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Old 18-06-2009, 03:48 PM   #1
VTracy
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Default A REAL wandering moon of Earth?

Who knew? I didn't.




http://www.space.com/scienceastronom..._facts-10.html




NUMBER 6
Sister moons


The Moon is Earth's only natural satellite. Right? Maybe not. In 1999, scientists found that a 3-mile- (5-kilometer-) wide asteroid may be caught in Earth's gravitational grip, thereby becoming a satellite of our planet.

Cruithne, as it is called, takes 770 years to complete a horseshoe-shaped orbit around Earth, the scientists say, and it will remain in a suspended state around Earth for at least 5,000 years.
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Old 18-06-2009, 06:40 PM   #2
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Originally Posted by VTracy View Post
Who knew? I didn't.




http://www.space.com/scienceastronom..._facts-10.html




NUMBER 6
Sister moons


The Moon is Earth's only natural satellite. Right? Maybe not. In 1999, scientists found that a 3-mile- (5-kilometer-) wide asteroid may be caught in Earth's gravitational grip, thereby becoming a satellite of our planet.

Cruithne, as it is called, takes 770 years to complete a horseshoe-shaped orbit around Earth, the scientists say, and it will remain in a suspended state around Earth for at least 5,000 years.
Its a Near Earth asteroid.Near-Earth asteroids (or NEAs) are our closest neighbours in the Solar System - some of these objects have been known to pass closer to the Earth than the Moon. These objects, which range in size from 30-40 km (eg. 1036 Ganymed and 433 Eros) to a few metres, are important to our understanding of past and present Earth impact rates, and are likely to prove useful as bases and/or mineral sources as humans move into near-Earth space. Each year, it is at its closest in the autumn and at this point it will pass almost directly beneath the Earth's South Pole! What does "Cruithne" mean? The Dalriada Celtic Heritage Trust inform us the Cruithne were "the first Celtic racio-tribal group to come to the British Isles, appearing between about 800 and 500 B.C., and coming from the European continent. They were also known as the Picts. BKW informs me that Cruithne was also the name of a legendary king of the Picts.







Cruithne is on an spiraling horseshoe orbit, similar to the one sketched above, but its behaviour is even stranger. First, it is an even more tightly-wound spiral, with kidney-bean shaped loops. Secondly, one part of the horseshoe actually overlaps the position of the Earth when viewed from above.

Last edited by eaglewingone; 18-06-2009 at 07:29 PM.
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Old 18-06-2009, 07:47 PM   #3
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I believe I first came across this 'moon' in Stephen Baxter's 'Time' novel (part of the Time | Space | Origin trilogy). I thought it was just fiction at the time.

I may have to re-read it now!

Phil.
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Old 18-06-2009, 08:08 PM   #4
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I believe I first came across this 'moon' in Stephen Baxter's 'Time' novel (part of the Time | Space | Origin trilogy). I thought it was just fiction at the time.

I may have to re-read it now!

Phil.
Sounds like a interesting book.
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Old 18-06-2009, 08:26 PM   #5
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You'd hardly call it a moon... moons do laps around planets, not laps near them... cool, but not a moon
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Old 18-06-2009, 08:28 PM   #6
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Be careful! I thought it was a hard read. I can't really explain without spoiling it, suffice to say I seem to remember the timeline of the narrative is disjointed (for good reason) which makes piecing together what's actually going on and in what order, somewhat tricky.

As if that wasn't enough, the same people and events are revisted in the follow up novel, 'Space'. I'm struggling through that and have 'Origins' and a set of short stories 'Phase Space' to read after that.

Damn - I really must get me some reading trousers this weekend!

Phil.
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Old 18-06-2009, 08:32 PM   #7
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I had heard about Cruithne a good while ago.

Info on the other hand is hard to track down.
Well, info that is reliable.
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Old 18-06-2009, 10:19 PM   #8
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You'd hardly call it a moon... moons do laps around planets, not laps near them... cool, but not a moon
Its an asteroid or Near Earth Asteroid. But the moons of Mars, Phobos and Deimos are just captured asteroids from the Asteroid Belt and they are considered moons of Mars. The definition of celestial objects have been changing in the last 10 years.

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Old 18-06-2009, 11:26 PM   #9
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It can't escape our gravity and goes around us.

I'd say that definition makes it a Moon.
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Old 18-06-2009, 11:43 PM   #10
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This article may help the debate:

What is a Moon? Definition Lags Behind Soaring Satellite Tally


In the old days of astronomy, before Galileo, there was just the Moon. Then scientists had to accept the clear and visible evidence of four objects orbiting Jupiter, satellites the master saw through a crude telescope in 1610.

Things have only grown more complex, especially of late.

With a spate of discoveries in the past three years, the number of known moons in the solar system has jumped to 118 as of this writing. That figure won't stand long. Astronomers expect the tally to double, at least, in the next few years. Most won't be surprised if it eventually triples. And that's not counting the small stuff -- boulders the size of football stadiums and countless smaller rocks that are surely trapped in orbit around the big outer planets.

Meanwhile, the complexity of moon types and behavior grows with the tally, and astronomers are struggling to sort out what it all means.

Several interviews with moon hunters and top theorists reveal that the definition of a moon is not clear and that there has been almost no discussion, professional or casual, about whether there should be any lower size limits set to separate real moons from miniature imposters. And nobody is in a rush to do anything about it.

Orbiting rocks and atoms

Last month, astronomers announced they'd found the smallest known satellite, a moon just 0.62 miles (1 kilometer) in diameter orbiting Jupiter. It is one of dozens of small moons that behave strangely. Most go backwards compared to the orbits of the larger Galilean moons. Some take long, elliptical paths and stray far above and below the planet's plane of rotation. Few are round.

Saturn, Uranus and Neptune all have similar satellites.

Other configurations are more complex. Saturn's rings contain myriad chunks of ice and rocks that scientists don't consider moons. Yet embedded in the rings are distinct moons. Size -- and the ability to be noticed -- has so far played a role in being designated a moon.

Yet some moon-like objects are so small they are invisible. Hydrogen atoms at the outer fringes of Earth's atmosphere orbit the planet in an arguably moon-like manner.

"We don't call those satellites," says Alan Stern of the Southwest Research Institute (SwRI), making a firm delineation between moon and non-moon at least at the very lower end of the size spectrum. Dust grains, too, are out, Stern says. From there on up, however, nobody has thought much about where to draw the line, if at all, on what constitutes a moon.

"There is no accepted definition," Stern said.

So what would Stern call a 6-inch rock orbiting a planet? "I would call it a 6-inch rock." But, he acknowledges, "It's technically a satellite."

By that strict definition, there could be thousands of moons in our solar system, or millions if you count the debris in Saturn's rings. Nobody is proposing such an approach. Stern thinks it might be useful to create a separate category called "minor moons" to distinguish the small stuff from what is conventionally thought of as a moon.

Stern's suggestion was made to this reporter and is not an official proposal, but someone might need to propose something official pretty soon. NASA's Cassini spacecraft, due to arrive at Saturn next year, may force the issue.

For now, the smallest known satellite of Saturn is Pan, at 12.4 miles (20 kilometers) wide. Pan sits within Saturn's rings and was spotted in photos taken by the Voyager spacecraft.

The Voyager survey was incomplete, says Joe Burns, a Cornell University mathematical theorist and astronomer who is on the Cassini imaging team. Cassini will probably find much smaller moon-sized rocks in Saturn's rings, Burns said, and "no jury has voted" on whether to call these imminent discoveries moons or not.

"I think we'll have to confront that," he said.

Rich vernacular

When humans first put hunks of fabricated metal into orbit around Earth a few decades ago, astronomers needed to differentiate them from stuff that was already in space, so the term natural satellite came into use. In hindsight, it might not have been an overly inspired choice of words.

"I believe that the public is often confused when the term 'satellite' is used," said Matthew Holman, an astronomer at the Harvard-Smithsonian Center for Astrophysics who is a co-discoverer of three moons around Neptune. "The public normally equates 'satellite' with artificial satellite.'"

From a lay person's perspective, Moon terminology has only become more obscure.

Astronomers have developed a loose classification system that roughly separates moons into three types based on size and distance from their host planet: irregular moons, regular moons, and inner moons (sometimes called ring moons).

Irregular moons

Irregular moons are of unknown origin. They might be captured asteroids or comets, or perhaps they're pieces of young planetary hopefuls that didn't survive to orbit at the big table of nine, researchers believe. Many irregulars travel in packs that indicate they were once parts of larger objects.

The irregulars are typically small and orbit at great distances and often on odd trajectories. Their orbits are stretched and tilted in just about every way you can imagine. They soar high above and below the plane in which regular moons orbit. They come close to the planet and then zoom far away on elliptical trajectories. More often than not their orbits are retrograde -- opposite the direction of regular moons and of the planet's spin.

About the only thing they don't do is orbit in a plane perpendicular to that of the planet's orbit. Research led by Burns, the Cornell mathematician, has shown that the Sun's gravity causes such an orbit to be unstable, so any object captured in one would crash into the planet or escape.

Most irregulars have probably been in orbit since their host planet was very young, which astronomers cite as a good reason to call them moons, despite their odd and varied behavior. Nobody knows how they got captured, however.

Burns said it is very difficult for a planet to capture a moon. Unless something slows an incoming asteroid or alters its trajectory -- like a highly improbable pass at just the right angle through an atmosphere -- it will either hit the planet or fly on past.

"If it comes from infinity, it's going to go back to infinity," he said.

Perhaps, Burns speculates, the irregulars ran into an extended envelope of gas that might have surrounded a giant planet shortly after its birth 4.5 billion years ago. That would explain why irregulars tend to be small, because a smaller object has more surface area in relation to its mass, and is more likely to be captured than a large object.

Scientists aren't sure how the giant planets formed, let alone what conditions were like around them back then. So figuring out moons will help astronomers understand planets, researchers agree.

Regular moons

While irregular moons define the outer orbital reaches of a planet's domain in space, the next region in is dominated by regular moons. Examples include Saturn's moon Titan and Jupiter's Galilean satellites: Io, Europa, Ganymede and Callisto.

These classic, large and round moons tend toward entirely stable, simple, nearly circular orbits. All move through a plane in space that is roughly equal to the planet's equatorial plane. This conformity leads theorists to believe the moons formed out of the same nebula of gas and dust that built the planet.

Other so-called regular moons were probably carved by collision. Our own Moon likely developed after a Mars-sized object slammed into Earth. Some Uranian satellites might have been formed by an impact that flopped the planet over on its side.

Pluto's moon Charon is thought to have been created by collision, too, says the SwRI's Stern, who leads NASA's New Horizons mission to study the planet and set for launch in 2006. Interestingly, the gravitational midpoint around which those two objects revolve is not inside Pluto, but instead out in empty space. Some researchers prefer to call this a binary planet system (others don't consider Pluto a planet at all).

The Pluto-Charon system is not unlike pairs of other rocks orbiting each other out there on the fringes of the solar system, a region known at the Kuiper Belt. The dancing Kuiper Belt Objects, so-called KBO binaries, often involve a primary object with a smaller satellite and further stretch usage of the word moon.

"The KBO binaries aren't rocks, they are worlds," Stern says. "In fact, most are large enough to be rounded by gravity and therefore mini-planets in their own right."

More than 30 asteroids are known to have moons, too.

Inner moons

Inside the orbits of the regular moons, things get crazy.

Inner moons, sometimes called ring moons because they often travel amidst other debris, are almost surely the youngest. They're also the most likely to disappear. Here's why:

Planets, especially the large ones, act like gravitational brooms, gradually clearing the solar system of comets. That's bad news for any moon close to the broom.

"It's more dangerous down there," says Kevin Zahnle of NASA's Ames Research Center.

If you are a moon, the odds of being hit by a comet grow the closer you are to your host planet. Zahnle imagines flies buzzing around a hunk of raw meat. Pretend your hand is a moon, and pass it a few feet from the meat: you might not be hit by a single fly. Pass your hand a couple inches from the meat, and many flies might hit it.

Making matters worse, orbital mechanics dictates that inner moons travel more swiftly than outer moons, so collisions are more fierce, making it less likely that a moon will survive a hit. "And if you're smaller to begin with, it's more precarious to begin with," Zahnle adds, because you don't have the self-gravity to survive an encounter.

All this means that the life of an inner moon is very unstable.

"These are the most ephemeral and are presumably young," Zahnle said. He suspects inner moons might be well less than a billion years old, but cautions that there are no data to support that view. More needs to be learned about the population of comets, he said, and how frequently they impact the giant planets.

So are these precarious little objects moons? Definitely, Zahnle said. He adds, however, that in a ring system, the smaller rocks are constantly running into each other, their movement governed more by collision than by traditional orbital mechanics. Zahnle would tend toward calling these things debris, not moons.

Outcast moons

Outside the traditional categories of satellites are two sorts of objects that really stretch definitions.

Ahead of and behind Jupiter are two packs of asteroids -- tens of thousands of them -- that orbit the Sun but are also gravitationally bound to Jupiter. Astronomers call them Trojan asteroids. They are technically satellites of Jupiter, some astronomers say, but others, including Burns, consider them mere companions to Jupiter.

"The one property that I think a satellite must possess to be a satellite is an orbit that encircles its planet," Burns said.

Earth, Mars and Neptune are each known to control one or more Trojan objects in similar fashion.

Earth has acquired a sort of enhanced Trojan, too. A rock called 2002 AA29 orbits the Sun while also carving a horseshoe-shaped path around Earth. Every few centuries it gets close enough to Earth to be considered more than a Trojan, becoming what astronomers now call a quasi-satellite.

To astronomers, all this is quite clear, but there might be room for further definition.

"I do not think a new classification [system] is needed," said Earth's quasi-moon co-discoverer Martin Connors of Athabasca University in Canada. "For technical purposes we may wish to define sub-categories."

A moon by any other name ...

Nobody has found more moons than David Jewitt of the University of Hawaii. Along with colleague Scott Sheppard, Jewitt's team has found three dozen satellites orbiting Jupiter and has more in the observational pipeline. Jewitt is not keen to redefine moon terms or set any lower size limit.

"To me, they're all natural satellites," he said. "Is a small dog not a dog because it is small?"

If there is some confusion now, Jewitt says, then that reflects the healthy progress of discovery, and greater clarity will be the ultimate result. Jewitt realizes, however, that from a public perspective there can be some "pain" associated with no longer being able to look satellites up in a textbook. "Many people are not comfortable with a world that changes quickly."

One thing that hasn't changed, not since well before Galileo's time: There is still only one natural satellite you can step outside and see with your naked eye, the one we call, simply, the Moon.

http://www.space.com/scienceastronom...on_040103.html
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Old 19-06-2009, 01:59 PM   #11
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Great info eagle, thanks for sharing!
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Old 20-06-2009, 01:33 AM   #12
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Great info eagle, thanks for sharing!
You are most welcome.
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