StarDate

As missions to the Moon go, Clementine was small, cheap, and low-key. Yet its accomplishments were big and long-lasting. Its maps of the lunar surface are still some of the best around. And scientists have worked long and hard to expand on its discovery of frozen water at the Moon’s poles.

Clementine was launched 25 years ago today. It was designed by the Defense Department to test sensors, batteries, and other technologies. It was targeted for the Moon, with a follow-up trip to an asteroid.

Clementine arrived at the Moon a few weeks later. During more than two months in orbit, it took almost a million pictures of the lunar surface, in multiple wavelengths. Those images produced the first map of the entire lunar surface. And they revealed the composition of the surface.

The probe also measured the elevation of the surface. And by tracking its radio waves, scientists measured the Moon’s “lumpy” gravity. Clementine also bounced radio waves off the poles. A radio telescope on Earth measured the reflected waves, which revealed ice hidden inside craters.

Clementine left lunar orbit in May of 1994 and headed for an asteroid. But a faulty thruster drained away its fuel, bringing its mission to an end.

And its first target has a bright companion the next couple of nights. The star Spica is to the lower right of the Moon as they climb into view after midnight tonight, and closer to the upper right of the Moon tomorrow night.

Script by Damond Benningfield

The disk of our home galaxy, the Milky Way, is enormous. It spans a hundred thousand light-years or more, and contains hundreds of billions of stars. Most of what makes up the galaxy, though, lies outside the disk. All of it is faint and thinly spread, and most of it is invisible.

This region of the galaxy is known as the halo. It appears to be shaped like a ball, with material distributed fairly evenly throughout its volume.

Quite a few stars inhabit the halo, though not nearly as many as in the disk. Almost all of them are extremely old. Hot, bright stars live short lives, so any that were born in the halo have long since expired. That leaves only stars that are less massive than the Sun — stars that are cool and faint. Many of them live in large, massive balls known as globular clusters.

The halo also contains a huge amount of hot gas. Some studies have said the amount may be equal to the mass of the entire disk.

But the halo’s main constituent isn’t visible to any telescope. It’s dark matter. It produces no detectable energy of any kind. We know it’s there, though, because it exerts a gravitational pull on the material we can see. It probably consists of subatomic particles. So far, though, efforts to discover these particles have been fruitless. Even so, dark matter may account for 85 percent of the galaxy’s mass — making the Milky Way a true heavyweight among galaxies.

Script by Damond Benningfield

Textbook views of the Milky Way show a long bar of stars in the middle, with several spiral arms wrapping around it. But that picture is incomplete. And some of it might not be right. In fact, astronomers are still trying to develop a complete and accurate diagram of our home galaxy.

The problem is that they’re trying to map the Milky Way from the inside. It’s like trying to map a million square miles of forest from a single spot deep inside it. Not only can you not see the rest of the forest, you don’t even know how far it extends.

Astronomers began trying to map the Milky Way a few centuries ago. In the late 1700s, for example, William Herschel counted all the stars he could see through his telescope and used their brightnesses to plot their distances. But giant clouds of dust permeate the galaxy. They absorb the light of the stars behind them, making them invisible — hiding large swaths of the Milky Way’s structure.

In the 20th century, astronomers began using radio telescopes to measure clouds of gas, which are abundant in the spiral arms. Radio waves penetrate the dust, so they made it possible to map the galaxy’s overall layout.

Today, new radio surveys provide ever more detailed views of the Milky Way. And astronomers haven’t given up on counting stars. The Gaia spacecraft has mapped more than a billion stars. That provides details on the structure of some of the galaxy — part of the ongoing effort to map our galactic home.

Script by Damond Benningfield

The star Regulus stands just a whisker away from the Moon tonight. They climb into good view by about 8:30 or 9, with the lion’s bright heart to the right of the Moon.

Regulus is a bit more than 79 light-years away. That means the light you see from Regulus tonight actually left the star more than 79 years ago — at the start of World War II. So when a particle of light from Regulus hits your eye, it’s ending a journey of 79 years.

Of course, as with many things astronomical, it’s all relative. For the particle of light itself — a photon — the trip took literally no time at all.

That’s because the photon was traveling at the speed of light — 186,000 miles every second. Nothing can travel faster than that. And only photons can travel at that speed. That’s because photons have no mass — they weigh nothing at all. If anything else were to travel at lightspeed, it would become infinitely massive. So physical objects are limited to speeds just below lightspeed.

As an object moves faster, time appears to slow down for it as viewed by an outside observer — its clock would tick more slowly. So if you could accelerate a starship to just a whisker below lightspeed, it could travel for thousands of years as measured by a clock back on Earth — but just a few years or even less as measured by its own clock.

So as you look at Regulus tonight, remember that the photons are completing a journey of both 79 years — and no time at all.

Script by Damond Benningfield

Gamma Cassiopeia is a busy star system. The main star is surrounding itself with a disk of gas and dust. The star is interacting with an invisible companion. And it’s building up to an impressive demise.

Gamma Cass is the middle point of the letter M or W formed by the stars of Cassiopeia, which is high in the northern sky at nightfall. Gamma Cass is the most distant member of that pattern, at 550 light-years. The main star — the one visible to the eye alone — is between 15 and 20 times the mass of the Sun. And it’s about 35,000 times brighter than the Sun.

The star spins at about a million miles an hour at its equator. That causes it to bulge outward, so it’s shaped more like a lozenge than a ball. That high speed causes the star to fling gas from its surface, forming a disk around the star.

Its companion probably is the corpse of a once-mighty star. Some of the gas from the main star may fall onto this dead companion. As it does so it heats up and produces X-rays.

Gamma Cas is only about eight million years old, yet it’s nearing its end. In a few million years more, it’s likely to explode as a supernova — ending the life of this busy star.

Incidentally, Gamma Cas has a nickname: Navi. It was bestowed in the 1960s by the crew of Apollo 1. It’s the middle name of commander Virgil Ivan Grissom spelled backward. After the crew died in a launchpad fire, NASA placed Navi on the starcharts used by later crews to navigate to the Moon.

Script by Damond Benningfield

In a bit of a coincidence, a total lunar eclipse will shine through American skies for the second January in a row. The entire eclipse will be visible from all of North America, with Hawaii seeing most of the action as well.

Lunar eclipses occur when the full Moon passes through Earth’s long shadow. But the Moon’s orbit is tilted a bit with respect to Earth’s orbit around the Sun. So during most months, the full Moon passes a bit above or below the shadow. When the geometry is just right, though, the Moon plunges through the shadow, darkening its surface.

It doesn’t turn completely dark, though. Instead, sunlight filtering through Earth’s atmosphere casts a deep orange or red glow. The color shows up easily in photographs. But when an eclipse is viewed live, the intensity of the color depends on the viewer’s color vision, weather conditions, and other factors.

This eclipse gets under way at 9:34 p.m. Central Time tonight. That’s when the lunar disk first touches the edge of Earth’s dark inner shadow, known as the umbra. The early portion of the eclipse is only partial, as the Moon moves more deeply into the shadow. It will take about an hour to become fully immersed in the shadow, creating the total eclipse. The Moon will remain fully eclipsed for an hour. It will exit the umbra about an hour after that, allowing the full Moon to return to its full glory.

Tomorrow: A bright, busy star system.

Script by Damond Benningfield

At the ends of their lives, stars like the Sun do a double dip. They puff up to giant proportions, then shrink, then puff up again, even bigger than the first time.

The star at the end of the tail of Draco, the dragon, is in the midst of that second period of puffing up. It’s about 70 times wider than the Sun, and almost 900 times brighter.

Lambda Draconis is low in the north at nightfall right now, with the rest of the dragon stretching to its left. The star wheels high overhead later on.

Long ago, Lambda Draconis converted the original hydrogen in its core to helium. That caused the core to shrink and get hotter. And that caused the first puffing up, as the star’s outer layers were pushed away from the core. The star then burned the helium to make even heavier elements. So for a while, the star shrank and faded.

Now, though, the core has gotten even smaller and hotter, which is causing the star’s outer layers to puff up even more than the first time.

Before long, though, those layers of gas will begin flying out into space, leaving Lambda Draconis behind. That will surround the dying core with a big, colorful bubble of gas — but only for a little while. Within a few tens of thousands of years, the bubble will dissipate, and only the star’s dead core will remain — a hot but tiny stellar ember known as a white dwarf.

The Sun will go through these phases, too — beginning in about five billion years.

Script by Damond Benningfield

We give full Moons all kinds of names. The full Moon of January, for example, traditionally is known as the Old Moon, Wolf Moon, or Moon After Yule. A second full Moon in a month is known as a Blue Moon. And a full Moon that occurs when the Moon is especially close to Earth is a supermoon.

And there’s a supermoon coming on Sunday night. The Moon will be full only half a day before it reaches its closest point to Earth for its current orbit — and its third-closest point for the year.

And it coincides with a total lunar eclipse. The Moon will pass through Earth’s long shadow. That will make it appear dark orange or red. The entire eclipse will be visible across all of North America, beginning Sunday evening.

The Moon’s orbit around Earth is lopsided, so the Moon’s distance varies by about 30,000 miles. Its closest approach is known as perigee. And the Moon will reach that point on Monday — about 222,000 miles from Earth.

Perigee doesn’t always occur at full Moon. Instead, on average it shifts by a couple of days per cycle. So it can occur when the Moon is full, new, or any phase in between.

The way “supermoon” is used varies. Some use it to mean the closest full Moon of the year — which will occur next month. Others use it for full Moons that are within a certain distance of Earth.

However it’s used, enjoy the supermoon — one that’ll be fully eclipsed — an event that’s come to have its own name: a Blood Moon.

Script by Damond Benningfield

Glaciers keep secrets. They hide rivers, hills, and other surface features. They conceal the bodies of animals that roamed the landscape tens of thousands of years ago, and enfold the debris of crashed airplanes. And a glacier in Greenland has hidden evidence of a cosmic impact — a 20-mile-wide crater blasted out by an asteroid.

Scientists used several lines of evidence to discover the crater, which is below the Hiawatha glacier on Greenland’s northwestern coast. They used radar scans made by airplanes to see through the ice to the ground below.

Those observations revealed a circular basin below the glacier. A peak in its middle rises 15 stories above the basin floor — a common feature in impact craters.

Scientists then collected material that had been washed from beneath the glacier by a river. The samples contained beads of glass that were forged in extremely high temperatures. They also contained quartz crystals that had been squeezed by extremely high pressure.

The evidence points to an impact by an asteroid about a mile in diameter. When it hit, it released as much energy as 700,000 one-megaton nuclear bombs. It would have obliterated any life within a wide range, and produced lesser effects hundreds of miles away. And after effects would have lasted for centuries.

The impact probably happened within the last hundred thousand years, although the exact date is unclear. The glacier will keep that secret for a little while longer.

Script by Damond Benningfield

The solar system gets the new year off to a beautiful start tomorrow. The three brightest objects in the night sky will team up before and during dawn: the Moon and the planets Venus and Jupiter.

Venus is the brilliant “morning star,” and it stands close to the lower left of the Moon. Jupiter stands well to the lower left of Venus, not far above the horizon at first light.

Other than the fact that both of them are planets, Venus and Jupiter don’t have very much in common. Venus is a ball of rock a little smaller than Earth, while Jupiter is a ball of gas that’s about 11 times wider than Earth.

One thing they have in common is that both are topped by layers of bright clouds — one reason that both planets look bright. And the clouds on both of them produce lightning.

Spacecraft have heard the “crackle” of lightning in radio waves from both planets. And on Jupiter, craft have actually seen the lightning.

Jupiter’s lightning is produced in the same way as on Earth — in giant thunderstorms. Plumes of water vapor rise high into the atmosphere, where they condense to form heavy clouds.

On Venus, the clouds are made of sulfuric acid. The droplets of acid can generate an electric charge, producing lightning. The bolts probably travel between clouds, not down to the ground. But there could be lightning at the ground, too, around active volcanoes — lighting up the surface of our closest planetary neighbor.

Script by Damond Benningfield