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The moon has been with us for billions of years, almost since the formation of Earth. From then on, Earth has also had countless other satellite companions. In fact, we probably have one right now.
But these objects are not full-fledged moons. They're more like temporary, extremely small moons. Called irregular natural satellites, they’re boulders from the large population of near-Earth asteroids that get snagged by our gravity. They orbit the Earth for a few months, then escape and move on.
A new study on the Web site arXiv.org says that such satellites should be very common. [Mikael Granvik, Jeremie Vaubaillon and Robert Jedicke, "The population of natural Earth satellites"] At any given time we probably have one temporarily captured orbiter of about a meter in size.
These space rocks are awfully hard to see. But astronomers seem to have spotted one such transient satellite in 2006. The asteroid, dubbed 2006 RH120, was a few meters in diameter. It was captured by Earth for about a year and then broke away back to interplanetary space.
But not all temporary orbiters escape. About one percent of them actually impact Earth during their captivity, according to the study. Fortunately, the real moon keeps a respectful distance.
—John Matson
[The above text is a transcript of this podcast.]
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17 Comments
Add CommentVery interesting. I wonder though, does the moon revolve around us, or the Earth around it? Do we rotate counterclockwise or if we stand on our heads, the opposite direction? It seems that our perspective is as valid as not having one at all.
Reply | Report Abuse | Link to thisWell, for you first question, it has nothing to do with our perspective. Gravity dictates that the moon revolves around us and not the other way around. As for your second question, yes, it is a matter of perspective. It is also irrelevant.
Reply | Report Abuse | Link to thisActually, I believe that the Earth and Moon orbit around the common center of their mass, which is near the surface of the Earth. Mutual gravitational attraction determines the path of an orbit. When the sun and planets are in a major alignment, the center of mass of the system moves toward the side of the sun the alignment is on. The short answer, then, is that the Earth and Moon are in a mutual orbit around each other, and the E-M system orbits the sun.
Reply | Report Abuse | Link to thisThe article is interesting but frustrating because of what is omitted from it. As a former professor of Math/Physics, I can understand the physics/dynamics of capture of such an asteroid, but I have more difficulty understanding what would then cause the asteroid to "break away" rather than impacting the Earth as any low orbiter is bound to do eventually because of the drag of Earth's tenuous gaseous envelope. Similarly, if such asteroids more easily than not escape the gravity of Earth even after many revolutions, I wonder why such escape is not true for at least some satellites which humans have placed in orbit?
Reply | Report Abuse | Link to thisWhile writing this comment, my subconscious has supplied the likely answer (which, unfortunately the article omitted) which is that the vast majority of such temporarily captured asteroids are (totally unlike the moon) in highly eccentric orbits and at some point the moon's gravity gives them a sufficient boost to up to escape velocity. And the reason why human satellites do not escape is that their orbits are more circular and their highest orbital velocities not sufficiently near escape velocity.
I believe your "likely answer" is the correct one.
Reply | Report Abuse | Link to thisDEAR PAULWAKFER, I HAVE HEARD THE THEORY? THAT THE MOON IS MOVING AWAY FROM THE EARTH--IS THERE ANY ACTUAL DATA TO SUPPORT THIS? AND IF THIS IS TRUE THEN IT COULD BE LOGICALLY DEDUCED THAT THE MOON WAS CLOSER TO THE EARTH AT AN EARLIER POINT IN TIME. WHICH THEN GOES BACK TO YOUR ORIGINAL THOUGHT---WHY WOULD THE MOON MOVE AWAY FROM THE EARTH... ANY IDEAS ON THIS? (ANY THOUGHTS ABOUT "PLANET X"/"NIBIRU" EFFECTS?)
Reply | Report Abuse | Link to thisThe reason the moon's orbit is getting larger is because the earth-moon system is dissipating energy in the form of tidal drag. Lower energy means a larger orbit.
Reply | Report Abuse | Link to thisIt's not just a theory. It has been measured consistently from the Lunar Laser Ranging experiment for over 40 years. I used to process the data from that.
DEAR BILLSINCL, SO FROM THAT DATA, IT COULD BE ASSUMED THAT THE MOON WAS CLOSER TO THE EARTH---DO YOU HAVE ANY IDEAS/COMPUTATIONS ABOUT JUST HOW CLOSE WAS THE MOON TO EARTH--ITS' CLOSEST POSITION TO EARTH... IT WOULD SEEM THAT THE FURTHER FROM EARTH, THE MORE RAPID WOULD BE THE MOVEMENT AWAY FROM EARTH... WOULD THAT BE A "STRAIGHT LINE" ACCELERATION OR WOULD VARIABLES (UNKNOWN?)ALTER THE RESULTS... I DO NOT BELIEVE THAT THE MOON WAS "EJECTED" FROM THE EARTH VIA SOME TYPE OF COLLISION/EXPLOSION. AND COULD YOU PLEASE EXPLAIN EXACTLY WHAT "TIDAL DRAG" IS.
Reply | Report Abuse | Link to thisWall of words plus shouting => didn't read.
Reply | Report Abuse | Link to thisDear myusername, please do not shout.
Reply | Report Abuse | Link to thisI find it hard to see how an asteroid is first trapped, by interactions with both earth and moon, I assume in fairly random orbits, then ejected by the same forces, without at least an equal number entering the atmosphere and falling to earth. Can you explain that without higher math?
Reply | Report Abuse | Link to thisAnd while you're at it, how does the gravitational sling effect work to send space probes off on distant missions? Could we use it repeatedly to reach relativistic velocities, maybe when planets are positioned right?
Please copy answers to danrob@efn.org
Daniel35...
Reply | Report Abuse | Link to thisI think the word trapped is slightly miss used. These bodies are only coming into a highly complex dance around the earth and moon before moving on. I think if they hit one of the lagrangian points at the right velocity, they might "stick".
My best guess on your other question of "gravitational sling effect" is that the planets are not moving any where near fast enough to impart the requested velocities. (according to wikipedia, jupiter is orbiting the sun at 13.07 km/s)
The article implies that these wannabe earth satellites are NECESSARILY temporary. Is it not possible that one could become permanent?
Reply | Report Abuse | Link to thisIs it possible that one could become a temporary satellite of the moon? Does the moon have a satellite?
Is it possible for one of these objects to get into a figire-8 orbit around the earth & moon?
Fulla questions and no answers.
It may be possible, but not very likely. Or maybe one should say, the probability of seeing a mini-satellite in a semi-stable orbit decreases with the time-span the semi-stable orbit is semi-stable over. Three-body systems tend to be unstable, particularly when the system does not readily approximate two two-body systems (like the Sun-Earth-Moon which to a good approximation can be split in the Earth-Moon system and the Sun-(E-M) system).
Reply | Report Abuse | Link to thisThere are exceptions of course, in particular the stable Lagrangian points - but even these are probably not stable in the long term, due to interference from other objects in the Solar system. A meter-sized rock doesn't require much nudging to change it's trajectory significantly.
The subject of the "second" tiny moon orbiting planet Earth has been the subject of speculations, publications, claims of watch and even cites in romans since the XIXth Century, just SciAm is giving a serious scientific support for the new. Yeah !, the moon slows down in its orbit, thus meaning that it's going outwards and away from our planet. Does anybody know if the orbit of Sun around the gravitational center of Milky Way galaxy is accelerating, constant, or deccelerating ?. If it's accelerating, this will mean the planet will finally go down to sunk in the black hole sited in the center of the galaxy, and if it's slowing, it means the Solar System will go away wandering in the deep space someday.
Reply | Report Abuse | Link to thisDEAR Ralf123, PUTTING YOUR PERCEPTIONS/INTERPRETATIONS ASIDE--CAN YOU ANSWER ANY OF THE QUESTIONS I SUBMITTED?
Reply | Report Abuse | Link to thisDEAR iWind,PUTTING YOUR PERCEPTIONS/INTERPRETATIONS ASIDE--CAN YOU ANSWER ANY OF THE QUESTIONS I SUBMITTED?
Reply | Report Abuse | Link to this