Saturn has many, varied moons – over 60 have been discovered so far. One of the larger ones, Dione, is shown here in this image from the Cassini orbiter, pictured as it moved across the face of its parent planet.
Dione orbits Saturn at around 377 400 km, roughly the same distance as that between Earth and the Moon. It has a diameter of around a third that of the Moon, and is just under 1.5 times as dense as liquid water. While the moon is thought to be mostly water ice, this higher density indicates that it must have a core of dense material – most likely silicates, the same kind of rock that makes up Earth’s mantle.
In this image, Dione is seen passing across the face of Saturn, a phenomenon known as a transit. The dark line cutting across the middle of the frame marks Saturn’s rings – these are not illuminated from this perspective, which was about 0.3º below the ring plane.
Transits occur when one celestial body passes in front of another. They are seen most often when moons pass in front of their parent planets, or planets (or even moons) in front of their parent stars. Transits are important events in astronomy, allowing observers to investigate the transiting body’s atmosphere and orbit in greater detail.
We can see a number of transits from Earth, such as when Mercury and Venus pass between the Sun and Earth, and show up clearly as black dots moving across the Sun’s bright disc.
Using powerful telescopes, we can also study more distant planets in other star systems as they pass in front of their stars. Scientists rely heavily on transits in their hunt for and study of exoplanets.
Dione orbits within Saturn’s magnetosphere, a region of space surrounding the planet that is filled with highly energetic atomic particles. These particles rain down on Dione and smash into its surface. Dione is too small with too little gravity to hold on to an atmosphere of its own, but this continuous high-energy bombardment releases molecules from the moon’s surface that form a thin, atmosphere-like layer. This tenuous atmosphere was discovered during two of Cassini’s close flybys of Dione, on 11 October 2005 and 7 April 2010.
Cassini acquired this visible-light view using its narrow-angle camera when it was 2.3 million km from Saturn on 21 May 2015. The image scale is 14 km per pixel.
A view of the glacier atop Africa’s highest peak, as observed by ESA’s Proba-V minisatellite.
The dormant volcano known as Mount Kilimanjaro is Africa’s highest mountain, at 5895 m. It is also the tallest free-standing mountain in the world, rising about 4900 m above its surrounding plain.
Located close to the equator at 3°S, only its summit is covered with snow and ice. The ascent towards the top is a journey through most of the world’s climate zones, from the tropical to the Arctic. On the way the landscape shifts from tropical rain forest to moorland, alpine heather to desert and finally snow and ice.
This 100 m-resolution false-colour image from Proba-V’s main Vegetation camera on 14 June 2015 shows Kilimanjaro enveloped by clouds to the south and north. The gradual decrease of vegetation with altitude can be seen by the colours changing from green to brown and finally light blue, representing the summit’s glacier.
Launched on 7 May 2013, Proba-V is a miniaturised ESA satellite tasked with a full-scale mission: to map land cover and vegetation growth across the entire planet every two days.
Its main camera’s continent-spanning 2250 km swath width collects light in the blue, red, near-infrared and mid-infrared wavebands at 300 m resolution and down to 100 m resolution in its central field of view.
VITO, the Flemish institute for technological research, processes and then distributes Proba-V data to users. VITO has produced an online gallery highlighting some of the mission’s most striking images so far, including views of storms, fires and deforestation.
This MRI scan shows a knee joint with cartilage covering the articulating joint surfaces to help the bones slide smoothly.
Cartilage responds slowly to changes in joint loading because it does not have any blood vessels, lymphatic system or nerves to feed and grow tissue, so nutrients are absorbed slowly.
Everyday loading of our skeleton is important to keep cartilage healthy because the motion and loading of the joint are needed to get nutrients into the cartilage, but little is known about cartilage in bedridden people on Earth.
To find out more, the Institute of Biomechanics and Orthopaedics of the German Sport University Cologne in Germany is studying astronauts.
As astronauts float in space for up to six months their legs are hardly used in weightlessness. The researchers are analysing biomarkers in up to 10 astronauts before and after flight to chart cartilage metabolism, thickness, volume and water content in knee joints.
This is the first time such a study is being done on healthy people. As cartilage responds so slowly, a similar study with healthy individuals on Earth would require that they do not move for many months, which is impossible.
The goal is to learn more about how the knee cartilage of the astronauts suffers from their trip into space. From here, researchers are hoping to understand the role of mechanical loading for cartilage health and the development of osteoarthritis.
This natural-colour Sentinel-2A image features Mexico City and surroundings.
Home to nearly nine million people, Mexico City is the densely populated, high-altitude capital of Mexico.
Sentinel’s optical camera clearly shows the difference between the densely built city centre and the vast surrounding vegetation.
The brownish-grey patch in the right corner of the central part of the image is a flat area with some agriculture, criss-crossed by canals. Mexico International Airport is also visible, and further along the dark green rectangle is Lake Nabor Carrillo.
This is a reservoir, encompassing more than 14 163 ha, which is 41 times larger than New York’s Central Park.
Mexico City sits in the Valley of Mexico, also called the Valley of Anáhuac, a large valley in the high plateaus at the centre of Mexico, at an altitude of 2240 m. This valley is in the Trans-Mexican Volcanic Belt, which is at least 2200 m above sea level. Mountains and volcanoes surround it, with elevations reaching beyond 5000 m.
The city rests mainly on the heavily saturated clay of what used to be Lake Texcoco. This soft base is collapsing through the over-extraction of groundwater, and the city has sunk as much as nine metres in some areas since the beginning of the 20th century.
Clouds are scattered throughout the image, under which lie various national parks and some of the still-active volcanoes, such as Popocatépetl at 5426 m.
Launched on 23 June, Sentinel-2A is a polar-orbiting, multispectral high-resolution imaging mission for land monitoring to provide images of vegetation, soil and water cover, inland waterways and coastal areas. While supplying our planet with such vital data, it can also deliver information for emergency services.
Also featured on the Earth from Space video programme, this image was captured by Sentinel-2A on 6 August.
The victorious group of the 2015 edition of the Copernicus Masters took home prizes worth more than € 300 000 in total. These included cash, consulting, data packages and other assistance, designed to help the winners refine their ideas for a possible market launch at one of ESA’s 11 Business Incubation Centres.
Exhibit on "Space for Climate" put in place by ESA and CNES at the Champs-Elysées Avenue in Paris, from 18 October to 27 October 2015. ESA and CNES mobilised to share with citizens the key role of space in controlling Climate Change.
This image shows the galaxy Messier 94, which lies in the small northern constellation of the Hunting Dogs, about 16 million light-years away.
Within the bright ring around Messier 94 new stars are forming at a high rate and many young, bright stars are present within it – thanks to this, this feature is called a starburst ring.
The cause of this peculiarly shaped star-forming region is likely a pressure wave going outwards from the galactic centre, compressing the gas and dust in the outer region. The compression of material means the gas starts to collapse into denser clouds. Inside these dense clouds, gravity pulls the gas and dust together until temperature and pressure are high enough for stars to be born.
This single frame Rosetta navigation camera image of Comet 67P/Churyumov-Gerasimenko was taken on 18 October 2015 from a distance of 433 km from the comet centre. The image has a resolution of 36.9 m/pixel and measures 37.8 km across.
The original image and more information is available on the blog: CometWatch 18 October
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Week In Images
19-23 October 2015