StarDate

The Moon probably was born in a “big whack” – a massive collision between the young Earth and another planet. Scientists are still working out the details of how that worked, but they mostly agree on the overall picture.

But it took a long time to reach that conclusion. For much of the 20th century, in fact, scientists debated three other possible scenarios for the Moon’s birth.

One said that Earth and Moon were born together – they both coalesced from the disk of gas and dust that encircled the newborn Sun. But the motions of the Earth-Moon system seemed too high for such a formation. And there are big differences in how Earth and the Moon are put together – Earth has a massive iron core, while the Moon has a small core.

Another scenario said that Earth captured the Moon as it wandered close by. But that doesn’t work well, either. It’s hard for one body to snag another, and especially hard when the two objects have such a similar mass.

The most popular idea said the Moon was a spin-off – the young, molten Earth was spinning so fast that it flung off a big blob of itself. But the physics of such a formation are pretty daunting as well.

It wasn’t until the 1980s that scientists began to settle on the current model: the Moon was born in a big whack.Look for the crescent Moon quite low in the southwest as twilight fades this evening. The brilliant planet Venus – the evening star” – stands close by.

Script by Damond Benningfield

Two years ago, a NASA spacecraft gave a small asteroid a big whack. And two years from now, a European craft will study the aftermath of that impact. It should reveal how the asteroid has changed, and why the impact was so effective.

The impact took place in September of 2022. A projectile as big as a van slammed into Dimorphos, the smaller member of a binary asteroid. It’s about 500 feet across, compared to half a mile for the larger member, Didymos.

The original mission was a test of a way to deflect an asteroid on a collision course with Earth. And the test worked – it changed the smaller asteroid’s orbit by much more than expected.

But to fully evaluate the results, scientists need to know more about the asteroid, and about what happened to it after the test. And that’s the job for Hera, the European mission. It’ll study the asteroid’s composition, and see how the impact changed its shape. Early studies of the impact say Dimorphos is a “rubble pile” – chunks of rock barely held together by gravity. It could have formed when material spun off the surface of Didymos, then stuck together.

Hera is scheduled for launch this month. It’ll fly close to Mars in March, using the planet’s gravity to get a “kick” toward its target. Hera should enter orbit around the asteroids in two years. And it will deploy two small sub-satellites – one to study each member of the binary asteroid.

Script by Damond Benningfield

Our home galaxy, the Milky Way, doesn’t have enough satellites – smaller galaxies in orbit around it. Unless it has too many. That’s the conundrum facing the scientists who study how galaxies are born.

So far, astronomers have discovered about 60 galaxies orbiting the Milky Way. Most of them are much smaller than the Milky Way. That makes them faint and hard to find. But models of galaxy formation say the Milky Way should have more than 200 satellites. The difference has been called the “missing satellite problem.”

The models are based on ideas about dark matter. It appears to make up about 85 percent of all the matter in the universe. But it doesn’t produce any energy that we can see. It reveals its presence only through its gravitational pull on the visible matter around it.

In the early universe, dark matter should have congregated in big clumps. They pulled in normal matter, which formed stars and galaxies. Smaller clumps should have given birth to smaller galaxies around the big ones. But the number of smaller galaxies hasn’t matched those predictions.

A recent search of a small patch of sky discovered several new small galaxies. Based on that, researchers suggested the Milky Way could have about 500 satellites.

Many of these little guys could have been absorbed by the Milky Way over the eons. Or perhaps hundreds of them still await discovery – tiny galaxies orbiting the giant Milky Way.

Script by Damond Benningfield

Scientists have been playing a game of hide-and-seek for decades. They’re looking for most of the mass of the universe, which is hidden from view. So far, they haven’t found it. So they keep widening the search – coming up with new explanations for what the missing mass might be, and how they might find it.

Their quarry is dark matter, which appears to account for about 85 percent of all the matter in the universe. It doesn’t produce any energy of its own, and it almost never interacts with “normal” matter. But astronomers “see” its presence through its gravitational pull on the visible matter around it.

For a long time, the best explanation seemed to be some type of especially heavy particle. But experiments that look for it have come up empty.

So scientists have devised other possible explanations. One is an especially lightweight particle – as little as a billion billionth of the mass of the lightest bit of normal matter. Another is primordial black holes – tiny but heavy objects created in the Big Bang. And one of the wildest explanations is “tachyons” – hypothetical particles that would move faster than light.

Some scientists argue that dark matter doesn’t exist at all. One team, for example, suggests the universe is much older than the currently accepted 13.8 billion years. If so, then dark matter wouldn’t be needed – and neither would the game of hide-and-seek.

More about dark matter tomorrow.

Script by Damond Benningfield

Sometimes, nature is kind and generous. In April, for example, it produced a total solar eclipse that was seen by tens of millions.

At other times, though, nature is more impish. An example of that comes up tomorrow. There’s another solar eclipse. But this time, the Moon will be farthest from Earth for the entire year. So the Sun will encircle the Moon with a wide ring of fire – an annular eclipse. And not many people will see it – almost its entire path is over the Pacific Ocean.

During an annular eclipse, the Moon lines up directly in front of the Sun, as it does during a total eclipse. But the Moon is farther from Earth than average, so it can’t completely cover the Sun. Tomorrow’s eclipse peaks less than an hour before the Moon reaches its greatest distance from Earth for the entire year – more than 252,500 miles – almost 14,000 miles farther than the average distance. So the Moon will block no more than 87 percent of the Sun’s disk.

Even so, it should be a great sight for anyone along the path – as long as they have the proper eye protection.

The eclipse begins south of Hawaii, then sweeps toward the southeast. It nicks the tip of South America, then ends over the South Atlantic. Hawaii will see a partial eclipse, beginning at sunrise. And North America? Well, nature’s not as generous this time. None of the eclipse will be visible from anywhere on the continent.

Script by Damond Benningfield

This is the time of year when we start thinking about cold snaps – frontal systems that cause sudden drops in temperature, and often bring clouds.

A recent study suggests that our entire planet went through a major cold snap a few million years ago. It was caused by a giant cloud – a dense clump of material between the stars.

The space between stars contains many clouds of gas and dust. In fact, we’re inside one right now – the Local Interstellar Cloud. But it’s quite thin, so it doesn’t have much impact on the solar system.

The study says we might have passed through a much thicker cloud two million to three million years ago. Its material could have been 15 thousand times denser than the current cloud.

That could have drastically squeezed the magnetic “bubble” generated by the Sun. Today, the bubble extends more than a hundred times the distance from Earth to the Sun. But in the denser cloud, it could have been squeezed to just a quarter of the Earth-Sun distance.

Since Earth would have been outside the Sun’s protective bubble, it would have been bombarded by more cosmic rays. And dust from the interstellar cloud might have settled into our atmosphere.

The researchers say that could have had an impact on Earth’s climate – perhaps making it much cooler. That could have killed off some life, and perhaps altered the development of human ancestors – possible impacts of an interstellar cold snap.

Script by Damond Benningfield

Royalty creeps across the southern sky on autumn nights: Fomalhaut, the brightest star of Piscis Austrinus, the southern fish. To the people of ancient Persia, it was one of four “royal” stars, along with Aldebaran, Regulus, and Antares. Each star ruled the sky during a different season. For Fomalhaut, it was the autumn sky.

Fomalhaut probably was a pretty easy choice. It’s the only bright star in a large region of the sky. And at this time of year, it’s in view pretty much all night.

Fomalhaut also was considered the protector of the winter solstice. About 5,000 years ago, the star aligned in the same direction as the Sun at noon on the solstice.

The Fomalhaut system appears to consist of at least three stars. The brightest is Fomalhaut A – the one that’s visible to the unaided eye. The star is about twice as big and heavy as the Sun, and quite a bit brighter. It’s about 25 light-years away.

A giant disk of dust encircles the star. For a few years, it looked like there might be a planet inside the disk. Astronomers even gave it a name: Dagon. But it turned out to be just a big clump of dust – the likely debris from a collision between two big asteroids.

Fomalhaut stands quite low in the southeast as the sky gets good and dark, and due south around midnight. Don’t confuse it with brighter Saturn, well to its upper left. Fomalhaut is in view all night – a royal star for autumn nights.

Script by Damond Benningfield

The Moon is a thin crescent in the dawn sky tomorrow. The Sun lights up only about one-tenth of the lunar hemisphere that faces our way. That’s the part of the Moon where it’s daylight. It’s nighttime across the rest of the disk.

But that doesn’t mean you can’t see that part of the Moon. It forms a ghostly presence, illuminated by earthshine – sunlight reflecting off of Earth. And the earthshine is pretty bright now, because Earth would appear 90 percent “full.” Because our planet is much bigger and more reflective than the Moon, it shines many times brighter than a full Moon appears from Earth. Tomorrow, the earthshine will be so intense that you can easily make out lunar features through a pair of binoculars.

And because the same side of the Moon always faces Earth, our planet sticks around in the same spot in the lunar sky, day and night. So future Earthwatchers will see it go through an entire cycle of phases every 29 and a half days, just as the Moon does. But from any given spot on the Moon, Earth won’t move – it’ll be “locked” at the same point in the sky.

Look for the crescent Moon in the east beginning an hour or two before dawn. It has a bright companion: Regulus, the star that marks the heart of the lion. It stands close to the right or upper right of the Moon. The Moon will be an even thinner crescent on Monday, but still easy to make out – thanks to the light of the silvery Earth.

Script by Damond Benningfield

If you’re a super-advanced civilization and you’re running out of energy, there’s a simple solution: Dismantle part of your planetary system to build “megastructures” around your star. Known as Dyson Spheres, they’d gather most of the star’s energy, giving you an almost limitless supply.

A recent study has found possible evidence of them. Most or all of the candidate objects are likely to have a natural origin: distant galaxies, or clouds of debris around a star. But astronomers are following up just in case.

The “spheres” wouldn’t completely surround a star. Instead, they’d consist of many huge panels in separate orbits around the star. They’d emit heat, so they’d glow in the infrared. So astronomers have been looking for the right kind of infrared glow for years.

In the recent study, a team scoured observations of about five million stars within a thousand light-years of Earth. They used many filters to narrow the list. And they came up with seven candidates. All of them are red dwarfs – stars that are much smaller, cooler, and fainter than the Sun. Such stars live a long time – long enough for a civilization to develop and grow – and surround the star with a Dyson Sphere.

Two of the candidate stars are near the “head” of one of the fish of Pisces. The figure is low in the east at nightfall, to the left of the bright planet Saturn. The candidates are much too faint to see without a telescope.

Script by Damond Benningfield

The Milky Way Galaxy is like a big blender filled with many ingredients. But it hasn’t been blended enough for all the ingredients to completely stir in. So it’s possible to make some of them out – to see some of the raw ingredients that created the galaxy.

Astronomers recently reported the discovery of what could be two of the earliest ingredients – ribbons of stars that could be the remnants of two small galaxies. If so, they were stirred into the Milky Way about 12 billion years ago, when the galaxy was just taking shape.

The possible remnants were named Shiva and Shakti, after a Hindu god and goddess. They were discovered by Gaia, a space telescope that’s measuring the composition and motion of more than a billion stars.

Each remnant consists of a ribbon of stars that spans thousands of light-years. The stars in each ribbon appear to move through the galaxy together. And they all have a similar make-up. That suggests they were born together – as members of small galaxies.

The larger Milky Way pulled those galaxies in, then ripped them apart. Today, most of their stars have been stirred in with the rest of the Milky Way. But a few may hold on to some of their original identity – some of the earliest building blocks of the Milky Way.

If you have dark skies, look for the glowing band of the Milky Way arching high overhead as night falls – a giant blender that’s not completely stirred up.

Script by Damond Benningfield