StarDate

The largest object in the asteroid belt isn’t like its neighbors. It contains a lot more frozen water, along with minerals that formed in a wet environment. Its crust may cover a deep layer of ice with pockets of salty liquid water. And some deep craters near its poles hold big slabs of ice that might have formed fairly recently.

Ceres is about 600 miles in diameter. It’s classified as a dwarf planet – the only one in the inner solar system. It’s in the middle of the asteroid belt, between the orbits of Mars and Jupiter.

But its composition suggests it wasn’t born there. Instead, it might have been born much farther from the Sun, between Jupiter and Saturn. Those giant worlds moved around when the solar system was young, pushing smaller objects into new orbits – possibly including Ceres.

A spacecraft studied Ceres from orbit for more than three years. It found patches of ice inside craters that never see the Sun. A recent study said those patches could be quite young.

The study looked at how Ceres is tilted on its axis, and how the tilt changes. Researchers found that the tilt has been at the correct angle to keep sunlight out of the craters for as little as 6,000 years. Another body might have slammed into Ceres, blasting out some of its water and forming a thin atmosphere. Some of the water then froze inside the craters – creating some chilly deposits on the largest body in the asteroid belt.

Script by Damond Benningfield

The two brightest objects in the night sky barely peek into view in the evening twilight tonight. But you don’t have much time to look for them – Venus and the Moon set by the time it gets dark.

There’s building evidence that Venus is an active world. Volcanoes appear to erupt on its surface, and giant blocks of rock may shift position.

There’s no doubt that Venus has been active in the past. More than 90 percent of its surface is covered by volcanic rock. In fact, the surface might have been completely flooded with molten rock as recently as half a billion years ago. And the surface is pockmarked by tens of thousands of volcanic features.

Spacecraft have revealed evidence of ongoing activity. The most recent example was reported earlier this year.

Scientists compared images of Venus taken by the Magellan spacecraft in 1990 and ’92. Magellan used radar to peer through the planet’s clouds and map almost its entire surface.

The new analysis found possible changes in the way two volcanic mountains reflected the radar waves. That suggests that both mountains erupted between the times they were observed. One of them produced a lava flow that covered about 12 square miles, the other, 17 square miles. Combined with other observations, that suggests that Venus is still an active world.

Venus is the brilliant “evening star.” It’s quite low in the west at sunset, a little above the crescent Moon.

Script by Damond Benningfield

If Planet Nine exists, it’s taunting astronomers. Several studies of the orbits of small bodies far from the Sun hint at the presence of a massive planet in the outer solar system. Yet the possible world has remained hidden.

Planet Nine was suggested in 2014, based on the orbits of some big chunks of rock and ice beyond the orbit of Neptune, the Sun’s most-distant major planet. The orbits appear to be influenced by the gravity of a body many times as massive as Earth.

The most recent evidence of Planet Nine comes from a study that modeled the orbits of 29 objects beyond Neptune. All of the orbits are tilted and stretched out. At their closest, these objects pass just inside the orbit of Neptune, which is 30 times farther from the Sun than Earth is. But they average more than a hundred times the Earth-Sun distance, so it takes many centuries for them to orbit the Sun.

Astronomers calculated the likelihood of so many objects following similar paths under two scenarios. In one, they modeled the gravitational effects of the Sun’s known planets, plus the pull of the rest of the galaxy’s stars and gas clouds. In the other, they added the effects of a possible Planet Nine. The model with Planet Nine produced a far better match to the orbits.

So far, a search of observations made along the planet’s possible path has come up empty. So if Planet Nine exists, it’s playing a game of hide-and-seek far from the Sun.

Script by Damond Benningfield

Comets are some of the most beautiful objects in the cosmos – balls of ice and rock with tails that can stretch for millions of miles. But they’re also some of the most fickle objects in the cosmos – they don’t always meet expectations. And some fall apart – vaporized by the Sun.

All of that is a preamble to the story of Comet C/2023 A3 Tsuchinshan-Atlas. Over the next few days, it might be visible through binoculars in the dawn sky, mainly from more southerly latitudes. And it could shine even brighter in the evening sky next month. Because we record our programs in advance, though, we can’t tell you how closely it’s matching the forecasts.

The comet was discovered last year. And early predictions called for it eventually to shine as brightly as Venus, which is climbing into view as the “evening star.” It’s no longer expected to get that bright. But it still could become bright enough to see with the eye alone.

The comet will pass closest to the Sun late this month, and closest to Earth next month – about 44 million miles away.

Now, the comet is passing through the constellation Sextans. That makes it a better target for skywatchers in the southern hemisphere. It’ll be easier to see from the northern hemisphere in the latter half of October, when it’ll climb fairly high at nightfall. So keep your eyes open – and your fingers crossed – for a good showing from a comet.

Script by Damond Benningfield

Two important sky paths intersect on the eastern and western horizons as night falls now: the ecliptic and the celestial equator.

The ecliptic is much easier to trace. It marks the Sun’s path across the sky. That path held great significance for many cultures, so they gave extra weight to the constellations it crossed – the constellations of the zodiac.

As night falls, the ecliptic arcs near a couple of the zodiac’s more prominent star patterns. It passes close above the “teapot” outlined by the stars of Sagittarius. And it nicks the “head” of Scorpius, with most of the scorpion’s body below it.

The Moon and planets all stay close to the ecliptic as well. As twilight fades this evening, for example, brilliant Venus is quite low in the west, with fainter Saturn about the same height in the east.

The equator is harder to see. It’s the projection of Earth’s equator on the sky, so it divides the sky into northern and southern hemispheres. Its closest bright star in early evening is Altair, which is high in the southeast. It’s in the northern half of the sky.

The equator maintains the same angle across the sky every hour of every day. But the ecliptic changes position as the night rolls on. It also shifts position from day to day. The two paths intersect along the horizon at some time during every day of the year. But they come together at nightfall only a couple of times a year – near the fall and spring equinoxes.

Script by Damond Benningfield

One popular sci-fi trope is invasion by alien “bugs” – microscopic organisms from other worlds that could harm life on Earth. But scientists also are concerned about things working the other way around: Earth bugs contaminating other planets. So landers are sterilized to prevent them from carrying any hitchhikers. And when the life of an orbiter ends, it’s targeted to crash into a place where it won’t cause any harm.

One world where that’s not a concern is Mercury. The Sun’s closest planet has no air – only a few atoms captured from the solar wind, or knocked off the surface of Mercury by the solar wind. And even that is a hard vacuum by Earth standards.

Mercury is zapped by solar energy. That heats the dayside to as high as 800 degrees Fahrenheit. And ultraviolet energy would destroy any Earth-like microbes.

So when the only Mercury orbiter to date wrapped up its mission, in 2015, it was allowed to crash into the planet.

The next Mercury mission is scheduled to fly past the planet next week, and enter orbit late next year. It may crash into Mercury when its mission is through as well – gouging a new crater, but leaving the planet uncontaminated.

Mercury is just creeping into the dawn sky. It looks like a fairly bright star. But it’s so low that you need a clear horizon to spot it. It’ll be a bit easier to see tomorrow because it’ll perch close to the right of the “fingernail” crescent Moon.

Script by Damond Benningfield

Star clusters are packed with stars, but not with planets. So far, only a few dozen of the more than 5600 confirmed exoplanets have been found in clusters. And two of those orbit a single star, in the Beehive Cluster.

The dearth of planets could simply mean that planets are hard to find. Only a small fraction of the stars in the Milky Way Galaxy reside in clusters, and not many clusters are close to us. And clusters can be tightly packed, making it hard to study their individual stars.

But because the stars are so tightly packed, it might be hard to make planets. As stars fly past each other, their gravity could stir up the raw materials for making planets, blocking their birth. And even when a planet is born, a close encounter could kick it away from its star. So maybe there really aren’t many planets in clusters.

The star in the two-planet system in the Beehive is a little smaller and cooler than the Sun, and four billion years younger. One of its planets is about twice the mass of Jupiter, the giant of our own solar system. And it’s so close to the star that it’s extremely hot. The other planet is even bigger, but much farther from the star, so it’s quite cold. So neither planet is a likely home for life.

The system is too faint to see without a telescope. But the Beehive is visible. Under dark skies, it looks like a hazy patch of light. At dawn tomorrow, it’s close to the right or upper right of the Moon.

Script by Damond Benningfield

No matter which system you go by, the names of the two brightest stars of Gemini aren’t quite right.

In mythology, the stars are named Pollux and Castor. They represent a pair of twin half-brothers. They had the same mother, but one had a mortal father, while the other was the son of Zeus, the king of the gods of Olympus. But the stars don’t look like twins. Pollux is almost twice as bright as Castor, and it looks orange, versus Castor’s pure white. They were called the twins mainly because they’re both bright, and they appear close together.

The stars themselves aren’t physically related. And they’re nothing alike. Pollux consists of a single giant star, while Castor is a system of six stars.

A second naming system is also off. In this system, Castor is Alpha Geminorum, while Pollux is Beta Geminorum.

The naming system was devised by Johann Bayer. In a star atlas published in 1603, he assigned each star to a constellation. And he named the stars using the letters of the Greek alphabet. “Alpha” usually was applied to a constellation’s brightest light, and “Beta” to the second-brightest. But in Gemini, they’re reversed. No one is quite sure why that’s the case. No matter the reason, though, the names of the bright “twins” of Gemini are a bit off.

The stars appear near the Moon early tomorrow. Pollux stands above the Moon at first light, with Castor a little farther to the upper left of the Moon.

Script by Damond Benningfield

When a delivery truck drops off its final package of the day, it isn’t abandoned – it’s prepped for more deliveries the next day. And it’s the same thing with several spacecraft. After delivering their main cargo, they’ve been given new missions – new addresses to check out.

The list has included three craft that delivered samples to Earth – bits of two asteroids, and some dust from a comet. The craft dropped capsules containing the samples into Earth’s atmosphere as they flew by. And another craft fired a cannonball into a comet.

After finishing their main missions, the spacecraft were still working. So they were given new missions. One of them is headed for a rendezvous with the asteroid Apophis, which will skim just above Earth in 2029. Another is headed for a comet.

Along the way, some of the missions have turned their cameras toward star systems with known planets. The craft can stare at a system for days or weeks. That allows them to measure tiny dips in a star’s light as a planet passes in front of it. Astronomers piece that together with information from other sources to refine a planet’s dossier – its size, mass, distance from its star, and more. One craft even found evidence of a second planet in a system, although it hasn’t been confirmed.

So even though some of these missions have long since dropped off their packages, they’re still delivering important discoveries about the universe.

Script by Damond Benningfield

Giant spiders crawl around the walls of an ancient city on Mars. And if that doesn’t sound like the plot of a bad movie, then what does? It turns out, though, that this plot is true – in a way. The “city” consists of walls of volcanic rock or hardened sand. And the “spiders” are explosions of dark dust from below the surface.

The region is called Inca City. It was discovered in 1972, in pictures from a Mars orbiter. It’s a grid of intersecting lines that resembles the outline of an ancient Incan city. The walls are miles long and hundreds of feet tall.

Scientists aren’t sure what created the lines. They could be hardened sand dunes. The most recent idea says they’re volcanic rock. Inca City may lie inside an old impact crater. Molten rock could have bubbled up through cracks in the crater floor. The floor was covered up, but the Martian winds have swept it clean, exposing the ridges.

Inca City is near the edge of the southern polar ice cap. Frozen carbon dioxide covers the region in winter. In spring, it vaporizes. Carbon dioxide a few feet below the surface can vaporize first, blowing holes in the ice above it. Dark dust in the plumes then settles to the surface – forming “spiders” around the walls of Inca City.

Look for Mars before and during dawn now. It looks like a bright orange star. Tomorrow, it’s to the upper right of the crescent Moon. The brilliant planet Jupiter stands above them.

Script by Damond Benningfield