Posts Tagged ‘Cassini’

Cassini sends home picturesque view of Enceladus

February 26, 2016 Leave a comment

NASA’s Cassini spacecraft made its last fly-by of Enceladus on Dec. 19. Since then it has been transmitting to Earth images taken of the ocean moon, including this beauty received Feb. 15:

Enceladus - last fly-by, Dec. 19, 2015 - received at Earth Feb. 15, 2016 - courtesy NASA, JPL, SSI, Justin Cowart

This view of Enceladus is from about 83,000 kilometers. The moon is captured in winter. It’s north pole is to the upper left, while its south pole is obscured by darkness in the lower right. Courtesy NASA, Jet Propulsion Laboratory, Space Science Institute, Justin Cowart.


NASA releases Cassini image of three Saturnian moons

February 23, 2016 Leave a comment

As NASA’s Cassini probe continues its exploration of Saturn and its moons, it sends home some amazing images of that distant region of our solar system.

Yesterday, NASA released an image showing three of the gas giant’s moons: Tethys, Enceladus, and Mimas. Here it is:

Three moons, obtained Dec. 2015, released Feb. 22, 2016

Tethys (above the rings), Enceladus (just below center and below the rings), and Mimas (below and to the left of Enceladus) appear in this image obtained by Cassini in visible light on Dec. 3, 2015. Courtesy NASA, Jet Propulsion Laboratory – California Institute of Technology, Space Science Institute.

According to NASA:

The view was acquired at a distance of approximately 837,000 miles (1.35 million kilometers) from Enceladus, with an image scale of 5 miles (8 kilometers) per pixel. Tethys was approximately 1.2 million miles (1.9 million kilometers) away with an image scale of 7 miles (11 kilometers) per pixel. Mimas was approximately 1.1 million miles (1.7 million kilometers) away with an image scale of 6 miles (10 kilometers) per pixel.

Tethys’ diameter is about 1,066 kilometers. The cratered moon orbits Saturn from a distance of about 294,600 kilometers. That is about 20 percent farther from Saturn than the Moon is from Earth. Tethys is slightly less dense than liquid water, which suggests that its structure is mostly ice. This frigid moon is tidally locked to Saturn – it does not rotate and only one side of Tethys faces Saturn.

Enceladus is smaller than Tethys, with a diameter of about 500 kilometers. The host of a sub-surface liquid water ocean, Enceladus is covered by water ice and  reflects nearly all the sunlight that hits it. That makes it a very cold satellite. The surface temperature on the moon is about -201 degrees Celsius. It orbits Saturn from a mean distance of about 238,000 kilometers, which is about the same distance as Earth’s satellite is from our planet.


This whimsical poster depicting Enceladus is part of NASA’s Visions of the Future project. You can see more at Courtesy NASA, Jet Propulsion Laboratory – California Institute of Technology.

Mimas is the smallest of Saturn’s major moons. It is heavily cratered, with a giant crater called Herschel stretching across about one-third of its surface. That feature has led Mimas to sometimes be called the “Death Star moon” because it evokes the fictional planet-destroying spacecraft in the Star Wars films. Mimas’ diameter is about 400 kilometers; it orbits Saturn from a mean distance of about 200,000 kilometers.

Looking back at Cassini: A last look at Dione

February 23, 2016 Leave a comment

NASA’s Cassini probe is in its last year of operation, continuing to explore the Saturn system, and it seems appropriate to look back at some of the amazing images the spacecraft has sent home to Earth.

This one was taken on August 17, 2015, during the probe’s last flyby of Saturn’s moon Dione.

Dione, Aug. 2016

Icy Dione, as seen by Cassini last August. You can see some of the rings of Saturn, as well as the solar system’s second largest planet, in the background. Courtesy NASA.

Dione (Di-OH-nee) is a small moon, with a diameter of only about 1,120 kilometers, and it orbits Saturn at about the same distance as the Moon orbits Earth. Covered by ice for which one of Saturn’s rings is the main source, Dione has many craters. Some are as large as 100 kilometers across.

Like Earth’s sole satellite, Dione does not rotate. Only one side of the moon ever faces Saturn.

Giovanni Cassini discovered Dione in 1864. The moon takes its name from the mythological Greek goddess who was said by Homer to be the mother of Aphrodite.

Cassini probe finds evidence of ocean on Enceladus

April 3, 2014 Leave a comment

Enceladus, the icy moon of Saturn known for its spectacular geysers, likely has an ocean of liquid water beneath its surface.

Researchers relied on gravity and topography data obtained by NASA’s Cassini probe in concluding that Enceladus could be the third known outer solar system moon that features liquid water.

The ocean is a regional water body near the moon’s south pole. It seems to lie about 30 to 40 kilometers below the moon’s surface, covered by a thick ice sheet, and likely has a depth of about 10 kilometers.

“This means it is as large as or larger than Lake Superior,” David J. Stevenson, a professor of planetary science at the California Institute of Technology and a co-author of the paper documenting the discovery, said.

Scientists depended on measurements of the gravity field of Enceladus to infer the presence of the ocean. As Cassini flew 100 or fewer kilometers over the moon’s surface during three flyovers, the spacecraft’s speed changed by a few millimeters per second.

The tiny slowing of Cassini was detected by antennae on Earth as the radio waves transmitted by the probe during the flyovers shifted slightly, just as sound waves do as their source moves farther away from a receiver.

“The detection of such small velocity variations gave us information about the accelerations the spacecraft was subject to,” Marzia Parisi, an aerospace engineer and research fellow at Sapienza University of Rome and one of the authors of the paper, said.

An acceleration of an object can include both an increase and a decrease in its speed.

The acceleration of a spacecraft that is flying over a celestial object is affected by a variety of factors, including changes in the object’s gravity.

“This could arise because of the absence of mass that is a depression of the ice surface, but when you look at the actual surface, you can see that although there is indeed a depression, it is much larger than that needed to explain the gravity,” Stevenson said. “So that means that there must be a compensating positive mass excess under the south polar ice. The natural way to do this is to have a layer of water because water is more dense than ice.”

In the case of the Cassini flyovers, another likely impact on the spacecraft’s velocity was drag caused by passing through water vapor plumes that are emitted from the moon’s south polar region.

The impact of those factors had to be considered before the research team could conclude that the cause of Cassini’s acceleration change was a fluctuation in mass tied to the presence of liquid water beneath the moon’s surface.

Parisi explained that the researchers were able to take account of those factors in their modeling.

The data does not explain how an ocean could exist on Enceladus, but it is logical to assume that Saturn’s gravitational pull has a great deal to do with it.

One significant implication of the discovery of an ocean on Enceladus is the possibility that it could be hospitable for microbial life.

“The added value of our work is that a large, potentially habitable environment has been found in an unexpected place in the solar system, where the energy needed to produce liquid water from ice is not provided by solar radiation,” Luciano Iess, a professor of aerospace engineering at Sapienza University of Rome and the lead author of the paper, said.

It is likely to be many years, if not decades, before humanity can find out whether, in fact, microbes exist beneath Enceladus’ icy surface.

Cassini will continue to explore the Saturnian system for about three more years. The probe will fly over Enceladus three more times, but will not further investigate Enceladus’ gravitational field to gain further insights into the ocean beneath its surface.

Europa, a Jovian moon, and Titan also have oceans, though on Titan it is composed of liquid methane.

NASA’s planetary science budget includes funds to plan a trip to Europa.

The paper documenting the discovery of an ocean on Enceladus will be published in the April 4, 2014 edition of Science.


Image courtesy Wikimedia.

artist's conception - courtesy NASA, JPL-CalTech






Graphic courtesy NASA, Jet Propulsion Laboratory-California Institute of Technology. Note the illustration of the hypothesized sub-surface ocean, appearing as a blue region just above the water vapor jets.

Space Photo of the Year

December 31, 2011 Leave a comment

It is difficult to choose the most fantastic space-related photograph of the year. There are, after all, many contenders. We had, for example, many “first-time” pictures of features on the Red Planet, our neighbor Mars. This one shows the west rim of huge Endeavor crater, which has a diameter of about 14 miles. The crater is characterized by geological features that are older than any others Opportunity has investigated during its mission.

Photo courtesy NASA/JPL-Caltech/Cornell/ASU

Then there was the first image ever obtained of the huge asteroid known as Vesta. NASA’s Dawn mission is in the midst of an exploration of the asteroid belt. This full view of Vesta was taken from a distance of 5,200 kilometers. The asteroid has a mean diameter of about 530 kilometers and is the second-largest object in the asteroid belt, second in size only to the dwarf planet Ceres.

Photo courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Sticking with the space theme, there are many other amazing images to see, starting with this dramatic one from the MESSENGER probe that shows just a small part of Mercury:

Photo courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington


We cannot forget the incredible images obtained by the Hubble Space Telescope, either. This one, obtained by the telescope in Feb. 2011, shows a nebula (a region of space in which stars are being formed) that is about 2,000 light years away from Earth:

Photo courtesy NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

But my favorite picture from 2011 is this one, which was obtained by NASA’s Cassini probe. It shows a storm encircling the huge planet. The storm is the most intense, and the largest, ever observed by Cassini. It produces a significant amount of radio noise, which indicates that there is lightning occurring in Saturn’s atmosphere. One odd feature of storms on Saturn is that they occur when a huge amount of energy is released all at once, which is unlike the meteorological pattern on other planets.

Photo courtesy NASA/JPL-Caltech/SSI


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