METEOROLOGIE - Le jour où des grêlons d’1 kg sont tombés : 92 victimes

La majorité des grêlons qui tombent sous les orages et les averses ne pèsent que quelques grammes. Mais il y a quelques années, c'est un grêlon d'un kilogramme qui a été retrouvé, et homologué.

Dans les situations orageuses les plus violentes, sous les supercellules par exemple, il arrive que des grêlons pesant plusieurs centaines de grammes tombent. Lorsqu'un grêlon mesure plus de 5 centimètres, il peut alors peser jusqu'à 500 grammes. Mais dans les cas les plus extrêmes, des grêlons de 800 à 900 grammes, avec un diamètre de 15 centimètres ou plus, sont possibles.

Ces grêlons géants sont rares d'une manière générale, et ils tombent le plus souvent aux États-Unis, en Argentine, au Brésil, au Chili, ou encore dans certains États d'Afrique, comme en République du Congo.

Cependant, le record mondial, homologué par l'Organisation météorologique mondiale, a été enregistré au Bangladesh : le 14 avril 1986, un grêlon pesant 1,02 kg a été ramassé. Ce jour-là, l'orage de grêle a été catastrophique : les grêlons géants ont causé la mort de 92 personnes, et ravagé les cultures de la ville Gopalganj, au sud du pays. L'événement paraît extraordinaire, mais il est tout à fait probable : le Bangladesh subit une météo très agitée, et les orages ultra-violents sont courants.

FuturaSciences

ASTRONOMY - Rubin's Galaxy

 2025 June 13

Rubin's Galaxy
Image Credit: NASA, ESA, B. Holwerda (University of Louisville)

Explanation: In this Hubble Space Telescope image the bright, spiky stars lie in the foreground toward the heroic northern constellation Perseus and well within our own Milky Way galaxy. In sharp focus beyond is UGC 2885, a giant spiral galaxy about 232 million light-years distant. Some 800,000 light-years across compared to the Milky Way's diameter of 100,000 light-years or so, it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. Part of an investigation to understand how galaxies can grow to such enormous sizes, UGC 2885 was also part of An Interesting Voyage and American astronomer Vera Rubin's pioneering study of the rotation of spiral galaxies. Her work was the first to convincingly demonstrate the dominating presence of dark matter in our universe. A new U.S. coin has been issued to honor Vera Rubin, while the Vera C. Rubin Observatory is scheduled to unveil images from its first look at the cosmos on June 23.

ASTRONOMY - 25 Brightest Stars in the Night Sky

2025 June 11

25 Brightest Stars in the Night Sky
Image Credit & Copyright: Tragoolchitr Jittasaiyapan

Explanation: Do you know the names of some of the brightest stars? It's likely that you do, even though some bright stars have names so old they date back to near the beginning of written language. Many world cultures have their own names for the brightest stars, and it is culturally and historically important to remember them. In the interest of clear global communication, however, the International Astronomical Union (IAU) has begun to designate standardized star names. Featured here in true color are the 25 brightest stars in the night sky, currently as seen by humans, coupled with their IAU-recognized names. Some star names have interesting meanings, including Sirius ("the scorcher" in Latin), Vega ("falling" in Arabic), and Antares ("rival to Mars" in Greek). You are likely even familiar with the name of at least one star too dim to make this list: Polaris. 

ASTRONOMY - Enceladus in True Color

 2025 June 10

Enceladus in True Color
Image Credit: NASA, ESA, JPL, SSI, Cassini Imaging Team

Explanation: Do oceans under the ice of Saturn's moon Enceladus contain life? A reason to think so involves long features -- some dubbed tiger stripes -- that are known to be spewing ice from the moon's icy interior into space. These surface cracks create clouds of fine ice particles over the moon's South Pole and create Saturn's mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown in true color from a close flyby. The deep crevasses are partly shadowed. Why Enceladus is active remains a mystery, as the neighboring moon Mimas, approximately the same size, appears quite dead. An analysis of ejected ice grains has yielded evidence that complex organic molecules exist inside Enceladus. These large carbon-rich molecules bolster -- but do not prove -- that oceans under Enceladus' surface could contain life.

ASTRONOMY - Between Scylla and Charybdis: A Double Cosmic Discovery

2025 June 9

Between Scylla and Charybdis: A Double Cosmic Discovery
Image Credit: M. Drechsler, Y. Sainty, A. Soto, N. Martino, L. Leroux-Gere, S. Khallouqui, & A. Kaeouach; Text: Ogetay Kayali (Michigan Tech U.)

Explanation: Can you identify this celestial object? Likely not — because this is a discovery image. Massive stars forge heavy elements in their cores and, after a few million years, end their lives in powerful supernova explosions. These remnants cool relatively quickly and fade, making them difficult to detect. To uncover such faint, previously unknown supernova remnants, a dedicated group of amateur astrophotographers searched through sky surveys for possible supernova remnant candidates. The result: the first-ever image of supernova remnant G115.5+9.1 — named Scylla by its discoverers—glowing faintly in the constellation of the mythological King of Aethiopia: Cepheus. Emission from hydrogen atoms in the remnant is shown in red, and faint emission from oxygen is shown in hues of blue. Surprisingly, another discovery lurked to the upper right: a faint, previously unknown planetary nebula candidate. In keeping with mythological tradition, it was named Charybdis (Sai 2) — a nod to the ancient Greek expression "caught between Scylla and Charybdis" from Homer’s Odyssey.

ASTRONOMY - Facing NGC 3344

 2025 June 8

Facing NGC 3344
Image Credit: ESA/Hubble & NASA

Explanation: From our vantage point in the Milky Way Galaxy, we see NGC 3344 face-on. Nearly 40,000 light-years across, the big, beautiful spiral galaxy is located just 20 million light-years away in the constellation of Leo Minor. This multi-color Hubble Space Telescope close-up of NGC 3344 includes remarkable details from near infrared to ultraviolet wavelengths. The frame extends some 15,000 light-years across the spiral's central regions. From the core outward, the galaxy's colors change from the yellowish light of old stars in the center to young blue star clusters and reddish star forming regions along the loose, fragmented spiral arms. Of course, the bright stars with a spiky appearance are in front of NGC 3344 and lie well within our own Milky Way.

MUSIC - Johannes Brahms - Lullaby

"Lullaby"

ASTRONOMY - Perseverance Selfie with Ingenuity

 2025 June 7

Perseverance Selfie with Ingenuity
Image Credit: NASA, JPL-Caltech, MSSS

Explanation: On the Mars rover's mission Sol 46 or Earth date April 6, 2021, Perseverance held out a robotic arm to take its first selfie on Mars. The WATSON camera at the end of the arm was designed to take close-ups of Martian rocks and surface details though, and not a quick snap shot of friends and smiling faces. In the end, teamwork and weeks of planning on Mars time was required to program a complex series of exposures and camera motions to include Perseverance and its surroundings. The resulting 62 frames were composed into a detailed mosaic, one of the most complicated Mars rover selfies ever taken. In this version of the selfie, the rover's Mastcam-Z and SuperCam instruments are looking toward WATSON and the end of the rover's outstretched arm. About 4 meters (13 feet) from Perseverance is a robotic companion, the Mars Ingenuity helicopter. Perseverance has now spent over 1,500 sols exploring the surface of the Red Planet. On Earth date January 18, 2024, Ingenuity made its 72nd and final flight through the thin Martian atmosphere.

ASTRONOMY - NGC 6302: The Butterfly Nebula

2025 June 6

NGC 6302: The Butterfly Nebula
Image Credit & Copyright: Mike Selby

Explanation: The bright clusters and nebulae of planet Earth's night sky are often given the names of flowers or insects, and its whopping 3 light-year wingspan, NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the central star of the planetary nebula is transforming into a white dwarf star, becoming exceptionally hot, and shining brightly in ultraviolet light. The central star is hidden from direct view by a torus of dust, but its energetic ultraviolet light ionizes atoms in the nebula. In this sharp, telescopic view composed with narrowband image data, the ionized hydrogen and doubly ionized oxygen atoms are shown in their characteristic red and blue-green hues to reveal a stunning complex of knots and filaments within the nebula's wing-like bipolar outflows. NGC 6302 lies about 4,000 light-years away in the arachnologically correct constellation of the Scorpion (Scorpius). 

ASTRONOMIE - Mars et Régulus : suivez leur curieux tandem bicolore !

Depuis son passage au plus près de la Terre en janvier dernier, Mars continue de s’éloigner de la Terre. Son éclat va en diminuant, mais elle est encore brillante : sa magnitude est de 1,4 à la mi-juin. On peut donc facilement la repérer à l’œil nu dans le ciel du soir, même quand la nuit n’est pas encore complète. La planète rouge est à chercher au-dessus de l’horizon ouest, dans la constellation du Lion qui est assez facile à reconnaître.

Du 15 au 19 juin, Mars frôle l’étoile principale du Lion : Régulus. Leur distance angulaire sera de l’ordre du degré, et descendra même à 0°45 le soir du 17 juin. Par coïncidence, les deux astres se trouvent avoir la même luminosité apparente, ce qui donne à cette paire un aspect intrigant. Quant à leurs couleurs respectives, elles sont bien différentes : Régulus est bleutée, tandis que Mars, recouverte d’oxyde de fer, est bel et bien rougeâtre ! Cette différence de couleur est parfaitement visible à l’œil nu. Et si vous possédez des jumelles, n’hésitez pas à les braquer sur le duo : l’éclat des deux astres en sera renforcé et les couleurs vous paraitront encore plus évidentes !

Ce rapprochement est aussi l’occasion d’apprécier le mouvement de Mars dans le ciel : celui-ci est perceptible de jour en jour grâce au repère fourni par Régulus qui, elle, semble immuable. En effet, Mars se trouve à environ 270 millions de kilomètres, ce qui est très peu par rapport à Régulus : cette étoile est située à 79 années-lumière de la Terre, c’est-à-dire presque 3 millions de fois plus loin ! Ceci explique que nous percevions les mouvements de Mars autour du Soleil (combinés à ceux de la Terre), alors que Régulus et les autres étoiles semblent fixes.

Stelvision