Skip to comments.Earth Must Have Another Moon, Say Astronomers
Posted on 12/22/2011 7:05:56 AM PST by Lonesome in Massachussets
But this was not an object of ours. Instead, 2006 RH120, as it became known, turned out to be a tiny asteroid just a few metres across--a natural satellite like the Moon. It was captured by Earth's gravity in September 2006 and orbited us until June 2007 when it wandered off into the Solar System in search of a more interesting neighbour.
(Excerpt) Read more at technologyreview.com ...
I downloaded the linked article in PDF. Maybe when I get done chewing on that I'll report back.
Captured and released? How exactly did it get "released" to wander off in search of a more interesting neighbor?
bump for later.
It didn’t exactly get released. It just left in disgust.
I think I see it now. It only completed two orbits before moving on - yes?
Did it “wander” off or was it slingshotted? Sounds like it was out for a Sunday stroll and then just moseyed off.
FR's resident Rapturists are slapping their foreheads asking "How could we have missed THAT sign of the imminent End-Of-The-World?"
It’s all the moon’s fault. Without the moon, something not already in Earth orbit can only come by parabolically or hyperbolically (some which crash into the Earth). But a deflection by the moon can put it into a stable orbit, at least until the moon kicks it out again. Which it is likely to do, given how many other moons we have besides the main one, accumulated over billions of years of moon collection time.
The any object orbiting a planet is perturbed by the gravity of every other particle in the solar system. For objects orbiting the earth, the significant perturbing bodies are, more or less in order: the moon, the sun, Venus and Jupiter. Depending on their relative locations, they can provide just enough of a push to free a weakly bound object from the grasp of earth's gravity and continue in orbit around the sun.
Jupiter, of course is the king of capture, remember Shoemaker-Levy? It was captured in 1968, but not observed until 1994, shortly before its perturbed orbit caused it to collide with Jupiter.
The opposite of capture occurs, too. An asteroid approaching a planet's orbit from behind can be given a boost that increases its velocity enough to hurl it out of the solar system. Numerous solar system missions take advantage of the gravitational boost available from Jupiter and the other planets.
Thanks for the nice, succinct explanation.
Damn.....even the asteroids want our welfare money. Go over to Mars! They have free healthcare!
PS: That also explains why the earth and the inner planets do not have significant ring structures and all the outter gas giants do. The relative strength of the perturbing forces, especially the sun, are so much stronger.
Both of Mars’ moons are believed to have been recently (like the past few million years) captured. Phobos is in retrograde orbit, opposite the direction of Mars’ rotation. The effect of tidal drag continually reduces the radius of its orbit and it is expected to collide with mars in a few million more years. (Tides actually increase the radius of our moon’s orbit by about one centimeter per year increase the length of the earth’s day by about 0.0015 seconds per century. The transfer of energy from earth’s rotation to the moon’s orbit is about one gigawatt.)
Too bad it didnt beam up Hussein before it left...
My thoughts exactly. Does it have it's own source of power, if not how did it move out of our gravity field. If it does, how did it get that source of power and is it man made?
Planets can capture objects that cross their orbits. This thing, obviously, was orbiting the earth-moon system, but it came within one lunar distance of something, either the earth or the baricenter. The most likely perturbation that released it from orbit around the earth-moon barycenter was probably due to the moon.
It must have crossed the border from Mexico.
Okay, now that we have all of that orbital eccentricity stuff explained, how about explaining that face on Mars!?
If you look for accurate ephemerides on the web, you will find instead of earth, a mysterious object called EMB. This is the earth-moon barycenter, which must be regarded as the object orbiting the sun as a single mass.
To speak of 2006 RH120 as being "perturbed by the moon" is like saying that Apollo 8, e.g., was orbiting the earth and "perturbed by the moon". I notice that the cited article is ambivalent on this point, which is amusing in light of the flap over the planetary status of Pluto. The embarrassing fact is that the earth-moon system is a double planet.
I was keeping it simple. The EMB is always below the surface of the earth, in any case, saying that an object orbiting the EMB is an object in orbit around the Earth, perturbed by the moon is just another way of looking at it. For that matter, planets orbit the barycenter of the solar system, which can be more than two solar diameters from the center of the sun.
For earth satellites, GPS for instance, the second harmonic of the earth’s gravity field, J2, is about 127 times more significant that the moon. The J2 perturbation is on the order of 0.00024 times the J0 component, the effect a point mass equal to the mass of the earht, located at the center of the earth. The effect of the moon is only about 0.0000019 times the J2 component, on average.
Two effects: the human brain is hardwired to perceive faces and confirmation bias.
OK, I was doing fine until the “second harmonic of earth’s gravity field”. I don’t get it and would like to. You clearly have a clue. Care to share?
Too simple, I would say, for a case where an object is "captured" by the earth-moon system, executes a chaotic trajectory within its sphere of influence, and is then "ejected". I don't think the moon can be considered a perturbing influence in such a case, when it is a full partner with earth in the festivities.
The representation of the earth’s gravitational field you learned in high school, as being spherically symmetrical is not quite true.
In Newtonian mechanics the graviational field resulting from a spherical object of uniform density can be replaced for points outside the object by a point mass located at its center. You may realize, that the actual earth is not a homogenous sphere. If it were (and it didn’t rotate) a plumb bob would always point to the center of the earth. In fact, the earth, or the sea level surface, can be well approximated by an ellipsoid of rotation, with the polar radius about 56 kilometers less than the equatorial radius. (A cross section of the ellipsiod passing through the line joining the North and South pole, will be an ellipse.)
The actual direction that a plumb bob points, ignoring rotation, is always perpendicular to the equipotential surface. This means that away from the poles and the equator the plumb bob does not point at the center of the earth, in the northern hemisphere it points slightly to the south of the center, in southern hemisphere to the north. The effect is greatest at 45 degrees latitude, and the deflection is about a tenth of a degree.
If you were a satellite in a low orbit around the earth, and you happened to be at 45 degrees north latitude you would experience a gravititational acceleration, not towards the center of the earth, but about a tenth of a degree to the south of it. The effect can be resolved into two components, one toward the center of the earth, the J0 component and one at an angle, due to the earth’s equatorial bulge (that extra 56 kilometers around the equator). We call that the J2 component, or Second Zonal Harmonic. (There is no first Zonal Harmonic, the zeroth harmonic is due to the sphercial component of the field.) The effect of the zeroth harmonic decreases as R-squared, the J2 Harmonic as R-cubed. At the surface of the earth it is already only about one one-thousandth as strong as the zeroth component and for objects far enough away it can be ignored.
The latest earth gravity model, EMG96 has over 250,000 harmonics! The first good estimate of the the Earth’s J2 component was not available until we started tracking Sputnik in 1957. There are proposed models with over two million components. Good luck with that.
J2 is an expression of the gravitational quadrupole moment of the earth, due to its oblateness. It alters the shape of a spherical equipotential surface by a few miles out of the 4000 miles of the earth’s radius. The tidal effect of the moon raises and lowers these surfaces by a few feet.
But this is near the surface of the earth. We’re considering an object wandering extensively around the earth-moon sphere of influence.
Well, comet Elenin didn’t work out as planned. :-)
What I don’t get it is, how is this a “second moon”? Doesn’t a body have to at least hang around and orbit a few times to qualify as a moon, or even as a satellite?
But then that’s why I’m not an astronomer.....
Check out this video for a very kewl illustration of the effect of the J2 Harmonic on a satellite orbit:
Mike Gruntman has a whole slew of kewl videos on his website:
or check out his actual website:
This is just hype to pique the interest of the unwashed masses. Everything has to be some kind of mind blow.
Thanks Lonesome in Massachussets.
An “extra, extra” ping to the APoD list members.
Yes, I insist. Have as many as you like, we’ll make more.
You are too kind. :-)
Otherwise we’d have to do the one-cheek sneak, and no one wants that. ;’)
Well, at least it didn’t post a vanity to attempt establishing its conservative credentials before it became a sleeper troll.
Gee, it should be waking up any day now...
Better linkie thingy:
Hey, that gives me an idea... if there’s ever an asteroid headed for Earth impact, one of your Zot messages may just do the trick...
Sure! Anything I can do to help.
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