This isn’t a sound effect from a sci-fi movie. Instead, it’s the “voice” of the auroras on the planet Saturn – radio waves that have been shifted to wavelengths we can hear. They were recorded by the Cassini spacecraft as it closed in on Saturn two decades ago. The radio waves are produced by the complex interplay between Saturn’s magnetic field and the solar wind – a steady flow of charged particles from the Sun. Motions deep inside the planet generate the magnetic field, which is about as strong as Earth’s. But because Saturn is much bigger than Earth, its magnetic field is much bigger as well – it fills a huge volume of space. The magnetic field forms a teardrop-shaped “bubble” around Saturn. That bubble deflects much of the solar wind. But some of the particles make it through. The lines of magnetic force guide some of them toward the magnetic poles. They spiral in, emitting radio waves as they do so. And when the particles hit the upper atmosphere, they create auroras – shimmering curtains powered by the Sun. Saturn is putting in its best appearance of the year. It’s at opposition – it lines up opposite the Sun. It’s closest to Earth, so it shines brightest. And it’s in view all night. The giant planet looks like a bright golden star. It’s low in the east at nightfall, and climbs high across the south later on. Script by Damond Benningfield

Saturn puts in its best appearance of the year over the next few nights. On Sunday, it’ll reach opposition – it will line up opposite the Sun. It’s closest to Earth at opposition, so it shines brightest. And it’s in view all night. It looks like a bright golden star. It’s low in the east at nightfall, and climbs high across the south later on. Saturn reaches opposition every 12 and a half months – the result of the combined motions of Saturn and Earth. Earth orbits the Sun at an average speed of about 67,000 miles per hour. Saturn is more than nine times farther from the Sun. And thanks to the laws of orbital motion, it moves only a third as fast as Earth. Johannes Kepler formulated those laws four centuries ago. He determined that the planets follow elliptical orbits; instead of perfect circles, the orbits are stretched out. There’s a relationship between the planet’s distance and its orbital period – the time it takes to make one full turn around the Sun. And a planet moves fastest when it’s closest to the Sun, and slowest when it’s farthest. Earth’s distance varies by only about three percent, so there’s not much change in its orbital speed. But Saturn’s distance varies by more than 10 percent, so there’s a bigger change in its speed. All of this works together to bring Saturn into alignment every 12 and a half months – shining at its best. More about Saturn tomorrow. Script by Damond Benningfield

To modern eyes, the stars of Sagittarius form the outline of a teapot. But in Greek mythology, the constellation was far more extensive. It represented a centaur – half-man, half-horse – holding a bow and arrow. And the brightest star in the constellation plays a role in both of those pictures. Kaus Australis is at the lower right corner of the teapot. It also represents the southern end of the bow – in fact, that’s the meaning of its name. It’s actually a binary – two stars in a wide orbit around one another. One of the stars is like the Sun. But from the system’s distance of about 145 light-years, it’s much too faint to see with the eye alone. The star we can see is much bigger and heavier than the Sun, and about 500 times brighter. And it spins in a hurry – once every 1.6 days, compared to almost four weeks for the Sun. If it were spinning just a little faster, it would rip itself apart. In fact, it’s closer to that self-destruct point than any other star yet seen. As a result of its rotation, the star is squashed – it’s about a third wider through the equator than the poles. And because they’re closer to the star’s core, the poles are thousands of degrees hotter than the equator. Astronomers can’t explain the star’s high-speed rotation. The star could be siphoning gas from a much-closer companion that’s hidden from view – spinning up the tip of the archer’s bow. Script by Damond Benningfield

Galaxies aren’t good neighbors. They can stretch and pull the galaxies around them, rip them apart, and even gobble them up. A case in point is the Magellanic Clouds – the largest satellite galaxies of the Milky Way. The Large Magellanic Cloud is about 165,000 light-years away. It’s about a third as wide as the Milky Way, and perhaps one-tenth as massive. The small cloud is a little smaller and farther away. Both of them are being distorted by the Milky Way’s gravity. And both may be incorporated into the Milky Way billions of years from now. But they’re also interacting with each other. In fact, a recent study says the gravity of the large cloud may be ripping the smaller one apart. Researchers have studied the system for more than a decade from an observatory in Chile. They’ve measured the motions of millions of stars. And they found that the stars in the Small Magellanic Cloud aren’t moving the way they expected. Most models say the galaxy forms a rotating disk, like the Milky Way. But the observations revealed that most of the stars are moving outward – away from the center of the galaxy. And that applied even to the stars in the center itself. The most likely cause is the pull of the Large Magellanic Cloud. Its gravity is dragging the stars away from their galactic home. That could eventually rip the smaller galaxy apart – leaving only some shredded remnants for the Milky Way. Script by Damond Benningfield

If you’re searching for life on another world, you don’t want to find life that’s hitchhiked from Earth. But preventing contamination isn’t easy. Over the past few decades, we’ve identified quite a few worlds in the outer solar system that could be habitable. These worlds are coated with frozen water. But they could have oceans of liquid water below the crust. Those oceans could supply the minerals and the energy needed for life. So scientists are especially interested in them. But they want to make sure that any life they find really is native. So every mission to these worlds goes through a careful process of sterilization. But building and launching a spacecraft requires hands-on contact by hundreds of people. They build the instruments, assemble the spacecraft, test it, and attach it to its booster. Much of the work is done in high-level cleanrooms. Along the way, the spacecraft and its components may be cleaned with chemicals, baked at high temperatures, or zapped with radiation – all to prevent Earthly “bugs” from catching a ride. One especially interesting target is Enceladus, a moon of Saturn. It has a buried ocean, but some of its water shoots into space. Some of it falls back onto the surface – perhaps making it easier to find native life on this icy world. Saturn is close to the right of our own moon in early evening. It looks like a bright star. It’ll stay close to the Moon all night. Script by Damond Benningfield

The Moon is full today. But it’s not just any old full Moon. It’s the best-known of them all: the Harvest Moon – the subject of books, music, and lots of fall festivals. In centuries past, the Harvest Moon was much more than a pop-culture event. It provided enough light for farmers to harvest their crops well into the night. And at high northern latitudes, it helped out for several nights in a row. Officially, the Harvest Moon is the full Moon that’s closest to the fall equinox. That put it around harvest time – especially at higher latitudes, where autumn frosts soon would blanket the night. Without artificial lights to help them out, farmers relied on the Moon to add hours to their work time. The Moon is bright enough to help for several nights before and after it’s full. And thanks to the angle at which it climbs into the sky, for latitudes north of about Milwaukee or Minneapolis it rises only a few minutes later each night. So farmers in such high northerly climes didn’t have to wait around for the Moon to rise – they could keep on harvesting through the twilight and into the night. Tonight, the planet Saturn is quite close below the Moon at nightfall, and stays close throughout the night. It looks like a bright star. Like the full Moon, it’s about to line up opposite the Sun – setting up its own prime viewing time. We’ll have more about Saturn and the Moon tomorrow. Script by Damond Benningfield

Neptune has lost its vibrancy. The planet itself hasn’t changed – only our perception of it – the result of a bit of creative license exercised 37 years ago. Neptune is the Sun’s fourth-largest planet – four times the diameter of Earth. But it’s also the most-distant major planet – 30 times farther from the Sun than Earth is. Our only close look at Neptune came in August 1989, when Voyager 2 flew just 3,000 miles above its cloudtops. It transmitted hundreds of pictures of the planet. They revealed white bands of clouds racing through Neptune’s atmosphere, and a massive storm – the Great Dark Spot. In most of the pictures, Neptune has a deep cerulean palette. But a couple of years ago, a team reprocessed the images. The original project team had enhanced the planet’s color and contrast. That revealed details in the atmosphere that otherwise wouldn’t show up. But it also concealed Neptune’s true appearance: a pale blue-green. So Neptune isn’t as vibrant as it seemed – but it’s still an amazing world far from the Sun. Neptune is closest to Earth today for the entire year. It shines at its brightest, and it’s in the sky all night. You still need help to see it. But its location is easy to spot. As night falls, it’s about half way between the full Moon – the Harvest Moon – and the planet Saturn, which looks like a bright star to the lower left of the Moon. Script by Damond Benningfield

The largest moon of Neptune was an agent of chaos. It wasn’t born with Neptune itself. Instead, it was captured by the giant planet. As it spiraled in, it might have hit one of Neptune’s existing moons. And it scattered the others – knocking some of them out of orbit. Only one of them might have survived. There’s a lot of evidence of that idea. Triton is about the same size and composition as Pluto. It contains more than 99 percent of the total mass of Neptune’s moons and rings. And it orbits in the opposite direction from Neptune’s rotation. There’s no way for a moon to form in that kind of orbit, so Triton must have formed elsewhere in the solar system. It was captured when Neptune was young. It might have caromed off an existing moon. Or it might have had a companion. When the duo passed close to Neptune, it was ripped apart – Triton entered orbit, while its sibling was kicked off on its own. Triton then bludgeoned its way through the system. A recent study says the only surviving moon is Nereid, Neptune’s third-largest moon. Researchers looked at it with Webb Space Telescope. They found that it’s made of the same materials as the moons of Uranus – not the other moons of Neptune. That suggests it was born with the planet, and survived Triton’s chaotic arrival. Neptune is at its brightest this week. It’s in the east at nightfall, to the upper right of the bright planet Saturn. But you need a telescope to see it. More tomorrow. Script by Damond Benningfield

It’s not often that someone hands an astronomer a major discovery. But that’s what happened 180 years ago today. German astronomer Johann Galle, the director of Berlin Observatory, received a letter from Urbain Le Verrier. The French astronomer had calculated the likely position of a planet beyond Uranus, which at the time was the Sun’s most-distant known planet. Galle looked for the new planet that night – and found it: the planet Neptune. Le Verrier had calculated that Uranus wasn’t orbiting the Sun as expected. He decided that it was being nudged out of position by the gravity of another planet, farther from the Sun. He calculated the planet’s location, then sent his results to Berlin. British astronomer John Couch Adams had made similar calculations. He passed his results to his own colleagues. One of them searched for the planet, and saw it, in 1845 – but he didn’t realize it. So Adams usually is credited as a co-discoverer. But some research in recent years has suggested that his calculations were off, so there’s a debate about who discovered the Sun’s eighth planet. Appropriately enough, Neptune is at its best this week. It lines up opposite the Sun, so it’s closest to Earth, and shining at its brightest. But it’s so far away that you need a telescope to pick it out. It’s not far to the upper right of the bright planet Saturn, which is low in the east at nightfall. More tomorrow. Script by Damond Benningfield

The Sun speaks to us. It tells us about conditions deep inside it, far below its surface. That helps scientists understand how the Sun is put together, how it works, and how it changes. Listening to the Sun is called helioseismology. It works in the same way that seismology works on Earth. Motions inside the Sun generate sound waves. Those waves can travel all the way around the Sun. And they can travel deep inside it. They cause the surface to vibrate. And scientists can measure the vibrations – the Sun’s voice. The voice is complicated. It’s producing many frequencies of sound – like a diva singing many octaves of notes all at the same time. Some of the waves penetrate all the way to the core, where the Sun generates energy. Others stay close to the surface. Scientists have to unscramble this cacophony to understand what the waves are telling us. One thing that scientists have learned recently is that an important region in its magnetic field has changed over the past four decades. Today, that region is much closer to the surface than it was 40 years ago, suggesting that our star is undergoing some changes. The magnetic field is especially important to Earth. Magnetic storms can knock out satellites and power grids, disrupt radio waves, and cause other problems. So we may be able to better protect our technology by listening to the voice of the Sun. Script by Damond Benningfield