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    Re: JPL announces Earth has another Moon!
    From: Frank Reed
    Date: 2017 Oct 24, 08:15 -0700

    Some folks following this story may miss the point of the latest press release, especially following Brad's admittedly tongue-in-cheek post. This object was discovered about a year and a half ago, and it was recognized at the time as a "quasi-satellite" of the Earth. Just to be clear, it is definitely not a second moon, though that's a good starting point for the discussion. Its orbit around the Sun is perturbed by the Earth in such a way that its motion is synchronized with the Earth over a period of a few centuries. It is also a rather explicit example of the Earth's gravitational dominance of this area of the Solar System --in fact, it is indeed a case of the Earth "clearing its neighborhood" in the loose terminology that has become associated with the definition of a major planet.

    The press release last week discusses research that has been conducted during the past year which has confirmed that the object is a natural object since there was a significant possibility that this was a large piece of "space history" (some of you may remember that there is at least one S-IVB stage from the Apollo missions in orbit around the Sun that has been observed near Earth in recent years). Here's the press release from the University of Arizona which hosted the research:
    https://uanews.arizona.edu/story/earths-new-buddy-asteroid-not-space-junk

    It is a tiny asteroid, which might reasonably be called a meteoroid to emphasize its small size, somewhere between 30 and 100 meters diameter. An interesting result of the current research is the object's rotation rate: about 28 minutes. Since the surface-skimming orbit around all objects, regardless of size, is in the range from 90 minutes to three hours or so, this single result proves that the object is a solid boulder. If it were a "rubble pile" or if it even had any pebbles or bits of sand on its surface (not too close to the poles of rotation), those bits of rock would be ejected from the surface with speeds greater than escape velocity merely due to the asteroid's rotation.

    Frank Reed

       
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