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 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?
LOL!!
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.
http://en.wikipedia.org/wiki/Spherical_harmonics
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:
http://www.youtube.com/watch?v=uZc0YJjyWGM
Mike Gruntman has a whole slew of kewl videos on his website:
http://astronauticsnow.com/vp/index.html
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 much.
Thanks Lonesome in Massachussets.
An “extra, extra” ping to the APoD list members.
:’D
Thanks!
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Must we?
Yes, I insist. Have as many as you like, we’ll make more.
You are too kind. :-)
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