StarDate

Spica cruises along with the Moon this evening. The leading light of the constellation Virgo is close below the Moon at nightfall, and sets before midnight.

Astronomers have studied Spica for millennia. In fact, Spica played a starring role in an important discovery about our own planet.

Around 127 B.C., Greek astronomer Hipparchus carefully studied Spica’s location. He noted its position relative to the Sun’s position at the fall equinox. He then compared his observations to those made by other skywatchers a few centuries earlier. And he found something surprising: Spica had shifted a bit. And in fact, other stars he measured showed that same change.

Hipparchus called that change the precession of the equinoxes. The entire celestial sphere — every single star — shifts by a little more than one degree per century.

That isn’t caused by the stars, though. Instead, it’s the result of a “wobble” in Earth’s rotation — like the wobble of a gyroscope that’s beginning to slow down. It takes about 26,000 years to complete a single wobble. During that time, Earth’s north celestial pole draws a big circle on the sky. Different stars along that circle take turns as the pole star.

The wobble also means that every star shifts position relative to the point of the equinox. So over the millennia, the entire celestial sphere rotates through the seasons — a discovery made possible in part by brilliant Spica.
 

Script by Damond Benningfield

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Our “universe” is only part of the universe — all of matter and energy, space and time. That’s because our view of the universe is limited. We see only a tiny sliver of it — a result of the age of the universe, the expansion of the universe, and the speed of light.

The fraction we see is known as the observable universe. And from what we see, it’s all the same. Everywhere we look, there’s the same mixture of galaxies and empty space, of stars and gas clouds. And everything seems to follow the same laws of physics.

But we don’t know what’s happening in the rest of the universe — everything beyond the “cosmic horizon” — the limit of our viewing range.

Logic suggests that everything beyond that horizon is just like everything in the observable universe. There’s no reason that the “bubble” of the universe we inhabit should be any different from the universe as a whole. But we just don’t know for sure — and we probably never will.

Scientists can try to draw some inferences, though. If they observe a galaxy cluster that’s 10 billion light-years away, for example, it has its own observable universe. Some of it overlaps our own, but some extends far outside it. If the cluster is being pulled toward the region we can’t see, then scientists might conclude that it’s been dragged by the gravity of something big and heavy — perhaps a giant galaxy cluster far outside the observable universe — something we’ll never see.
 

Script by Damond Benningfield

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The universe might be infinitely large. And even if it’s not, it’s probably so big that it might as well be infinite. But we see only a tiny fraction of that extent — a bubble known as the observable universe.

What we see is limited by the speed of light — 670 million miles per hour. That’s fast. But it’s not infinite. So light can travel only so far in a year, or a century, or a billion years. Because the universe was born 13.8 billion years ago, we can’t see any farther than light can travel in that time — 13.8 billion light-years. The farthest galaxies and quasars that we’ve seen are almost that far away.

Or they were that far away. We see these objects as they looked at a distance of more than 13 billion light-years. But keep in mind that the universe is expanding. So today, there’s a lot more space between us and those early objects. In fact, today the edge of the observable universe is about 46 billion light-years away.

Assuming Earth were still around, in tens of billions of years, we’d see what those objects look like today. And the limits of the observable universe might eventually reach about 60 billion light-years. But the universe is expanding so fast that we’d never see any farther — limiting us to a bubble of space and time inside a possibly infinite universe.

We’ll have more about the observable universe tomorrow.
 

Script by Damond Benningfield

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Red and green planets huddle close together the next few mornings like bulbs on a strand of Christmas lights. The red one is bright and easy to see, while the green one takes some work.

Orangey red Mars is the brighter of the two. Fainter Uranus is to the upper left at first light tomorrow, and shines pale green. You need binoculars to find it. The planets will change positions over the next few days, but they’ll stay closer together.

Uranus is one of two “ice giants” in the solar system — a category that scientists are still trying to figure out.

Uranus and Neptune are the Sun’s farthest major planets. They’re much bigger and heavier than Earth, but smaller than Jupiter and Saturn.

Until a couple of decades ago, Uranus and Neptune were classified as gas giants, like Jupiter and Saturn. But there are some big differences.

Jupiter and Saturn consist mainly of hydrogen and helium, the lightest of all the elements. Uranus and Neptune, on the other hand, contain mostly heavier elements.

Uranus and Neptune also are put together differently. They have a thin layer of hydrogen and helium at the top, with water, ammonia, and other compounds below. Those compounds are classified as ices, although they actually form a hot, slushy “ocean.”

And scientists aren’t sure where Uranus and Neptune were born. It could have been closer to the Sun, much farther from the Sun, or about where they are today: ice giants far from the Sun.
 

Script by Damond Benningfield

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They may not look it, but the stars at opposite ends of the stick-figure outline of Boötes, the herdsman, are a lot alike.

The star at the bottom is yellow-orange Arcturus, one of the brighter stars in the night sky. The star at the top, Nekkar, looks only a few percent as bright. But that’s only because it’s more than six times farther away — about 235 light-years.

Boötes is in the west as night falls. Its outline resembles a kite or an ice cream cone, with Arcturus at the bottom of the cone, half-the way up the sky. Nekkar — also known as Beta Boötis — is far to its upper right.

Both stars are in a final phase of life. They’ve “burned” all the original hydrogen in their cores, causing their outer layers to puff up. That makes them giants. Arcturus is a little bigger, but Beta Boötis is a little hotter, so their true brightness is about the same.

Two things that aren’t the same are their mass and age. Beta Boötis is only about 300 million years old, versus seven billion years for Arcturus. But Beta is three times heavier than Arcturus. That extra mass made the star consume its hydrogen much more quickly. As a result, it lived a much shorter “normal” lifetime than Arcturus.

The stars face the same fate. Eventually, they’ll blow their outer layers into space. That will leave only their hot but dead cores — white dwarfs — the same fate that awaits the Sun in several billion years.
 

Script by Damond Benningfield

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South Korea is getting ready to launch its first mission to the Moon. Among other things, it’ll scope out lunar resources — things like water ice, aluminum, and silicon — that could support future colonies. But it’ll also look for a resource that no one will be able to use for many decades, if ever.

Helium-3 is a form of the second-most common element in the universe. While “normal” helium has two protons and two neutrons, helium-3 has only one neutron.

Helium-3 could someday be used as a fuel for nuclear fusion. It would produce less radioactive waste than normal helium, and it should “burn” more efficiently.

Here on Earth, it’s quite rare — only about three atoms for every 10,000 atoms of regular helium. But it’s a lot more common on the Moon. That’s because there’s a fair amount of it in the solar wind. The Moon has no magnetic field or atmosphere to stop the solar wind, so its particles embed in the surface.

So some folks have dreamed of pulling the helium-3 from the lunar dirt and bringing it to Earth. And that requires good maps of the highest concentrations of the element.

There are all kinds of caveats, though. You’d have to sift through millions of tons of dirt to get enough helium-3 to be worth it. Shipping it to Earth would be expensive. And working fusion reactors are decades in the future at best. So efforts to find helium-3 are a long-term investment in the future.
 

Script by Damond Benningfield

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South Korea plans to join the club: countries that have sent missions to the Moon. KPLO — Korea Pathfinder Lunar Orbiter — is scheduled to launch as early as next month. If it succeeds, it’ll make South Korea only the seventh nation or group of nations to reach the Moon.

The all-time leader in successful Moon missions is the United States, with the Soviet Union not far behind. In fact, they had a monopoly on the Moon until late in the last century. But in the last 25 years they’ve been joined by China, Japan, India, and Europe.

And the club soon could get even bigger. The United Arab Emirates plans to launch a mission this fall. And Mexico, Australia, Turkey, the UK, and others plan to send craft to the Moon, too.

KPLO is designed mainly as a test flight. It’ll build the technology — in space and on the ground — for more-ambitious missions in the future.

The craft will do some science. It’ll orbit the Moon from pole to pole, at an altitude of about 60 miles. From there, it’ll map the surface to help find good spots for future landers and rovers. And its instruments will look for lunar resources. The list includes frozen water, uranium, silicon, and helium-3, which we’ll talk about tomorrow. Those resources might someday be put to use by South Korea — potentially the next member of the “we’ve-been-to-the-Moon” club.
 

Script by Damond Benningfield

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A mission to an asteroid will have to wait a while. It was scheduled to head for space as early as next week. But a series of technical problems has kept it on the ground for at least another year.

The spacecraft is named for its asteroid target: Psyche. The asteroid is in the asteroid belt, between the orbits of Mars and Jupiter. It’s shaped like a potato, with an average diameter of about 140 miles. It could be a fragment of the core of a small planet that was blasted apart in a long-ago collision.

That idea is based on Psyche’s composition — it contains a lot of iron and nickel. In fact, it’s the largest of all the known metal-rich asteroids. Scientists are especially interested in it because it could have formed in the same way as Earth did, and it could provide a peek into a planetary core that we can’t get here at home.

The Psyche spacecraft is about the size of a Smart Car, with solar panels big enough to cover a tennis court. It’ll use ion power to maneuver — electric thrusters that deliver low thrust, but that can fire for weeks or months without stopping.

Psyche will get a gravitational boost from Mars to help it reach the asteroid. It’ll spend almost two years orbiting the heavy space rock, eventually dropping as close as about 50 miles. It’ll take pictures and measure Psyche’s magnetic field and its composition — giving us a detailed look at a body that could tell us a lot about our own planet.

Script by Damond Benningfield

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In the “Star Trek” universe, most of the good real estate is on “Class M” planets — worlds like Earth, which can support human life. Present-day scientists don’t use that classification — not yet, anyway. But they do classify some asteroids as type M. And NASA is about to send a spacecraft to study the largest of them.

Psyche — named for the Greek goddess of the soul — is in the asteroid belt, between the orbits of Mars and Jupiter. It’s shaped a bit like a potato, with an average diameter of about 140 miles.

An asteroid is a member of type M if it has a high concentration of metals — especially iron and nickel. And that’s true of Psyche. Originally, scientists said it might be almost completely metal. They even estimated the value of that metal at around 10 billion billion dollars. More-recent observations say the proportion of metals could be around half, with much of the asteroid consisting of empty space.

Psyche might once have been part of a small planet. Its gravity was strong enough to melt the interior, so heavy elements — like iron and nickel — settled in the core. Lighter elements floated upward, forming the mantle and crust. Psyche would have been part of the core. A collision could have shattered the original body, blasting Psyche into space.

Studying Psyche could tell us more about how planets with metallic cores formed, especially the most famous class-M planet of all: Earth.

More tomorrow.
 

Script by Damond Benningfield

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The Moon and the planet Venus — the “morning star” — stage a beautiful encounter tomorrow. They’re low in the sky as twilight waxes. But they’re quite bright, so as long as you have a clear horizon, they’re easy to see.

Venus is completely dry. Even so, it has giant “continents” that tower high above the surrounding plains of volcanic rock. One is called Ishtar Terra. It’s about as big as Australia. And it contains the highest spot on the entire planet: a massive mountain range called the Maxwell Mountains.

The Maxwells tower more than seven miles above “sea level” on Venus — the elevation that geologists use for measuring the height or depth of the planet’s features.

What’s even more impressive is that one side of the Maxwell range is quite steep. The mountains climb four miles over a horizontal distance of just six miles — a slope unlike any seen anywhere else in the solar system.

Because of their elevation, the Maxwells are one of the coolest spots on the planet. The average temperature is more than 700 degrees, but that’s about 150 degrees below average. And the atmospheric pressure is about half of that at lower elevations.

Geologists aren’t sure why the Maxwells are so high. They could be squeezed upward as different sections of Venus’s crust mash together. Or they could be pushed upward as molten rock bubbles toward the surface. Either way, the Maxwell Mountains would be quite a challenge to climb.
 

Script by Damond Benningfield

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