StarDate

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