Afleveringen
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Future spelunkers might want to explore caves on the Moon and Mars. Scientists have mapped many cave openings on both worlds. On the Moon, caves could provide shelter from radiation and meteorites for astronauts. On Mars, they could provide shelter for microscopic life that was born on the planet itself.
Orbiting spacecraft have photographed some likely cave entrances on both worlds. They’re big holes in the ground. They may lead to larger chambers on the sides.
On the Moon, the caves probably were excavated by lava flowing below the surface. After the lava disappeared, parts of the empty tubes they left behind caved in, providing the openings. The side chambers could be good places to set up lunar habitats.
Most of the caves on Mars probably formed the same way. But a recent study found eight caves that might have been carved by water. The caves were seen in a region that’s marked by deep channels that carried water in the distant past. The surface water dried up long ago. But the caves could lead to buried pools of ice. If life ever evolved on Mars, it might have survived in those damp locations. So the caves could be a good place to check for life on the Red Planet.
The Moon and Mars line up with the star Aldebaran in tomorrow’s dawn sky. Mars looks like a bright star to the lower right of the Moon. Aldebaran is an even brighter star, about the same distance to the lower right of Mars.
Script by Damond Benningfield
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When we gaze into the night sky, it’s like looking at a projection on a giant dome – we see two-dimensional pictures, with no perception of depth. Even astronomers can have a hard time plotting that third dimension. And that can skew their understanding of how stars work.
Consider the Coathanger – a pattern of 10 stars that really does resemble a coat hanger. It’s in the constellation Vulpecula, the fox.
For decades, astronomers thought those stars formed a cluster. A cluster’s stars are all the same age and same distance, and they formed from the same ingredients. But some of the stars in a cluster are small and light, while others are big and heavy. Seeing how the different weight classes have evolved helps astronomers understand how all stars age.
But a study in 1970 found that only a few of the Coathanger’s stars were related. And a later study, which used a satellite to plot the distances to stars, found that none of them are related – they just happen to line up in the same direction. So plotting the third dimension robbed the Coathanger of some of its scientific value – but none of its beauty.
The Coathanger is a great target for binoculars. Sweep them from the bright star Altair, which is low in the east at nightfall, toward even brighter Vega, far to its upper left. The Coathanger is about a third of the way along that line – a beautiful grouping that’s not really a group at all.
Script by Damond Benningfield
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Zijn er afleveringen die ontbreken?
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61 Cygni has two distinctions. It moves across the sky faster than all but about a half-dozen other stars. And it was the first star to have its distance accurately measured.
The system is in Cygnus, the swan. The constellation is high in the east at nightfall, marked by Deneb, the swan’s bright tail. 61 Cygni is to the lower right of Deneb. Under dark skies, it’s just visible to the eye alone.
61 Cygni consists of two stars. Both are smaller, lighter, and cooler than the Sun, and much fainter. They orbit each other once every 650 years or so.
In 1804, Giuseppe Piazzi discovered that the system moves across the sky in a hurry. That suggested that the system is close by. So astronomers started trying to figure out just how close.
They looked at the star at intervals of six months, when Earth was on opposite sides of the Sun. That slight change in perspective causes nearby stars to move a bit compared to stars that are farther away.
Early attempts to measure that angle didn’t work – the equipment just wasn’t good enough. But in 1837 and ’38, Friedrich Bessel used a new instrument that provided a sharper view. It told him that 61 Cygni was 10.4 light-years away. That’s just one light-year off the true distance. So Bessel’s work provided the first good measurement of the distance to any star other than the Sun.
Script by Damond Benningfield
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The “evening star” nuzzles the lion the next few nights. Venus will pass quite close to Regulus, Leo’s brightest star. At their closest, they’ll be just one degree apart – the width of a pencil held at arm’s length.
Despite their proximity in the sky, Venus and Regulus are nowhere close to each other in reality – they’re separated by many trillions of miles.
Venus is a planet in our own solar system. Right now, it’s 92 million miles away. That’s about the average distance to the Sun – a distance known as the astronomical unit. It’s the basic “yardstick” for measuring the solar system. It’s a lot more convenient than miles or kilometers – a lot fewer zeroes to worry about.
But it’s not a great yardstick for measuring the distances between stars. Regulus, for example, is more than five million astronomical units from us – five million times farther than Venus. Yet it’s one of our closer neighbors.
So there are two other units for measuring those distances. The most common is the light-year – the distance light travels in one year – almost six trillion miles. And astronomers generally use parsecs; one parsec is three and a quarter light-years. So Regulus is about 24 parsecs from Earth.
Regulus is close to the upper left of Venus at nightfall this evening. Venus will slide past the star over the next two nights. They’ll be closest together on Wednesday – but close only in appearance.
Script by Damond Benningfield
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Saturn is almost 10 times farther from the Sun than Earth is. At that distance, the Sun looks only about one percent as bright as it does from Earth, so
Saturn is cold and dark. But it’s not quite as cold as you might expect. Saturn actually puts out more than twice as much energy as it receives – heat radiating from deep inside the planet.
Saturn is made mostly of hydrogen and helium – the lightest and simplest chemical elements. But its core contains a lot of rock, metal, and other heavy materials – more than 15 times the total mass of Earth. Gravity squeezes the core tightly, causing it to shrink. That produces heat, which rises to the surface and shines out into space.
But that doesn’t explain all of Saturn’s heat. The rest may come from an odd type of rainfall. Droplets of liquid helium may fall toward the core. As they fall, they produce friction, which produces heat.
Saturn’s internal heat drives much of the weather in its atmosphere, including storms that can be as big as continents – swirling clouds on a cold, dark planet.
Despite the lack of sunlight, Saturn looks bright in our sky because it’s huge – about nine times the diameter of Earth. It’s easy to see how bright the next couple of early mornings because it’s close to the Moon. Tomorrow, it’s to the lower left of the Moon at dawn. It’ll be a little farther to the right of the Moon on Wednesday.
Script by Damond Benningfield
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Earth will reach its farthest point from the Sun for the entire year around midday tomorrow. We’ll be about 3.1 million miles farther than we were at closet approach, in early January.
That far point is known as aphelion or ap-helion. The term comes from a mash-up of Latin and Greek. “Ap” is from a Latin word that means “far away,” while “helion” is a Greek word for the Sun.
The changing distance is a result of the shape of Earth’s orbit. Instead of a nice, round circle, it’s an ellipse – like a lopsided circle.
Over tens of thousands of years, the shape changes – the result of the gravitational influence of the Sun, Moon, and planets. The orbit seesaws between being a little more circular and a little more lopsided. Right now, we’re toward the more-circular end of the seesaw.
Today, aphelion always comes about two weeks after the summer solstice. But that won’t always be the case. Thanks to a slow wobble in Earth’s rotation, aphelion shifts an average of one day later every 58 years. So it occurred on the solstice about 900 years ago. And about 4600 years from now, it’ll occur on the fall equinox, in September.
Incidentally, the lopsided orbit has a big impact on the length of the seasons. Our planet moves slowest when it’s farthest from the Sun. That makes summer in the northern hemisphere almost five days longer than winter.
Script by Damond Benningfield
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Bellatrix, Spica, and Omega Herculis have something in common. They’re all good “semiquincentennial” stars. That has nothing to do with the stars themselves – only their distance. All three of them are roughly 250 light-years away. In other words, we see them as they looked roughly 250 years ago – around the time of the American Declaration of Independence.
One light-year is the distance light travels in a year – almost six trillion miles. So each of the anniversary stars is about 1.5 quadrillion miles away – the number 15 followed by 14 zeroes.
The distances were measured with a technique known as parallax. Astronomers look at a star when Earth is on opposite sides of the Sun. That causes a tiny shift in the star’s position against the background of more-distant objects. The size of that shift reveals the star’s distance. But there’s a slight margin of error, so the distances might not be exact.
The best measurements say that Bellatrix is a little more than 250 light-years away. Omega Herc is a little less. Spica is in the middle – almost exactly 250 light-years from Earth. So we see it as it looked around the year 1776.
Spica is the brightest member of that trio. It’s low in the southwest as night falls. Earth won’t see the light it emits tonight until the American quincentennial – 250 years from now.
Script by Damond Benningfield
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The next big meteor shower is weeks away – at least, here on Earth. But some predictions say that a big shower could take place this weekend on the planet Venus – a result of the long-ago break-up of an asteroid.
Regardless of where it takes place, a meteor shower happens when a planet flies through the orbital path of a comet or asteroid. Bits of rock and dust shed by the small body ram into the planet’s atmosphere at high speed. They heat up and vaporize, forming the glowing streaks known as meteors.
In this case, the source of the particles could be two asteroids. They have a similar composition, and they follow similar paths around the Sun.
A team of European astronomers recently plotted the orbits of the two bodies a hundred-thousand years into the past. The study found that, about 20,000 years ago, both asteroids passed especially close to the Sun many times. And the asteroids were so close to each other that they could have been one larger asteroid.
Warmed by the Sun, the surface of the asteroid could have cracked and splintered. The way the asteroid heated up could have caused it to spin faster. The weakened asteroid then split apart, spewing debris into space. Venus periodically slips through this trail of debris – perhaps triggering a meteor shower.
From Earth, only the brightest of those meteors might be visible through a telescope – streaking through Venusian skies.
Script by Damond Benningfield
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The planets Mars and Uranus will stage an especially close encounter the next couple of mornings. Uranus is quite faint, but its proximity to Mars can help you pick it out.
Uranus is a giant – about four times the diameter of Earth. But it’s so far away that it’s faint. Under especially dark skies, people with good eyesight can just make it out. Moonlight or light pollution mask it from view, so you need binoculars to find it.
And even then, it looks like a meager star, perhaps with a hint of blue-green. Methane in its upper atmosphere absorbs red light, so only the blue and green can reach us.
Mars is much smaller – only a bit more than half of Earth’s diameter. But it’s also much closer, which makes it easier to see.
Even with the eye alone, it’s no problem to make out the planet’s orange color. That’s produced by iron oxide in the rocks and the tiny dust grains that coat much of the surface. That color will become easier to make out over the coming months, as Earth and Mars get closer and closer.
For now, look for Mars low in the east-northeast beginning a little before dawn. It’s quite easy to make out. Tomorrow, Uranus will stand a little to the lower left of Mars, so both of them will fit in a binocular field of view. And Uranus will be even closer above Mars on Saturday – a faint giant appearing to almost touch the Red Planet.
We’ll talk about a much brighter planet tomorrow.
Script by Damond Benningfield
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A Japanese spacecraft is scheduled to pay a call on an asteroid this weekend. The encounter won’t last long – the craft will buzz by at about 12,000 miles per hour.
This is the second asteroid encounter for Hayabusa2, which launched in 2014. Its main mission was a detailed study of the asteroid Ryugu. It orbited the asteroid for a year and a half. It gathered a few grams of dust and pebbles and dropped them off at Earth in late 2020. Studies have shown that the samples contain all the key ingredients of DNA – the building blocks of life.
Hayabusa then continued its trek. And this weekend, it’ll pass by the asteroid Torifune – a chunk of rock about a quarter of a mile in diameter.
Torifune’s average distance from the Sun is just a fraction farther than Earth’s distance. But its path is lopsided, so it regularly crosses Earth’s orbit. It’s not currently a threat to hit our planet. But it could be sometime in the distant future.
Right now, the asteroid is at its closest point to the Sun, and about 60 million miles from Earth. Hayabusa will scan it with several instruments as it swings by. But the high speed makes that tough. The entire spacecraft has to turn to keep the asteroid in view. And it can’t turn fast enough to keep an eye on it through the entire encounter. So it’ll have to settle for a quick glimpse as it blazes by this potentially hazardous asteroid.
Script by Damond Benningfield
