StarDate

We’ve had an unbroken presence at Mars for a quarter of a century. It started with Mars Global Surveyor, which began settling into orbit 25 years ago today.

Before Global Surveyor, it had been 15 years since there was a fully successful Mars probe. The United States and Russia both launched missions during the interim, but they all failed.

Global Surveyor orbited Mars from pole to pole. It spent a year and a half dipping deeper into the upper atmosphere to sculpt its orbit. So it began full science operations in early 1999.

The craft mapped all of Mars many times over. That allowed scientists to study changes on the surface caused by winds, meteorite impacts, and outbursts of water or gases from below the surface.

It also watched the weather. Its pictures showed frontal systems, dust storms, and cloud formations. And the probe measured Mars’s gravitational and magnetic fields, revealing details about its interior.

Global Surveyor’s original mission called for one Mars year of observations — about two Earth years. It was given three extensions, each for another Mars year. But on November 2nd, 2006, it went silent. The craft had received conflicting commands from Earth, which knocked it out. Even so, Global Surveyor survived longer than any Mars craft ever had — beginning an unbroken presence at Mars that seems likely to continue for years.

More about Mars tomorrow.
 

Script by Damond Benningfield

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When you look into the night sky, it’s a lot like watching some “live” TV and radio programs: there’s a slight delay. TV and radio do it in part to comply with federal law — it allows them to screen out certain offensive material. The night sky does it to comply with the laws of physics.

Consider the Moon and Jupiter, the solar system’s largest planet, which keep company all night. Jupiter looks like a brilliant star. It’s to the left of the Moon in early evening, and closer above the Moon at first light.

Because of the time lag, we don’t see the Moon and Jupiter as they looked at the same moment. Instead, the light from Jupiter takes more than a half hour longer to reach us.

That’s all because of the speed of light, which is limited. It’s a very high limit — about 670 million miles per hour. In fact, nothing in the universe can go faster.

But even at that incredible speed, it takes time for light to cross the great distances between worlds. The Moon is closer to us than any other astronomical object — less than a quarter of a million miles away. So light from its surface takes about one and a quarter seconds to reach Earth.

Jupiter is a lot farther — about 370 million miles right now. So its light takes about 33 minutes to reach us. In other words, when you look at Jupiter, you’re seeing the giant planet as it looked about 33 minutes earlier — a time delay mandated by the laws of physics.
 

Script by Damond Benningfield

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The most famous of all full Moons lights up the sky tonight: the Harvest Moon. It keeps the fields bright, allowing farmers to continue harvesting crops well into the night.

There’s not a complete agreement on how to reckon the date of the Harvest Moon. One system is simple: It’s the full Moon of September. But that doesn’t necessarily match the best time of the month for harvesting. If the full Moon comes in the first few days of the month, it might be too early to really help. And occasionally, there can be two full Moons in September — on the 1st and 30th.

So the most common system says the Harvest Moon is the full Moon closest to the autumnal equinox. That means the Harvest Moon can come as early as about September 6th, or as late as about October 7th.

This year, the equinox takes place on September 22nd. That’s 12 and a half days after September’s full Moon, and 17 days before October’s. So Harvest Moon honors go to September.

There’s a common perception that the Harvest Moon is especially bright, but that’s not the case. Brightness is determined by the Moon’s distance. But the distance isn’t synched up with the Moon’s phases. This year, the Moon was closest for the month just a couple of days ago — about 12,000 miles closer than average. It’s only a couple of thousand miles farther tonight, so the Moon will be a little brighter than average — an especially bright Harvest Moon.
 

Script by Damond Benningfield

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The star system TV Columbae has perplexed astronomers for decades. It flares up, getting about 30 times brighter than average. After a few hours, it returns to normal. And that cycle has been hard to explain. But recent studies have offered a possible solution: a micronova.

TV Columbae is more than 1600 light-years away. It’s in the constellation Columba, the dove, which is too far south to see from most of the U.S.

The system consists of two stars. One is in the prime of life. The other is a white dwarf — the corpse of a once-normal star. It’s heavy but tiny — no bigger than Earth. The two stars may be less than a million miles apart. That’s close enough for the powerful gravity of the white dwarf to “steal” gas from its companion. The gas then piles up on the surface of the white dwarf.

The same thing happens in many other star systems. In those cases, the gas spreads across the entire surface of the white dwarf. And if the gas gets hot enough, it triggers a powerful explosion — a nova. That can make the system shine many thousands of times brighter than average.

The new studies suggest that something similar happens on TV Columbae and a few other stars. Instead of spreading across the entire white dwarf, though, the gas is concentrated in columns at the stars’ magnetic poles. The gas gets hot enough to explode, but there’s much less of it. So that triggers a smaller but still-mighty eruption: a micronova.
 

Script by Damond Benningfield

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Big space missions require big patience. A high-priority mission to one of the moons of Saturn, for example, won’t arrive for more than a quarter of a century — if it gets there at all.

Enceladus Orbilander received a high ranking in a blueprint of NASA science for the coming decade. It rated after a Mars sample return mission and a trip to the giant planet Uranus.

It ranked highly because Enceladus is considered a possible home for life. Geysers of water and ice erupt from cracks near the south pole. The water comes from a global ocean of liquid water below the moon’s icy crust. The ocean appears to have all the ingredients for microscopic life.

Orbilander would orbit Saturn for almost five years, gradually maneuvering into orbit around Enceladus. It would fly through the geysers, looking for signs of life. After a year and a half in orbit, it would land near the south pole. It then would spend a couple of years measuring material from the geysers that was falling to the ground.

Orbilander would cost $5 billion or more, so it’ll take some time to get it all together. At the earliest, it would launch in the 2030s, arrive at Saturn around 2045, and land in the 2050s — a goal that’s going to take some patience.

Saturn itself is close to the Moon tonight. It looks like a bright star to the upper left of the Moon at nightfall, and closer above the Moon as they set in the wee hours of the morning.
 

Script by Damond Benningfield

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A high-flying observatory is about to be grounded. SOFIA — the Stratospheric Observatory For Infrared Astronomy — has been operating for a decade. But its work no longer matches NASA’s science goals, so it’ll be shut down by the end of the month.

SOFIA consists of a Boeing 747 with a door in its side. At high altitude, the door opens to reveal a large infrared telescope. Water vapor in Earth’s lower atmosphere absorbs infrared light. At an altitude of seven or eight miles, though, the telescope is above 99 percent of the water vapor, providing a clear view of the infrared sky. And the observatory can travel around the world to view interesting targets.

Infrared wavelengths are especially useful for studying fairly cool objects: planets and comets, clouds of interstellar gas and dust, and the cocoons around young stars. SOFIA studied all of those objects and more.

The project has had a troubled history, though. It ran years behind schedule and tens of millions of dollars over budget. NASA threatened to cancel it. But the agency decided the science was worth the extra cost. SOFIA began full science operations in 2014.

But a review panel recently decided that the observatory’s returns could no longer justify its budget. In addition, SOFIA’s capabilities didn’t match up with exploration plans for the next decade. So NASA and Germany, a partner in the project, decided to retire SOFIA — with no plans to replace it.
 

Script by Damond Benningfield

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The celestial teapot will lose part of its handle tonight — one of its stars will be hidden behind the Moon. The disappearing act will be visible across most of the United States.

The event is known as an occultation — from a Latin word that means “to hide.” Such an event is possible because the star is close to the ecliptic — the Sun’s path across the sky. The Moon stays close to the ecliptic as well. So when the geometry is just right, the Moon crosses in front of the star, so the star winks out.

Astronomers can learn a lot about a star system by watching an occultation. If they know the star’s distance, then the amount of time it takes for the star to vanish can reveal its size. And if the light drops in steps, then it means the system has two stars or more.

Tonight’s occultation is of Tau Sagittarii. It’s at the lower corner of the handle of the teapot outlined by some of the bright stars of Sagittarius. The star is about 120 light-years away. It’s past the end of its “normal” lifetime, so it’s bigger and cooler than the Sun, and much brighter. But we don’t know what astronomers might learn about it by watching tonight’s occultation — the disappearance of a star.

It begins a little before midnight Central Time, and lasts an hour or so. The exact timing depends on your location. Skywatchers in the far west will miss all or part of the occultation because at least part of it happens before nightfall.
 

Script by Damond Benningfield

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You don’t always need a telescope to enhance the beauty of the night sky. For many objects, a pair of binoculars will do just fine. In fact, some objects look better through binoculars.

One example is the star cluster Messier 39. It’s high in the northeast at nightfall, to the lower left of Deneb, the bright star that marks the tail of Cygnus, the swan.

M39 is a bit sparse. Binoculars reveal about 30 stars, which are spread across about seven light-years of space. Seen from the cluster’s distance of a thousand light-years, that span is roughly the size of the full Moon.

And that’s why M39 is best seen through binoculars. A good pair will allow you to see the entire cluster in a single view. Three bright stars form a wide triangle, with the other members of the cluster roughly filling in the space between them. A telescope typically sees a smaller field of view, so it can’t take in the entire cluster all at once.

And if you have especially clear, dark skies, you might just make it out with your eyes alone. It looks like a faint, hazy smudge of light below the swan.

In the future, M39 will undergo periods where it’s a lot more prominent. A few of its brighter stars are nearing the ends of their “normal” lifetimes. When they reach that end, they’ll puff up to many times their current size, making them shine much brighter. That’ll make M39 stand out — no matter how you look at it.

Tomorrow: a star disappears.
 

Script by Damond Benningfield

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If you watch where the Sun rises and sets every day for a month, there’s generally not a lot of difference. But that’s not the case for the Moon. During its month-long cycle of phases, its rising and setting points race north and south along the horizon.

The Moon stays close to the ecliptic, which is the Sun’s path across the sky. During the year, as the Sun moves north and south, the angle of the ecliptic changes as well. The angle also changes during the day, as Earth rotates on its axis. So where the Moon rises and sets depends on the time of year and the Moon’s angle relative to the Sun, which defines its phase.

So depending on your latitude, the Moon can rise anywhere from roughly southeast to northeast. And although the Moon stays close to the ecliptic, its orbit around Earth is tilted. That allows it to stray about five degrees to either side of the ecliptic. As a result, the Moon’s extreme rising and setting points are five degrees to the north and south of those same points for the Sun.

So like a baby in a cradle, the Moon rocks back and forth along the horizon during its month-long cycle.

Tonight, from the latitudes of the 48 States, the Moon will set a little north of southwest, before midnight. It follows the bright star Antares, the heart of the scorpion, below the horizon. In a couple of weeks, though, the Moon will set a little south of northwest — one-fifth of the way around the sky.
 

Script by Damond Benningfield

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To make a planet that’s comfortable for life, a lot of things have to fall into place. The planet has to orbit the right kind of star. It has to orbit at the right distance. It has to be made of the right materials. And the list goes on and on.

One factor may be whether a planet has plate tectonics, with slabs of its crust moving around and over each other. On Earth, the motions of the crust help regulate temperatures. Volcanoes belch carbon dioxide, which helps keep the air warm. But the plates also help bury carbon dioxide in Earth’s interior, keeping the air from getting too warm.

It may also help if a planet has a moon — especially one as big as Earth’s. Our moon helps keep Earth stable as it orbits the Sun. It also creates the tides, which are a big factor in sustaining life.

Another factor may be how close the planet’s birthplace was to a recently exploded supernova. The massive stars that become one type of supernova create a lot of heavy elements. Those elements are needed to make the rocky planets like Earth that are the most likely homes for life. When the star explodes, it blasts these elements into space. They “seed” nearby stellar nurseries, providing the ingredients for planets and life.

As astronomers discover more planets in other star systems, scientists are adding to the list of conditions. So we’re likely to find that many more things have to fall into place to make a planet comfortable.
 

Script by Damond Benningfield

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