StarDate

Landing on the Moon is hard. Early this year, for example, an Israeli lander failed during final descent. And two months ago, a lander and rover launched by India failed just seconds from touchdown.

Since 1972, there have been only four successful landings on the Moon. The most recent came last December, when China dropped a lander and a rover on the lunar farside.

In large part, that’s because of a lack of interest. But the lack of missions is also because even today, landing on the Moon isn’t easy.

And it never has been. Even Apollo, which landed six teams of astronauts on the Moon, had problems. Apollo 11, which made the first landing, had to overcome a lot of glitches as it dropped toward the surface. Apollo 13 had to abort on the way to the Moon. And the mission between them, Apollo 12, almost didn’t even make it to Earth orbit. More about that tomorrow.

Yet interest in the Moon is running high. More than a half-dozen landers are set to head for the Moon in the next two or three years. The first is scheduled for launch as early as December — a Chinese mission that will pick up samples and return them to Earth.

In the meantime, look for the just-past-full Moon tonight. It has a close, bright companion: the star Aldebaran — the eye of the bull.

Script by Damond Benningfield

Gemini is known for its brightest stars, Pollux and Castor. They mark the heads of the mythological twins. The stars climb into view, in the east-northeast, by about 10 o’clock. Castor stands a bit above its brighter “twin.”

Gemini’s third-brightest star is at the bottom of the stick figure that outlines the twins — to the right of the twins as they rise. It’s easy to pick out, even under tonight’s full Moon. It represents one of the feet of Pollux.

Alhena is like many of the stars visible to the unaided eye. For one thing, it consists of two stars, not one. They’re so close together, though, that their light blurs into a single point. And for another, one of its stars is nearing the end of its life, so it’s especially big and bright. It’s this member of the duo that allows us to see Alhena, even though the system is more than a hundred light-years away.

This primary star is several times bigger and heavier than the Sun. It’s also later in life, so it’s puffing up to giant proportions. Right now, it shines more than a hundred times brighter than the Sun. Over millions of years, though, it’ll become even bigger and brighter.

Alhena’s other star is quite similar to the Sun — roughly the same size, brightness, and temperature. Since it’s less massive than its flashier companion, it’ll live billions of years longer. So it’ll still be shining steadily — if inconspicuously — long after the demise of its mate.

Tomorrow: a tough place to reach.

Script by Damond Benningfield

Astronomers use several techniques to discover planets in other star systems. By far the most prolific is the transit method. It looks for a planet to pass in front of a star, causing the star’s light to fade by a tiny bit. Planet-hunting space telescopes have used this technique to discover thousands of exoplanets.

A transit takes place in our own solar system early today. Mercury, the innermost planet, will pass across the face of the Sun. All or most of the transit will be visible across all of the United States except parts of Alaska.

You might wonder if astronomers in other star systems could detect Mercury’s transit. It all depends on the sensitivity of their instruments.

Our transit-hunting telescopes can detect a tiny drop in a star’s light — in many cases, less than a hundredth of a percent. Mercury’s transit will produce a smaller drop than that — about three-thousandths of a percent. So it would take really sensitive instruments to detect such a tiny change.

If they do see the transit, alien astronomers could use it to determine Mercury’s size, its distance from the Sun, and how long it takes to orbit the Sun. And when combined with other techniques, they could learn Mercury’s mass, density, and much more.

We recommend that you watch Mercury’s transit online or at a public venue that’s displaying it. Don’t look at the Sun directly, though — you need eye protection to gaze upon this rare astronomical alignment.

Tomorrow: a “double” foot.

Script by Damond Benningfield

The morning sky offers a couple of treats tomorrow. One is a close conjunction between a bright star and a planet. The other is a transit of a planet across the face of the Sun.

The conjunction is between Mars and Spica, the brightest star of the constellation Virgo. They’ll be low in the east-southeast during early twilight. Spica is the brighter of the two, with orange Mars close to the left.

The transit begins shortly after sunrise for those in the eastern third of the United States. And it’ll already be under way at sunrise for those who are farther west. It takes place as Mercury, the Sun’s closest planet, passes directly between Sun and Earth. Mercury will look like a tiny dot silhouetted against the Sun.

Mercury transits happen 13 or 14 times each century, in either May or November. November transits happen when Mercury is at its closest to the Sun and farthest from Earth. So as seen from Earth, it forms a smaller dot than in May.

The transit will last about five and a half hours. It’ll end shortly after 1 p.m. as seen from the east, and earlier from more westerly time zones.

As with any event involving the Sun, never look at it directly — it could damage your eyes. Instead, use eclipse glasses or other protection. And even with protection, Mercury will be only a tiny dot, and quite hard to see. For the best view, watch online, or visit a planetarium or other site that’s tracking it. Then enjoy the transit — the last until 2032.

Script by Damond Benningfield

When a star is in the prime of life, it can shine steadily for billions of years. As it begins to die, though, it can change in a hurry. A star near the neck of the swan, for example, pulses in and out like a beating heart. That causes its size and brightness to change dramatically.

Chi Cygni is about 600 light-years away, in Cygnus, the swan. The constellation stands high overhead at nightfall.

Chi Cygni is in the final stages of life. It’s burned through the hydrogen in its core to make helium. Now, it’s burning hydrogen and helium in shells around the core.

And that makes the star unstable. Its outer layers fall inward, which makes the surface hotter and brighter. It makes the layers around the core hotter, too. The hotter gas traps energy from the core, which then pushes outward, causing the star to get bigger. But as it does so, it gets cooler, so the star gets smaller. Each pulse takes about 13 and a half months.

At its biggest, the star is almost 350 million miles wider than at its smallest. And at visible wavelengths, it can get more than ten thousand times brighter.

Chi Cygni’s overall brightness doesn’t vary nearly as much, though. At its biggest, the star emits mostly infrared energy, which isn’t visible to the human eye. As it contracts and gets hotter, though, it produces mainly visible light. So at its peak, the star is just visible to the unaided eye. And in between, it’s in-visible — hiding along the neck of the swan.

Script by Damond Benningfield

A tiny black dot will crawl across the face of the Sun on Monday: the planet Mercury. Its passage will be visible across all of the United States except most of Alaska.

Such a passage is known as a transit. It’s possible only because Mercury is closer to the Sun than Earth is, so it regularly passes between Earth and the Sun. Mercury’s orbit is tilted a bit, though, so the little planet doesn’t transit the Sun on every crossing. Instead, transits occur only when the geometry is just right — when Mercury crosses the plane of Earth’s orbit at the right time.

Today, such crossings happen 13 or 14 times per century. They occur only within a few days of May 8th or November 10th. November transits happen when Mercury is farther from Earth, so it makes a smaller dot than during May transits.

All of Monday's transit will be visible from the eastern third of the U.S. For the rest of the country, it’ll be in progress at sunrise.

It begins at about 7:35 a.m. local time on the east coast, and ends at about 1:04 p.m. The times will be one, two, or three hours earlier as you move westward across the time zones.

Never look at the Sun directly, though. You need eclipse glasses or other protection to observe it. Even then, Mercury is so tiny that you probably won’t be able to see it. Instead, watch online, or visit a museum or other venue with safe Sun-watching equipment — for a glimpse of a transit across the face of the Sun.

Script by Damond Benningfield

The starry pictures that decorate the night sky are like a giant mural — they’re a complex canvas that never changes. For most of human history, though, that mural was drawn in different ways. Different cultures connected the starry dots to form their own pictures and tell their own stories. And even cultures that adopted older pictures often rewrote the stories that went with them.

Consider Aquarius, one of the constellations of the zodiac. It stretches from south to southeast at nightfall, with its brightest stars to the right of the Moon.

Today, the constellation represents a young man pouring water from a jug. And that’s been the basic depiction for thousands of years. But the story behind it has changed.

In ancient Babylon, for example, the picture represented the god who ruled over a portion of the Sun’s path across the sky. He held an overflowing water jug that was associated with flooding.

And in Egypt, the constellation was associated with the annual flooding of the Nile River. The story said the Nile overflowed when a boy dipped a jug into its waters.

The Greeks adopted many of the constellations from the Babylonians and the Egyptians, so they kept Aquarius. But they changed the story. One version says the boy was Ganymede, a beautiful young man who was stolen away by Zeus, the king of the gods. Whisked to Olympus, he became the cupbearer of the gods, and was immortalized in the stars — where he remains today.

Tomorrow: Crossing the Sun.

Script by Damond Benningfield

Water covers more than two-thirds of Earth’s surface. And scientists are still debating where it came from. A study earlier this year might help clarify things — at least until the next study comes along.

Earth probably wasn’t born with a lot of water. Conditions were so hot that water was vaporized. And lightweight elements like hydrogen — one of the components of H2O — were blown away from the inner solar system. That didn’t leave a lot of raw material for filling Earth’s oceans.

For a while, the leading idea said the water was deposited well after Earth was born — by icy comets. When astronomers looked at the chemistry of comets, though, they found that cometary water didn’t match the water on Earth.

So that led to a new idea: That Earth’s water came from asteroids. Although they’re made mainly of rock, these giant boulders contain a lot of water. And its composition is a better match for Earth’s water. So asteroid impacts became the new favorites.

But a few months ago, a new study found that comets might be a good source after all. Astronomers looked at a comet that releases a lot of water and gases into space. Its water was a close match to Earth’s. That suggests that most comets hold onto their Earth-like water. They spew water into space only from their surfaces, where the chemistry is different. But when one of them hits Earth, it spatters water from the inside — water that’s a lot like the stuff that fills our planet’s oceans.

Script by Damond Benningfield

When King Harold the Second of England headed toward a showdown with William the Conqueror in the year 1066, many of his people thought he was doomed. A bright comet had blazed across the sky earlier in the year. That was a sign of serious bad luck. And when Harold lost, the comet was depicted on a famous record of the battle — the Bayeux tapestry.

Comets are balls of rock, dirt, and frozen gases. When they get close to the Sun, some of that material fizzes off into space, forming a long tail.

Yet that profile wasn’t understood until recent centuries. Before that, anything in the night sky that changed was thought to be bad news. And because comets were prominent, they were thought of as messengers from the gods — or from demons. They inspired fear, and were considered bad omens.

Halley’s Comet has appeared in the night sky many times. And it’s been quite bright during many of those apparitions. So it’s received a lot of the bad press. It was blamed for earthquakes and the birth of two-headed animals in Switzerland, the Black Death in England, and the fall of the Alamo in Texas. And in 1910, people bought “comet pills” to protect them from its gases.

Comets can be scary — but only if they hit Earth. In 1910, a comet may have flattened hundreds of square miles of Siberian forest when it exploded high in the sky. But such encounters are rare. Most comets are nothing to fear. They’re beautiful objects that just happen to be passing our way.

More tomorrow.

Script by Damond Benningfield

Astronomers have cataloged thousands of comets — balls of rock and ice that sprout long tails when they get close to the Sun. Some of them have been known for centuries. But most of them were discovered only recently.

For a long time, finding comets was one of the main goals of every astronomer. Discovering a comet brought fame, job security, and even medals.

By the 20th century, though, comet-hunting had become a job for the amateurs. They tagged anywhere from a handful to perhaps a dozen or so every year.

Late in the century, though, astronomers began launching automated searches for comets and asteroids. The searches were designed to spot objects that might someday endanger Earth. But they’ve found comets and asteroids all across the solar system.

Most of the asteroids are in a broad band between the orbits of Mars and Jupiter. But the comets come from much farther out, so there aren’t many of them in the inner solar system.

The champion comet-hunter wasn’t designed to look for comets at all. Instead, a satellite named SOHO keeps an eye on the space around the Sun. It’s watching what happens in the Sun’s outer atmosphere.

Comets often swing close to the Sun. That makes them easy pickin’s for SOHO. Some of the comets flare brightly, then retreat to the outer solar system. But others vaporize under the Sun’s intense heat. So many of the comets seen by SOHO vanished almost as soon as they were discovered.

More about comets tomorrow.

Script by Damond Benningfield