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Cassini continues its extended tour of the Saturn system with the 17.5-day-long Rev127, the spacecraft's 128th orbit around the Ringed Planet. Cassini begins Rev127 on February 22 at its farthest distance from Saturn, called apoapse. At this point, Cassini is 2.37 million kilometers (1.47 million miles) from Saturn's cloud tops.
Cassini's ISS camera starts its observations for Rev127 the day after apoapse by taking images of Iapetus, 1.53 million kilometers (0.95 million miles) away. During this distant observation, ISS will observe the southern, sub-Saturn hemisphere of Iapetus. Over the next four days, ISS will take four more multi-spectral observations of Iapetus, with the distance to the satellite ranging from 1.55 to 1.92 million kilometers (0.97 to 1.12 million miles) away. On February 24, ISS will acquire a 3-hour observation of the small outer satellite, Skoll. This moon is too small and too far away for Cassini's narrow-angle camera to resolve it as anything more than a faint point of light. However, at 13.9 million kilometers (8.6 million miles) away, this is one of the best opportunities to image the moon for the rest of the Cassini mission. Even a distant observation like this can be used to better understand the properties of Skoll's surface by measuring its brightness at different phase angles (the angle between Cassini, the observed object, and the sun). Also on February 24, Cassini will observe Dione partially occult Enceladus and its south polar plume. During this observation, Dione will be 2.04 million kilometers (1.27 million miles) away from Cassini and Enceladus 2.27 million kilometers (1.41 million miles) away. Finally, ISS will acquire an astrometric observation known as a SATELLORB of several of Saturn's small satellites including Helene, Telesto, Pallene, and Methone.
On February 26, ISS will look at another outer, irregular satellite, this time taking a look at Tarvos from a distance of 25.5 million kilometers (15.8 million miles). On March 1 and 2, ISS will continue its photometric study of Enceladus' plume during three observations, looking at the icy satellite from a distance of 0.67 to 1.06 million kilometers (415,000 to 658,000 miles). ISS is observing Enceladus at different phase angles to better understand the particle size in the plume. Knowing the brightness and average particle size, scientists can better estimate the mass ejected by Enceladus.
On March 2, Cassini will perform its closest targeted flyby of Saturn's second largest moon, Rhea. Closest approach occurs at 17:41 UTC at an altitude of 100 kilometers (62 miles) above Rhea's surface. This encounter will provide an opportunity to acquire a nine-frame, multi-color and clear-filter mosaic across Rhea's sub-Saturn hemisphere. Highlights from this mosaic include six frames, centered near 5 degrees north latitude, 295 degrees west longitude, over Rhea's wispy terrain, marked by sets of tectonic faults and a large impact basin near the center of the trailing hemisphere. This mosaic will also cover portions of Rhea's equator in order to examine a series of faint streaks along the equator that may be the remains of a transient ring around Rhea.
Speaking of rings, one of the primary goals of this encounter will be to confirm the presence of a set of faint rings around this moon. Evidence for these rings, the first seen at a planetary satellite, came from the Magnetospheric Imaging Instrument (MIMI) data from Cassini's last close flyby of Rhea in November 2005. However, the rings have never been imaged, suggesting that, if they do exist, they are composed primarily of boulder-sized objects that are difficult for Cassini to image. The MIMI instrument will search for similar ring signatures (marked by dips in the level of charged particles, which are absorbed by ring particles) during closest approach on this flyby.
After the nine-frame mosaic is taken, the Composite Imaging Spectrometer (CIRS) will become "prime," observing Rhea's thermal emission as it enters Saturn's shadow 1.5 hours after closest approach. ISS will acquire two narrow-angle, clear-filter frames and three wide-angle frames.
On March 3 at 06:02 UTC, Cassini will reach the periapse of Rev127, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 115,630 kilometers (71,850 miles) above Saturn's cloudtops.
Later that day, Cassini will perform its first close flyby of the small moon Helene. Helene is the larger of the two Trojan moons that share the orbit of Dione. Closest approach for this flyby occurs at 13:41 UTC at an altitude of 1,817 kilometers (1,129 miles). During this encounter, Cassini ISS will acquire a single observation sequence of Helene, ROTCOLR003, consisting of three segments and 87 images. The montage at left shows what some of the images from this observation should look like. During the first segment, Cassini ISS will image the nightside of Helene from distance ranging from 15,558 to 24,515 kilometers (9,667 to 15,232 miles). Using Saturn-shine for illumination, this segment will allow Cassini to map the sub-Saturn hemisphere and observe the narrow, sun-lit crescent of Helene with the use of alternating long- and short-exposure images. The peak resolution for this segment is 90 meters (295 feet) per pixel.
During the next portion of the observation, Cassini will perform its closest approach to Helene. Because Helene is moving quickly across Cassini's sky during this time, the spacecraft will use the same "skeet-shoot" technique it used to image Enceladus during several flybys of that moon. In this case, out of seven pairs of narrow-angle- and wide-angle-camera images (plus two more solo narrow-angle-camera images or NACs), three of the NACs should catch a portion of Helene's surface. These images will be taken from altitudes of 1,894; 3,160 and 3,440 kilometers (1,176; 1,963; 2,137 miles) above Helene. The first two frames should allow for stereo coverage over the northern limb of Helene as Cassini passes over the southern, anti-Saturn hemisphere of the satellite. The third frame will cover a portion of the terminator and the dark limb (illuminated faintly by Saturn-shine), allowing for analysis of Helene's small-scale topography. The resolutions for the three frames will be 11, 19, and 20 meters (36, 62, and 66 feet) per pixel. Most of the WAC images acquired during this segment should include Helene in their fields of view, at an order of magnitude lower resolution than the NAC images.
During the final part of the observation, Cassini will target Helene as it recedes from the spacecraft. ISS will acquire images of the small moon using most of its NAC filters in order to determine its composition and photometric properties. During this segment, Helene will pass in front of Saturn, so the planet's clouds will provide a backdrop for most of the images taken. This portion of the observation will cover Helene's anti-Saturn hemisphere from distances ranging from 6,205 to 28,405 kilometers (3,855 to 17,650 miles). The pixel scale during this part ranges from 37 meters (120 feet) at the start of this tracking period to 167 meters (548 feet) at the end of the observation 40 minutes later.
On March 5, ISS will acquire a cloud monitoring observation of Titan's trailing hemisphere from a distance of 1.78 million kilometers (1.11 million miles). ISS will take follow-up observations on March 6 and 9 to allow for longer-baseline cloud tracking. If clouds are present, these observations will allow scientists to measure the speed of cloud motion and to monitor cloud formation and evolution. On March 6, Cassini will image the space around Iapetus known as its hill sphere, looking for dust that might be falling on the distant, icy moon. On March 7, 8, and 10, ISS will take three more SATELLORBs, this time imaging the following moons: Helene, Calypso, Pallene, Janus, Epimetheus, Telesto, Methone, Prometheus, Pandora, and Anthe. On March 11, ISS will look at another outer, irregular satellite, this time taking a look at Erriapus from a distance of 12.9 million kilometers (8 million miles). Finally, on March 11, ISS will observe the E ring from a vantage point close to the ring plane. The goal is to observe the vertical structure of this diffuse outer ring.
Cassini reaches apoapse on March 12, bringing Rev127 to an end and starting Rev128.
Image products created in Celestia. Dione and Enceladus basemaps by Steve Albers. Helene shape model and basemap by "Fenerit". All dates in Coordinated Universal Time (UTC).