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Cassini continues its extended tour of the Saturn system with the 17.5-day-long Rev126, the spacecraft's 127th orbit around the Ringed Planet. Cassini begins Rev126 on February 4 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 cameras start their observations for Rev126 two days after apoapse by taking a wide-angle-camera calibration observation, riding along with the Ultraviolet Imaging Spectrometer (UVIS). The instruments will be using the bright, B-type star Spica (Alpha Virginis) to better understand changes in the properties of UVIS and ISS. The next day, 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 14.4 million kilometers (9 million miles) away, this is the best opportunity 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). On February 11, ISS will perform a similar study of Enceladus' plume, observing the icy satellite from a distance of 1.48 million kilometers (923,000 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 February 12, the Visual and Infrared Mapping Spectrometer (VIMS) will take a mosaic of Saturn's nightside to monitor the dynamics of Saturn's clouds. ISS will ride-along during the observation, searching for lightning within storms on Saturn and looking at the nightside clouds as they are illuminated by the rings.
On February 13 at 16:55 UTC, Cassini will reach the periapse of Rev126, its closest point to Saturn in the orbit. At periapse, the spacecraft will be 115,200 kilometers (71,580 miles) above Saturn's cloud tops. ISS and the other Cassini investigations have a complex schedule planned for the periapse period. Starting at 5:58 UTC and running four hours, Cassini will be in the shadow of Saturn. During the eclipse, ISS will image the limb of Saturn in order to better understand the upper haze layers of the planet's atmosphere. A few hours later, Cassini will perform a non-targeted encounter with the Trojan moon, Calypso, at a distance of 21,257 kilometers (13,208 miles). Calypso shares the orbit of Tethys, orbiting on average 60 degrees behind the much larger moon. ISS will acquire 25 images over a 45-minute period during this encounter. These will be the highest resolution images yet acquired of this small (30-by-23-by-14 kilometers or 19-by-14-by-9 miles) moon, with the best pixel scale reaching 125 meters (410 feet) per pixel. During this observation, Calypso's sub-Saturn hemisphere will be illuminated and the leading hemisphere portion of that area will be visible to Cassini. The previous best was a set taken in September 2005 from a distance of 101,000 kilometers (63,000 miles). See PIA07633 to view those images.
Thirty minutes after periapse, ISS will perform a non-targeted encounter with Saturn's innermost large icy satellite, Mimas. The altitude for this encounter is 9,510 kilometers (5,910 miles) -- the closest Cassini has ever gotten to this cratered moon. For this encounter, ISS will acquire three mosaics along with another observation where ISS will be riding along with the Composite Infrared Spectrometer (CIRS). ISS' first mosaic of Mimas, GEOLOG001, will start 30 minutes after closest approach when Cassini is 14,800 kilometers (9,200 miles) away from Mimas. GEOLOG001 is a seven-frame, multi-spectral mosaic of the region surrounding the crater Herschel. Herschel, at 130 kilometers (80 miles) across, is the largest impact basin on Mimas, so large that it caused significant amounts of stress on the lithosphere of the small moon and so distinctive that it helped give the satellite the nickname, "The Death Star Moon." These high-resolution observations of the basin will be used to estimate the age of the crater. Scientists will count the number of smaller craters on the basin floor, compared to other regions on Mimas, to set limits on how old the basin can be. The second mosaic, GEOLOG002, will also be a seven-frame, multi-spectral mosaic, this time covering most of the visible surface of Mimas, shown above left. The best resolutions for these two observations will be 87 meters (285 feet) per pixel for GEOLOG001 and 191 meters (626 feet) per pixel GEOLOG002. Next, Cassini will ride-along with a CIRS FP3 temperature map of Mimas' day side, acquiring six narrow-angle-camera images during the scan. Finally, ISS will acquire a full-frame, multispectral observation (GLOCOL001) of Mimas' anti-Saturn hemisphere from a distance of 70,000 kilometers (44,000 miles). Saturn will provide a backdrop for this observation.
Next, Cassini's optical remote sensing instruments will turn their attention to Tethys. ISS will acquire a multi-spectral observation of Tethys' leading hemisphere from a distance of 175,000 kilometers (109,000 miles). The region includes the large impact basin, Odysseus, the largest on Tethys. The next day, February 14, ISS will ride-along with VIMS on a cloud dynamics observation of Saturn, taking a three-by-three, wide-angle-camera mosaic of the dayside of Saturn. Afterward, ISS will take a look at a mutual event as Epimetheus partially occults Janus. Like similar observations taken over the last month, Epimetheus and Janus will actually be fairly close to one another, with 16,000 kilometers (9,900 miles) separating them, after completing an orbital swap on January 21.
On February 15, ISS will acquire its first observation in conjunction with a non-targeted encounter with Titan that will have taken place late on February 14. This encounter will provide an opportunity to observe the center of the trailing hemisphere during a series of observations between February 15 and 18. The first observation, LOPHASE001, will be taken from a distance of 810,000 kilometers (500,000 miles), and just as the name implies, it is a low phase angle observation with Titan nearly "full." Over the next three days, Cassini will acquire three more cloud monitoring observations from distances ranging from 1.1 to 2.3 million kilometers (0.7 to 1.4 million miles) away. Titan isn't the only moon ISS will be focusing on during the outbound leg of Rev126. Between February 16 and 22, ISS will acquire five distant observations of Iapetus, taken from distances ranging from 1.47 to 2.0 million kilometers (0.91 to 1.24 million miles) away. These observations will focus on Iapetus' sub-Saturn hemisphere.
Cassini reaches apoapse on February 22, bringing Rev126 to an end and starting Rev127. Rev127 includes targeted flybys of the small moon Helene and the much larger moon Rhea.
Image products created in Celestia. Mimas and Tethys basemaps by Steve Albers. All dates in Coordinated Universal Time (UTC).