, at 24 meters/pixel, will be of the bright icy terrain that lies between Baghdad Sulcus and its neighbor, Damascus Sulcus. Finally, at about four minutes 50 seconds after closest approach, the ground track will converge on a section of Damascus Sulcus that lies between two known eruption sites and corresponding hot spots. ISS has placed the seventh footprint off the ground track so that CIRS will be able to measure temperatures along the rift floor of Damascus while the ISS NAC obtains 28 meter/pixel resolution of the same area. With the skeet-shoot sequence complete, Cassini will once again track Enceladus and acquire an eight-tile mosaic of the south polar region in the CLR, UV3, GRN, IR1 and IR3 filters. Dwell times at each tile will be at least two minutes in order to let VIMS, riding along on this observation, reach sufficient signal-to-noise. VIMS will be doing compositional mapping at the highest spatial resolution, searching for trace compounds in the ice to determine what drives the plumes. These compounds include substances like ammonia and carbon dioxide, and hydrogen compounds such as hydrogen peroxide. VIMS will also (i) search for other phases of ice that may be stable on Enceladus, such as amorphous ice, (ii) conduct ice grain-size mapping (to determine the thermal history of the surface) and phase angle studies to map microphysical properties, (iii) determine the bolometric Bond albedo (the fraction of total incident radiant energy that is reflected) and (iv) search for hot spots. (VIMS does very well at detecting temperatures above 150K and as temperatures rise above that level, VIMS can detect small sub-pixel sources all the way up to and above 273 K.) UVIS will also ride along here and look for differences in ultraviolet albedo at high spatial resolution that can be used to characterize water ice grain sizes near and far from the tiger stripes. A systematic change in the grain size may be correlated with the surface age and types of processes the surface has experienced. At 21:34:49 UTC, 28.5 minutes after closest approach, CIRS will be the prime instrument for the next 3.5 hours, 2.5 hours of which Enceladus will be in eclipse beginning at 21:41:26 UTC. Because it is sensitive to light at mid- and far-infrared wavelengths between 9 and 1000 microns, CIRS is ideally configured to measure the temperature of the "warm" surface of Enceladus. Any solid object with a temperature above absolute zero glows, even the frigid surface of Enceladus. At temperatures as low as 60 K, the Enceladus surface can be 'seen' by CIRS even while the moon is in eclipse. This CIRS observation is comprised of four parts:
1. A 15-minute, fast global raster scan using its far-infrared focal plane FP1. About halfway through this first part, Enceladus goes into eclipse, slipping into Saturn's shadow at 21:41:26. Full-disk mapping with FP1 will help constrain total heat flow from the interior.
2. A 1.75-hour, slow global scan using its mid-infrared focal plane FP3. These scans will be compared with those obtained at high spatial resolution during the March flyby when CIRS obtained the first contiguous regional temperature map, which resolved the tiger stripes and identified several hot spots. These full-disk FP3 maps will also look for time variability in the south polar thermal emission and search for additional hot spots.
3. A 20-minute stare for ISS and VIMS.
4. A 52-minute stare with FP1 to observe the warming of Enceladus as it emerges from Saturn's shadow on August 12 00:07:34 UTC to understand surface physical properties. ISS, UVIS, and VIMS will all ride along on this observation. UVIS will search for airglow, and ISS 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 NAC will be as high as 700 meters/pixel and the phase angle will be 73 degrees. Despite the fact that Enceladus is in eclipse, its surface is readily visible (as in this eclipse image from the March '08 flyby) by using long exposures illuminated by a variety of light sources including sunlight refracted through Saturn's upper atmosphere and ringshine reflected onto the planet and back at Enceladus (this view shows some of these light sources which are visible during an eclipse by Saturn). In March, Dione and Rhea were also sources of ambient light while Enceladus was in eclipse; this time, unlike March, Titan-shine will be a factor. At 01:06:19 UTC, four hours after closest approach, VIMS will be the prime instrument for an hour-long observation on which ISS, UVIS, and CIRS will all ride along. The ISS NAC resolution will be 1.4 km/pixel; the phase angle is 73 degrees. Cassini's ORS instruments are pointed at latitude 62 S, longitude 322 W. ISS will obtain multi-spectral NAC images as well as polarization in the UV3 and GRN filters.
At 02:06:19 UTC, five hours after closest approach, Cassini begins a 25-minute turn to aim its X-band antenna at Earth. Beginning at 02:36:19 UTC and continuing for the next five hours, Cassini will play back the data stored on its solid-state recorder from this flyby. RSS will again make gravity measurements of the icy moon using its Ka-band transmitter. In addition to collecting Cassini's data downlink, the Earth-based 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 determine Enceladus' gravity field. Once the RSS gravity pass and downlink are complete, Cassini will begin the 24-minute turn to point the ORS instruments back at Enceladus. UVIS will be the prime instrument for the final ORS observation of the flyby beginning at 13:06:19 UTC, eleven hours after closest approach. For nearly 5.5 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 multispectral and polarized 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 only two months later, in Rev. 88, on October 10. That encounter follows a trajectory very similar to the one described here, in which the spacecraft approaches from the north, passes 25 km above the surface, and recedes with a view of the eclipsed and sunlit south polar terrain. The Rev. 88 flyby includes an even deeper plume passage than this one--and still deeper plume passages are planned for flybys later during the Equinox Mission, in Revs. 120 and 130.
Learn more about the Cassini Imaging Cameras in this PDF document, published by the Imaging Team in the journal Space Science Reviews.
The green squares on the graphics presented here represent the narrow-angle camera field of view. On several of the graphics, the moon's opposite pole is visible through the 3-D model, where lines of the longitude are seen to converge.