http://ca.news.yahoo.com/earth-usually-more-one-moon-study-suggests-155802499.html

SPACE.comBy SPACE.com Staff
Space.com | SPACE.com – Wed, 4 Apr, 2012

Earth usually has more than one moon at any given time, according to the results of a new computer simulation.

The huge, bright and iconic moon so beloved by poets and romantics is joined by a rotating cast of captured asteroids that often measure just a few feet across. These mini-moons typically orbit Earth for less than a year before zipping back off into space, researchers said.

The scientists used a French supercomputer to simulate the movements of 10 million near-Earth asteroids as they passed by our planet. They then tracked the trajectories of the 18,000 space rocks that Earth's gravity captured in the simulation.

The team concluded that at least one asteroid with a diameter of 3 feet (1 meter) or more is likely orbiting our planet at any one time. There may be many smaller objects circling Earth, too, but the study didn't address them; it was tough enough to model the motions of the bigger space rocks. [Photos: Our Changing Moon]

"This was one of the largest and longest computations I’ve ever done," study co-author Jeremie Vaubaillon of the Paris Observatory said in a statement. "If you were to try to do this on your home computer, it would take about six years."

Eventually, minimoons break free of Earth's gravity and resume their paths around the sun. This happens at about nine months for the typical minimoon, but some of them may orbit Earth for decades, researchers said.

Most asteroids snagged by Earth's gravity don't zoom around our planet in neat circles, according to the simulation. Instead, they follow complicated, twisting paths, tugged this way and that by the gravitational pulls of Earth, the moon and the sun.

Observations have shown that Earth does indeed harbor minimoons. In 2006, for example, the University of Arizona’s Catalina Sky Survey discovered one about the size of a car. Known as 2006 RH120, the asteroid orbited Earth for less than a year after its discovery.

A better understanding of the number and nature of Earth's minimoons could have practical benefits, researchers said.

"Minimoons are scientifically extremely interesting," said co-author Robert Jedicke of the University of Hawaii at Manoa. "A minimoon could someday be brought back to Earth, giving us a low-cost way to examine a sample of material that has not changed much since the beginning of our solar system over 4.6 billion years ago."

Read more at: http://ca.news.yahoo.com/earth-usually-more-one-moon-study-suggests-155802499.html

Courtesy of Space.com

Interesting, but i think the term spacerock is more accurate than the term minimoon is.  Some of these objects are only the size of a beach ball.

Interesting, but i think the term spacerock is more accurate than the term minimoon is.  Some of these objects are only the size of a beach ball.

I agree. Let's call something a moon (or a planet) when it is large enough to become round(ish) due to it's own gravitational mass. I'd make an exception and would probably still be willing to call something a moon if it is orbiting around something that is itself not large enough to have become round under its own gravitational mass. So, to talk about a moon of an asteroid might still be valid when we're talking about space rocks. Would you agree with this definition?

BTW, note that my definition would return planetary status to Pluto. But, I just don't like arbitrary definitions. Pluto is massive enough to be round and is in the the primary orbit at its distance from the Sun. It is also made of an igneous rock (which happens to be ice but is a rock at that temperature).

So, to recap my main definitions, that which is large enough to be round under its own mass is either a planet or a moon, depending on whether it is the primary object in its orbit around the star in question or is orbiting some other, larger planet. Two equally large objects in orbit around each other and the star? Hmm... co planets?

Anyway, that always seemed less arbitrary than worrying about whether a planet were smaller than some large moon in another orbit or whether it would grow a tail if it hypothetically fell out of its stable orbit.

Neil deGrasse Tyson and others don't agree. So, I'm "wrong". But, I don't like to accept arguments from authority. I still stand by my definitions which seem more natural to me than arbitrary.

Does anyone else agree? Or, should I keep this opinion to myself from now on? (Not that I will. But I am curious to hear other opinions.)

Scott, I am all for returning planetary status to Pluto.  It orbits the Sun and it is round under it's own gravitational pull.  That's good enough for me.   I don't care if it doesn't "dominate its region" in space. 

Quote: Neil deGrasse Tyson and others don't agree. So, I'm "wrong". But, I don't like to accept arguments from authority. I still stand by my definitions which seem more natural to me than arbitrary." unquote.

Scott, but is not the definition of a definition, an accepted agreed delineation by multiple authorities?

But further to your preferred declarations, I would suggest other nomenclatures, like Planetoids, and Moonoids. Then any which are large enough to have active volcanism, that could then be called Vulcvanoids! [you can't use Hemorrhoids, since there is not any blood present, at least from current knowledge of Hematology!] Ha ha ha

Time for me to go for brunch!

"Making up your own words is not a good way to go thru life son." /// As Obtuser opines, I think you should carefully review the current terminology of the field before going off to create your own and intentionally lose touch with everyone else that has an interest in the subject matter.  I don't know astronomy but why does "asteroids" not match the issue perfectly? 

Of note--with only my spattering of casual exposure to astronomical issues and nomenclature, but my love of words and definitions, I had not heard the concept of objects large enough for their gravity to round them out.  That does seem like a telling difference that could be used as a way to describe the universe.  The note?==all these words and ideas don't change the universe at all.  Moon, moonoid, asteroid, asteriks, space rock, space junk--makes no difference to/of the universe.

what does change though is how we think about the universe.  Words:  the very stuff of thoughts.  Always worth thinking about, discussing, marveling.  Seems to me more than just moons can orbit a planet.  Like gravity, has that singular fact captured your imagination/attention?  I assume "by the physics of it all" that moons cannot have moons?  In the contest of various gravities, the moon's moon will either crash into the moon, the sponsoring planet/sun, or careen off into the universe.  I'll bet that means an object in space cannot orbit a moon even once==absent some unique set up pool shot==but maybe not even then??  Can two asteroids (irregularly shaped) be large enough to be in binary rotation with each other and not be large enough to round themselves out?

Fun stuff, but when you don't know enough to understand the whys of the simpliest questions......I wonder if their is a character to represent this?......♫?

Scott, but is not the definition of a definition, an accepted agreed delineation by multiple authorities?

But further to your preferred declarations, I would suggest other nomenclatures, like Planetoids, and Moonoids. Then any which are large enough to have active volcanism, that could then be called Vulcvanoids! [you can't use Hemorrhoids, since there is not any blood present, at least from current knowledge of Hematology!] Ha ha ha

Well, until the time at which Pluto was demoted to a dwarf planet, which somehow not a planet, there was no scientifically accepted definition. This was very recent. Interestingly, Ceres got promoted from asteroid (space rock) to dwarf planet. But, Ceres is not round under it's own mass. So, it seems that the new definitions of planet and dwarf planet and not at all a planet are very arbitrary. They are also very recent. I'm merely saying that I disagree with the authorities on this. As I've also stated, of course, the authorities win. I just don't have to like their definition. It seems more emotional than scientific. Who cares that Pluto is made of water ice? The gas giants have no solid core. Pluto is solid igneous rock. Why does the fact that that rock has the chemical composition is H2O matter? Does that not seem arbitrary and capricious?

Regarding dominance of it's region in space, only Jupiter would be a planet by that definition. Dominating its own orbit may be a valid qualification. Pluto does that. Some have suggested other issues with Pluto's orbit like the tilt and the ellipticity of the orbit that brings it closer than Neptune for some period twice per Plutonian year. Perhaps there is some validity to this. But, I would have left it a planet. I like Neil a lot. I still continue to disagree with him, and other real astronomers, on this.

So, while I recognize that I am not an authority and that my opinion will not carry any weight (or even mass), I still feel like expressing it. Is that wrong?

Re: vulcanoids, I like it. If the volcano leaves it's mark in the form of a caldera/hole, can we call them assteroids?

"Making up your own words is not a good way to go thru life son." /// As Obtuser opines, I think you should carefully review the current terminology of the field before going off to create your own and intentionally lose touch with everyone else that has an interest in the subject matter.  I don't know astronomy but why does "asteroids" not match the issue perfectly? 

Why the quotes? Who are you quoting? I don't see that string anywhere previously. Did Obtuser edit a post after you quoted him on this? Perhaps I'm just hypersensitive to putting words into the mouths of others these days. You know why.

As for why asteroids does not match the issue, they are typically not large enough to be round. Again, I'm just expressing an opinion here. The one that hit the earth and caused the moon's creation probably was round. What do we call an object large enough to be round but not in a stable orbit? I have no idea. If it's large enough to reach critical mass, we call it a star.

Scotty:

Why the quotes? Who are you quoting? I don't see that string anywhere previously. Did Obtuser edit a post after you quoted him on this? Perhaps I'm just hypersensitive to putting words into the mouths of others these days. You know why.  ///  So, making up words is ok, but making up quotes is not?  You do normally preference such usage and thats a good thing--a step towards using the dictionary?  My own usage was for humor and in the style of a headline from Fark.  My own private reference and humor as was orbiting asteriks.

As for why asteroids does not match the issue, they are typically not large enough to be round. Again, I'm just expressing an opinion here. ///  Sounds right to me--otherwise they are called rogue planets of which there are many?

 The one that hit the earth and caused the moon's creation probably was round.  ///  The Nova I saw called it a small planet.  It was round and orbiting the sun.  What is a planet?

What do we call an object large enough to be round but not in a stable orbit? ///  A planet in an unstable orbit?  A Rogue Planet??

 I have no idea. If it's large enough to reach critical mass, we call it a star. ///  Not unless it is mostly a gas and ignites.

Your devotion to word precision is sorely lacking......and we all can do better.

Quote: "If it's large enough to reach critical mass, we call it a star. ///  Not unless it is mostly a gas and ignites." unquote.

Since Jupiter is near critical mass, then it can be designated with an Astarrisk? NO?

Any more of this and I will change my handle to Obscurer!!!

Why the quotes? Who are you quoting? I don't see that string anywhere previously. Did Obtuser edit a post after you quoted him on this? Perhaps I'm just hypersensitive to putting words into the mouths of others these days. You know why.  ///  So, making up words is ok, but making up quotes is not?  You do normally preference such usage and thats a good thing--a step towards using the dictionary?  My own usage was for humor and in the style of a headline from Fark.  My own private reference and humor as was orbiting asteriks.

When you put quotes around something, it looks like you're quoting someone. I've had my fill recently of having quotes attributed to me that I did not say. So, as I said, I'm just being hypersensitive about it. But, it did look to me as if you were quoting Obtuser. Since he never said the words in quotes, that made me wonder why you were putting words in his mouth. Punctuation has meaning. You seem to have trouble when I deliberately and openly state that I would redefine a word to mean blah. But, you have no trouble using punctuation completely at random and without any concern for what it actually means. /// === > > > >

As for why asteroids does not match the issue, they are typically not large enough to be round. Again, I'm just expressing an opinion here. ///  Sounds right to me--otherwise they are called rogue planets of which there are many?

Rogue planet. I had not heard that term before.

The one that hit the earth and caused the moon's creation probably was round.  ///  The Nova I saw called it a small planet.  It was round and orbiting the sun.  What is a planet?

I've only ever heard the object called "a Mars-sized object". Rogue planet works for me.

What do we call an object large enough to be round but not in a stable orbit? ///  A planet in an unstable orbit?  A Rogue Planet??

Rogue planet still sounds good to me. As an added bonus, it explains where Sarah Palin evolved. Clearly evolutionary pressure was not as great on her home planet. Nor did it ever produce life as intelligent as say a chimp or a dolphin or a chipmunk.

I have no idea. If it's large enough to reach critical mass, we call it a star. ///  Not unless it is mostly a gas and ignites.

By definition, if it reached critical mass, it ignited. Therefore, it is a star regardless of its composition.

Your devotion to word precision is sorely lacking......and we all can do better.

So, let's do better.

Planet: http://en.wikipedia.org/wiki/IAU_definition_of_planet
Star: http://en.wikipedia.org/wiki/Star

Given these two definitions, I no longer have a problem with the IAU definition of a planet. I had not read the simple criteria before. The question about Pluto then comes down to whether it has cleared its orbit. I guess it hasn't. So, I retract my earlier statement and my own preferred definition and will defer to the IAU. They did an acceptable job. Whether it is my preference or not, I can no longer claim it to be arbitrary and capricious. Incidentally, part of my problem came from not realizing that Ceres actually is round. Oops. I'm going to go wipe this egg off my face.

As for a star, our own sun has huge amounts of iron and other elements. It is a second (or later) generation star, forming out the remnants of a prior star. All of the stuff in our solar system is from a supernova, else we would not have heavy elements (or heavy elephants) on earth.

Would you cease to call a star a star when it has exhausted most of its gas but is still burning? There are other nuclear reactions going on in our sun than just hydrogen into helium. It is also forming heavy elements like iron. I'm not even sure the gases on the sun are actually gases. I believe at that temperature and pressure, they are a plasma.

So, want to try using words more correctly? Sure. When are you planning to start?

Wasn't it just last year that another very distant very dim Pluto like planet was discovered as the 10th largest object orbiting our Sun?
http://science.nasa.gov/science-news/science-at-nasa/2005/29jul_planetx/
OK, so it was 2005, but it seems like that was just last year!

Scotty--admitting the egg on your face has taken away my best response.  Well done.  Yes, we, including myself, can always do better.  as I recall, the planet that hit earth was not a rogue but was part of that orbit clearing process allowed Earth to assume its stable position.  My fun with punctuation, and even words, does not do violence to ideas.  Big difference.

My fun with punctuation, and even words, does not do violence to ideas.

Yes. It does.

Scotty---really?  Well, thats not my intent and while I know people don't like it I do have to wonder if you are focused/experience the dislike and calling it violence? ////  Or--its a matter of degree?  How badly can punctuation affect an idea versus using an actual word incorrectly?  No--the two issues aren't even on the same continuum.

Once again, I think you are using the wrong word to describe your mental state/reaction.  You might be mislead and distracted, but that is "you" and not violence to the idea being discussed. 

I could conform but then I wouldn't be as happy.  Still, I don't like the idea of doing violence to ideas.  How many of us ever change?