Today is the December solstice – the start of winter in the northern hemisphere. It’s the darkest time of the year – many hours of darkness for watching the stars. But it’s also a great time for space science in Antarctica, where it’s daylight around the clock. NASA launches high-altitude balloons from a base near McMurdo Station, the continent’s largest settlement. Their payloads can keep an eye on the heavens for weeks as they circle around the south pole. When their work is done, they parachute to the ice. Scientists from the United States, Japan, and other countries hunt for meteorites in Antarctica. There aren’t more meteorites there, but on the ice, there’s a good chance that almost any rock came from beyond Earth. Over the decades, tens of thousands of meteorites have been found there. Astronomers take advantage of the daylight to repair and upgrade telescopes at the south pole. The collection includes instruments that study the “afterglow” of the Big Bang. The instruments can operate even in daylight, but the southern summer is the only time to do most of the maintenance work. The south pole also is home to IceCube – a collection of thousands of light detectors frozen in the ice. They look for neutrinos – particles that tell us about some of the most energetic events in the universe. IceCube can also operate all year – even under the midnight sun at the south pole. Script by Damond Benningfield

If you don’t like winter but you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator. Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse. It looks like a flattened circle, with the Sun slightly away from the center. Because of that shape, our distance to the Sun changes. And that’s where the laws of orbital motion come into play. Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time. To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January – the start of winter – and farthest at the start of summer, in July. So Earth moves around the Sun in a hurry during northern winter, making the season shorter. This winter, for example, starts at 9:03 a.m. Central Time tomorrow. That’s the moment of the December solstice, when the Sun stands farthest south for the year. The season ends 89 days later. By comparison, this past summer lasted almost 93 days – a longer season thanks to the science of orbits. More about the solstice tomorrow. Script by Damond Benningfield

The “she-goat” is a lot more than it seems. What looks like a single brilliant star is actually two sparklers. Both of them are much bigger, heavier, and brighter than the Sun. Capella is the brightest star of Auriga, the charioteer. Its name comes from a Latin phrase that means the she-goat. It’s 43 light-years away – just down the block by astronomical standards. Both stars of Capella are about two and a half times as massive as the Sun. And both are more than 70 times brighter than the Sun. But the stars are quite close together – less than the distance from Earth to the Sun. So we can’t see them as individual points of light, even with the biggest telescopes. Astronomers discovered Capella’s dual nature with a technique called spectroscopy. It breaks a star’s light into its individual wavelengths. The spectrum of Capella shows two patterns of light. The patterns move back and forth as the stars orbit each other. The patterns reveal details about both stars – their surface temperature, composition, and more. From that and other details, we know that both stars have moved beyond the prime phase of life. Now, they’re in the giant phase. Both stars have puffed up to about 10 times the diameter of the Sun – two big, brilliant stars for the she-goat. Capella is a third of the way up the northeastern sky at nightfall. It’s one of the brightest stars in the entire night sky, so you can’t miss it. Script by Damond Benningfield

The tales that describe many of the ancient constellations can be romantic, tragic, heroic, or majestic. Some, on the other hand, are just weird. An example is Auriga. The constellation is low in the eastern sky at nightfall, and climbs high across the sky later on. It’s marked by a pentagon of stars. It’s easy to pick out thanks to the brightest member of that figure, Capella – one of the brighter stars in the entire night sky. Although Auriga is described as a charioteer, the character usually isn’t depicted with a chariot. But he is shown with a goat and her two kids on his shoulder. There are several versions of his story. In one, he was an early king of Athens. He was raised by the goddess Athena. Among other things, she taught him how to tame horses. He was so good at it that he became the first person to harness four horses to a chariot, like the chariot that carried the Sun across the sky. Zeus, the king of the gods, was so impressed that he placed the charioteer in the stars. The goat is represented by Capella. It isn’t a part of any of the legends of Auriga from Greek or Roman mythology. It may represent the goat that suckled the infant Zeus, who placed her and her children in the sky in gratitude. The goat and kids may once have formed their own small constellation. Today, though, they ride on the shoulder of the charioteer – who rides on nothing at all. More about the charioteer tomorrow. Script by Damond Benningfield

At the cusp of the 20th century, it seemed like contact with another world was just a matter of time. In fact, the French Academy of Sciences announced a prize for such a feat 125 years ago today. The winner would receive 100,000 francs. There was only one catch: Mars didn’t count. The prize was established by Clara Guzman in honor of her son. He was a follower of astronomer Camille Flammarion, who wrote extensively – and fancifully – about the Red Planet. Guzman excluded Mars from the competition because it seemed just too easy. Percival Lowell had popularized the idea that Mars was crisscrossed by canals – built by Martians to bring water from the poles to the planet’s deserts. Inventor Nicola Tesla had reported hearing possible radio signals from Mars. And many others thought that vast dark areas on Mars were covered with vegetation. Many schemes were proposed to contact the Martians. One suggested creating giant geometric shapes in Siberia. Another suggested digging the shapes into the Sahara Desert, filling them with kerosene, and setting them on fire. None of the schemes ever materialized. And no one ever claimed the prize for contacting another world. So the French academy decided to award the prize for making physical contact. In 1969, it awarded the Guzman Prize to Apollo 11 astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins – the first men to set foot on another world. Script by Damond Benningfield

The planet Mercury is shrinking. It’s contracted by several miles since its birth. And it’s continuing to get smaller even now. Mercury is the closest planet to the Sun. It’s also the smallest major planet in the solar system – a little more than 3,000 miles in diameter – about the width of the 48 states. It has a core of iron and nickel, surrounded by dense layers of rock. And it’s topped by a thin crust. The surface of the planet is marked by lots of impressive cliffs. The biggest is more than 600 miles long and about two miles high. They formed as Mercury lost heat from its interior. As the planet cooled, it shrank. Estimates of how much it’s contracted have ranged from about a mile to about nine miles. A recent study narrowed the range a little bit. It measured the most dramatic features, then scaled that to the surface of the entire planet. The result suggests that Mercury has shrunk by about three to five miles as a result of its cooling. And when you add in some other causes, the total contraction is about four to seven miles. And Mercury is still getting smaller today. This incredible shrinking planet is quite low in the southeast in the dawn twilight for the next few days. It looks like a bright star, but you need a clear horizon to spot it. And because of the viewing angle, it’s easier to spot from more southern latitudes. Tomorrow, the Moon stands to its right or upper right. Script by Damond Benningfield

Here’s what we know for sure about the planet K2-18b. It’s about 125 light-years away. It’s bigger and heavier than Earth. It orbits a cool, faint star once every 33 days. It receives about the same amount of energy from its star as Earth gets from the Sun. And it has an atmosphere. After that, things get muddled. Astronomers aren’t sure about the structure of the planet or the make-up of its atmosphere. And ideas about whether it might be habitable are all over the place. The confusion highlights the challenges of studying planets in other star systems. K2-18b passes in front of its star on every orbit. And as it does so, the chemical “fingerprints” of its atmosphere are added to the starlight. Substracting the starlight provides a profile of the atmosphere. But the profile is hard to read. Many of the fingerprints are subtle, and can be produced by different compounds. Earlier this year, a team announced the discovery of compounds in the atmosphere that could be produced by microscopic life. Follow-up studies by other groups contradicted that finding. But the original study team has stuck by its conclusions. So it’ll take a lot more work to know for sure what’s going on at K2-18b. The K2-18 system is in Leo, which climbs into good view after midnight. K2-18 is to the right of Denebola, the star that marks the lion’s tail. But it’s too faint to see without a telescope. Script by Damond Benningfield

For stars that are similar to the Sun, the end comes in stages. And each stage is triggered by changes in the star’s core. One star that’s going through those changes is Diphda, the brightest star of Cetus, the sea monster. The star is several hundred million years old – billions of years younger than the Sun. For most of its life, it “fused” hydrogen atoms in its core to make helium. When the hydrogen was gone, it began fusing hydrogen in a shell around the core. That made the star puff up, so it was classified as a red giant. Now, it’s finished off the shell, so it’s fusing the helium in the core to make carbon and oxygen. This phase is generally lumped into the red-giant category. Technically, though, it has its own name: the red clump. In a hundred million years or so, Diphda will have used up all the helium. The star isn’t massive enough to fuse the carbon and oxygen to make heavier elements. Without that energy, the core will collapse to about the size of Earth. It’ll be extremely hot, though, so it’ll blow away Diphda’s outer layers. For a while, the star will enter one more phase: a planetary nebula – a colorful cloud of gas and dust. When the cloud disperses, only the dead core will remain: a white dwarf – the hot but tiny remnant of a star. Cetus spreads across the southeastern quadrant of the sky at nightfall. Diphda is near its lower right corner, roughly a third of the way up the sky. Script by Damond Benningfield

People collect all kinds of things, from baseball cards to Persian rugs. Over the past 40 years, some NASA aircraft have collected dust – grains of dust from beyond Earth. Many of the collection efforts have taken place during meteor showers. That’s included the Geminid shower, which is at its peak tonight. A meteor shower takes place when Earth flies through a trail of particles that were shed by a comet or asteroid. Many of the particles burn up in the upper atmosphere, creating the streaks of light known as meteors. But many more grains are too small to burn up. They float down through the atmosphere. Some of them stop at a height of about 10 miles. And that’s where the research aircraft head. Once there, they open up small boxes that catch whatever is drifting along – pollen grains, parts of bugs, bits of volcanic ash, and even exhaust from rocket engines. Analysis reveals whether the captured particles are from Earth or from outside. The cosmic particles can then be tied to the meteor shower that was under way. And that can tell scientists about the shower’s parent body – a sample-return mission that never leaves Earth. The Geminids are in good view tonight. The meteors are visible from mid-evening on. At its best, the shower might produce a hundred or so meteors per hour. And you don’t need to look in a particular direction to see them – just look up and wait for the fireworks. Script by Damond Benningfield

A couple of thousand years ago, a large asteroid or comet might have been blasted apart. And we’re still seeing the fireworks from its destruction – as the Geminid meteor shower, which will reach its peak tomorrow night. Most meteor showers flare to life when Earth passes through the orbital path of a comet. The comet sheds bits of rock and dirt, which spread out along its orbit. As Earth flies through this trail of debris, the solid grains ram into the atmosphere, forming the glowing streaks known as meteors. But the Geminids are a bit odd. For one thing, their parent body – 3200 Phaethon – appears to be an asteroid or a “dead” comet, not an active comet. For another, the meteor stream contains way more material than we’d expect to see from a body the size of Phaethon. A couple of years ago, scientists came up with a possible explanation. They used observations by a Sun-orbiting spacecraft that passed through the meteor stream. They then used computer models to calculate a possible cause for the stream. They concluded that a larger body could have been destroyed. That produced Phaethon and a couple of other large remnants. But it also produced a giant cloud of dust and pebbles. So while some of the material that makes up the Geminids comes from Phaethon, a lot of it also comes from that cloud – shrapnel that makes fireworks in Earth’s night sky. More about the Geminids tomorrow. Script by Damond Benningfield