Orion's First Flight

NASA is allowing people to virtually board the Orion Test Flight in December. All you have to do is sign up here:

Orion First Flight Boarding Pass

Other Saturn Moons

Moons of Saturn 2007

From top left to right: Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background, Hyperion in bottom right, Iapetus, and Phoebe

 NASA/JPL/Space Science

Wikipedia 

 

Saturn has more moons than any other planet, including Jupiter. Besides Titan, it has many satellites that are strange in there own way.

 

The second largest moon of Saturn is Rhea, which is less than half the size of the Earth's Moon. It was discovered by Giovanni Cassini in 1672. Of all bodies in the Solar System in hydrostatic equilibrium, it is the smallest, i.e. gravity and fluid pressure are in balance. It is mainly ice, and much like Titan and the Moon, it is tidally locked to Saturn. It is also the largest moon without an atmosphere.

Rhea

Image Credit:

NOAA

The third largest moon of Saturn is Iapetus. It was also discovered by Giovanni Cassini in 1671 and is the largest body in the Solar System not in hydrostatic equilibrium. It is two-toned with the leading face of the moon dark with a low albedo and the back end brighter, like the color of dingy snow. It almost resembles the yin-yang symbol. It is believed that the dark face comes from Iapetus colliding with dark carbon-rich or silicate-rich materials. It is also tidally locked with Saturn.

Iapetus

Image Credit:

NASA/Voyager

Dione is the fourth largest moon of Saturn, discovered by Cassini in 1684. Like Rhea and Iapetus (and all the moons in this post), it is an icy body. Again, it is tidally locked to Saturn and shares an orbital resonance with Enceladus of 1:2. It also has two co-orbiting satellites, Heleneand Polydeuces, located at the L4 and L5 Lagrange points for Dione. Though it is mainly water ice, its density leads us to believe that it has a rocky interior.

Dione

Image Credit:

NASA/Cassini

Tethys is the fifth largest moon of Saturn, and was discovered by Cassini in 1684. It is spherical and has a density very nearly that of water. We also know that Tethys is not very active as it is heavily crated and contains a long, deep ice crack about 500 km long and 3 km deep.

Tethys with Odysseus crater

Image Credit:

NASA/Cassini

Enceladus is the sixth largest moon of Saturn, but was not discovered until 1789 by William Herschel. As an icy body, it also has a very high albedo, making it very reflective. Not only is it an icy body, it is also an active body. The interior of Enceladus is kept hot by tidal forces between itself, Saturn, and Dione. Waper vapor geysers have been discovered erupting from the surface from the Cassini spacecraft. Some of the water vapor does fall back down as snow, but most escapes due to low gravity on Enceladus. Material in the E Ring is replenished by the erupting geysers. Under the surface ice, Enceladus is believed to have a liquid ocean which may contain organic molecules (molecules with carbon).

Enceladus

Image Credit:

NASA/Cassini

Mimas is the seventh largest of Saturn's moon, and the one that many people might recognize. It was discovered by William Herschel in 1789 and much like the other moons listed here, it is a small icy body, with many craters. It has a deep 2 km crack that was probably created early in its history. Mimas is the smallest body in the Solar System that is spherical due to self-gravity. It has a 2:1 resonance with the Cassini Division, which helps keep it clear of particles. Mimas has one really distinguishing feature, and that is a large crater in the northern hemisphere.

 

Mimas with Herschel Crater in upper left

Image Credit:

Solar Views

Death Star from Star Wars. I wonder where George Lucas got his inspiration?

 

Hyperion is the eighth largest of the major moons of Saturn, and was discovered by William Lassell, William Cranch Bond, and George Phillips Bond, all in 1848. It is the largest of the irregularly shaped moons and as seen below, looks like a potato. Its shape is best determined by the diameters along its three axes, 410 km by 260 km by 220 km.

Irregularly shaped Hyperion

Image Credit:

NASA/Cassini

Titan

Cassini Image of Titan showing the atmosphere of the moon

Image Credit:

NASA/Cassini

 

Titan is the largest Saturnian moon and the second largest in the Solar System behind Jupiter's moon Ganymede. Like Ganymede, it is also larger than Mercury. Christian Huygens, who discerned the rings of Saturn, also discovered Titan in 1655, making it the fifth satellite discovered with the telescope.

 

It is the only moon known to have a dense atmosphere, where atmospheric pressure is measurable on the surface. Though Europa, Ganymede, and Callisto may have liquid oceans below their outer crusts, Titan is the only body in the Solar System to have surface liquid besides the Earth. However, you wouldn't want to swim in those oceans as they are bodies of methane.

 

Because it has liquid methane oceans and lakes, much like Earth has a water cycle, Titan experiences a methane cycle. Surface methane evaporates and forms clouds in the Titan sky. The rain Titan experiences is methane.

 

Although Titan is smaller than Earth, its atmosphere is dense enough to create higher surface pressure than on Earth, at about 1.45 atmospheres (146.7 kPa). One atmosphere on Earth is the normal pressure at sea level which is 101.5 kPa (kilopascals). As shown in the above image, the atmosphere is extended with a composition of 98.4% Nitrogren (N₂), 1.4% methane (CH₄), and trace other molecules including water in the stratosphere (higher levels) and 95% N₂, 4.9% CH₄, and other molecules in the troposphere. Because of the methane clouds, the sky is very hazy on Titan so would have poor visibility on the surface when we make our first crewed mission to Titan sometime in the future.

 

The gravity on Titan is only 85% of that on the Moon even though it is larger because of its smaller density. If you were to stand on Titan, your weight would only be 15% of that on Earth.

 

Much like the Moon and Earth are tidally locked, Titan is tidally locked to Saturn, so for every orbit around Saturn, which is almost 16 Earth days, it only rotates once on its axis.



F Ring and Shepherd Satellites

The F-Ring, Prometheus (inner moon), and Pandora (outer moon). The A-Ring fills up the bottom half of the image with the Keepler Gap easily visible

Image Credit:

NASA/JPL/Voyager

 

Saturn's F-Ring is the outermost of the discrete rings discovered in 1979 by Pioneer 11. Compared to the other rings, it is very active with features changing in the structure of the ring on a timescale of hours. The F-Ring is 3,000 km from the A-Ring and is separated from the A-Ring by the Roche Division. Compared to the other discrete rings, the F-Ring is very narrow, only a few hundred kilometers thick. So how exactly does the F-Ring maintain its shape?

The ring is between the orbits of two satellites, Prometheus and Pandora. Prometheus orbits just inside the inner edge of the F-Ring and Pandor just outside the outer edge. These two satellites are able to use their gravitational influence on the ring to keep it stationary and in place. If these moons were not there, the F-Ring would have dissipated long ago.

Prometheus also creates kinks and knots in the ring from its orbit which show up in the ring when Prometheus is at apoapis (farthest distance from Saturn). Because Prometheus does have an elliptical orbit, at each successive apoapis, the knots and kinks are 3.2° ahead of the previous section.

Prometheus creating knots and streamers in the inner F-Ring

Image Credit:

Cassini/ISS/JPL/EPA/NASA

Saturn's Roche Division

Close up of the Roche Division. Visible from the bottom left to the upper right, the A Ring with the Encke Gap and the Keeler Gap, Atlas in the center, and the F Ring

Image Credit:

NASA

 

The Roche Division is the space between the A Ring and the F Ring in the ring system of Saturn.  Despite sharing the name with the Roche limit of Saturn and being near it, it is not named for the limit, but actually is named for Edouard Roche.

Like the Cassini Division, it is not empty, but contains material similar to the D Ring, E Ring, and F Ring, but very sparsely distributed.

The Cassini spacecraft discovered to small ringlets in the division, both near the orbits of a moon. One ringlet shares an orbit with the moon Atlas and the other is close to the orbit of Prometheus, which will be discussed in the next blog post.

October 2014 Lunar Eclipse

Legend

Intense red shading: Observers within this area can see the eclipse from beginning to end.

Red shading right/east of intense shading: Observers within this area can see the eclipse until moonset/sunrise.

Red shading left/west of intense shading: Observers within this area can see the eclipse after moonrise/sunset.

No coloring: Eclipse is not visible at all

Note: Actual eclipse visibility depends on weather conditions

 

Image and Legend taken from TimeandDate.com

 

On October 8, 2014, there will be a total lunar eclipse occuring. If you remember from the post about eclipses, lunar eclipes occur during the full moon phase when the Moon's orbit takes it into the shadow of the Earth. As you can see from the image above, Anyone in the intense red region in the middle of the map will get to see the entirety of the lunar eclipse. I live in Pittsburgh, USA, so I will only get to see the beginning just before the sun rises and the moon sets. If you live in China, you will get to see the ending, just after sunset/moonrise.

Another cool thing about this lunar eclipse is due to the sunlight filtering through the atmosphere, the Moon will appear reddish in color.

 

Here are the times of the lunar eclipse in Universal Time (UTC) -24 hour clock. Add or subtract the appropriate number of hours to get your local time.

• Penumbral eclipse begins (the Moon enters the penumbra of the Earth's shadow) - 8:17
• Partial eclipse begins (Moon enters the umbra of the Earth's shadow) - 9:18
• Full Eclipse begins (Moon entirely in the umbra) - 10:27
• Maximum Eclipse - 10:55
• Full Eclipse ends (Moon begins to leave the umbra) - 11:22
• Partial Eclipse ends (Moon leaves the umbra entirely) - 12:32
• Penumbral Eclipse ends (Moon leaves the penumbra entirely) - 13:32
 

The A Ring of Saturn

A close up of the A Ring. The Cassini Division lies between the A Ring and the B Ring, the Encke Gap (though the image says division) is within the A Ring, and the Roche Division separates the A Ring from the F Ring

Image Credit:

 Sky and Telescope

 

The A Ring of Saturn is the farthest of the main rings and the first one discovered. It is composed of material similar to that of the B Ring and therefore is bright. It is separated from the B Ring by the Cassini Division, and has similar structure as the B ring.

One of the main features of the A Ring is the Encke Gap, discovered by James Keeler when he was working at Lick Observatory near San Jose, California. (Keeler was working at Allegheny Observatory when he discovered the rings were not solid). The gap was named in honor of Johann Encke who had discovered that the A Ring was not uniformly bright. The gap is about 325 km wide and centered at 133,590 km from Saturn's center. It is kept clear by the orbit of a small moon, Pan, and contains at least three thin ringlets which are knotted due to the gravitational influence of passing moons.

Encke Gap (PIA06534)

Image Credit:

NASA/JPL/Space Science Institute

The Keeler Gap, named for James Keeler, was discovered by the Voyager probe and is about 42 km wide and 250 km from the outer edge of the A Ring. Daphnis orbits with the gap and keeps it clear, much like Pan with the Encke Gap. Daphnis actually is inclined with respect to the rings and actually causes waves at the edges of the gap.

Keeler Gap with Daphnis within. Notice the waves at the edges of the gap.

Image Credit:

NASA/JPL/Space Science Institute

 

Besides the gaps, there are many moonlets that orbit within the ring and helps create waves and spokes within the ring structure. They were first discovered in Cassini images and by 2008, over 150 moonlets have been identified. They were only discovered because of the influence their gravity has on the A ring. Based on evidence, there are possibly thousands of these small moonlets, no more than several kilometers in diameter. They orbit in a path that is only 3000 km wide at a distance of 130,000 km.

As you may have noticed, the rings themselves contain both divisions and gaps. The IAU (International Astronomical Union) defines a division as a separation between two distinct rings and a gap as a small opening in a ring itself. Hence the Cassini division divides the B Ring from the A Ring and the Encke Gap and Keeler Gap are gaps in the structure of the A Ring.