Mammatus Clouds over Nebraska

When do cloud bottoms appear like bubbles? Normally, cloud bottoms are flat. This is because moist warm air that rises and cools will condense into water droplets at a specific temperature, which usually corresponds to a very specific height. As water droplets grow, an opaque cloud forms. Under some conditions, however, cloud pockets can develop that contain large droplets of water or ice that fall into clear air as they evaporate. Such pockets may occur in turbulent air near a thunderstorm. Resulting mammatus clouds can appear especially dramatic if sunlit from the side. The mammatus clouds pictured here were photographed over Hastings, Nebraska during 2004 June. [via NASA]

Interstellar Comet 2I Borisov

From somewhere else in the Milky Way galaxy, Comet 2I/Borisov is just visiting the Solar System. Discovered by Crimean amateur astronomer Gennady Borisov on August 30, 2019, the first known interstellar comet is captured in these two recent Hubble Space Telescope images. On the left, a distant background galaxy near the line-of-sight to Borisov is blurred as Hubble tracked the speeding comet and dust tail about 327 million kilometers from Earth. At right, 2I/Borisov appears shortly after perihelion, it’s closest approach to Sun. Borisov’s closest approach to our fair planet, a distance of about 290 million kilometers, will come on December 28. Even though Hubble’s sharp images don’t resolve the comet’s nucleus, they do lead to estimates of less than 1 kilometer for its diameter. [via NASA]

Full Moon Geminids

The dependable annual Geminid meteor shower will be near its peak tonight (December 13/14) and before tomorrow’s dawn. As Earth crosses through the dusty trail of active asteroid 3200 Phaethon the meteors will flash through the sky from the shower’s radiant in Gemini. Gemini will be pretty easy for skygazers to find too as it won’t be far from a nearly full waning gibbous Moon. You don’t have look at the shower’s radiant to see meteors though. The almost full moonlight won’t hide the brightest of the Geminids from view either, but it will substantially reduce the rate of visible meteors for those who are counting. In fact, the 2019 Geminids should look a lot like the 2016 meteor shower This composite image from the 2016 Geminids aligns individual short exposures to capture many of the brighter Geminid meteors, inspite of a Full Moon shining near the constellation of the Twins. Along the horizon are the Teide Observatory’s Solar Laboratory (right) and the Teide volcano on the Canary Island of Tenerife. [via NASA]

Decorating the Sky

Bright stars, clouds of dust and glowing nebulae decorate this cosmic scene, a skyscape just north of Orion’s belt. Close to the plane of our Milky Way galaxy, the wide field view spans about 5.5 degrees. Striking bluish M78, a reflection nebula, is on the right. M78’s tint is due to dust preferentially reflecting the blue light of hot, young stars. In colorful contrast, the red sash of glowing hydrogen gas sweeping through the center is part of the region’s faint but extensive emission nebula known as Barnard’s Loop. At lower left, a dark dust cloud forms a prominent silhouette cataloged as LDN 1622. While M78 and the complex Barnard’s Loop are some 1,500 light-years away, LDN 1622 is likely to be much closer, only about 500 light-years distant from our fair planet Earth. [via NASA]

N63A: Supernova Remnant in Visible and X-ray

What has this supernova left behind? As little as 2,000 years ago, light from a massive stellar explosion in the Large Magellanic Cloud (LMC) first reached planet Earth. The LMC is a close galactic neighbor of our Milky Way Galaxy and the rampaging explosion front is now seen moving out – destroying or displacing ambient gas clouds while leaving behind relatively dense knots of gas and dust. What remains is one of the largest supernova remnants in the LMC: N63A. Many of the surviving dense knots have been themselves compressed and may further contract to form new stars. Some of the resulting stars may then explode in a supernova, continuing the cycle. Featured here is a combined image of N63A in the X-ray from the Chandra Space Telescope and in visible light by Hubble. The prominent knot of gas and dust on the upper right — informally dubbed the Firefox — is very bright in visible light, while the larger supernova remnant shines most brightly in X-rays. N63A spans over 25 light years and lies about 150,000 light years away toward the southern constellation of Dorado. [via NASA]

Looking Sideways from the Parker Solar Probe

Everybody sees the Sun. Nobody’s been there. Starting in 2018 though, NASA launched the robotic Parker Solar Probe (PSP) to investigate regions near to the Sun for the first time. The PSP’s looping orbit brings it yet closer to the Sun each time around — every few months. The featured time-lapse video shows the view looking sideways from behind PSP’s Sun shield during its first approach to the Sun a year ago — to about half the orbit of Mercury. The PSP’s Wide Field Imager for Solar Probe (WISPR) cameras took the images over nine days, but they are digitally compressed here into about 14 seconds. The waving solar corona is visible on the far left, with stars, planets, and even the central band of our Milky Way Galaxy streaming by in the background as the PSP orbits the Sun. PSP has found the solar neighborhood to be surprisingly complex and to include switchbacks — times when the Sun’s magnetic field briefly reverses itself. The Sun is not only Earth’s dominant energy source, its variable solar wind compresses Earth’s atmosphere, triggers auroras, affects power grids, and can even damage orbiting communication satellites. [via NASA]

Geminid Meteors over Chile

Are meteors streaming out from a point in the sky? Yes, in a way. When the Earth crosses a stream of Sun-orbiting meteors, these meteors appear to come from the direction of the stream — with the directional point called the radiant.  An example occurs every mid-December for the Geminids meteor shower, as apparent in the featured image.  Recorded near the shower’s peak in 2013, the featured skyscape captures Gemini’s shooting stars in a four-hour composite from the dark skies of the Las Campanas Observatory in Chile. In the foreground the 2.5-meter du Pont Telescope is visible as well as the 1-meter SWOPE telescope. The skies beyond the meteors are highlighted by Jupiter, seen as the bright spot near the image center, the central band of our Milky Way Galaxy, seen vertically on the image left, and the pinkish Orion Nebula on the far left. Dust swept up from the orbit of active asteroid 3200 Phaethon, Gemini’s meteors enter the atmosphere traveling at about 22 kilometers per second. The 2019 Geminid meteor shower peaks again this coming weekend. [via NASA]