Hi All, Ron's forwarded article on the tricky task of measuring the YORP effect on the NEA (54509) 2000 PH5 reminds us that nearly all asteroids in the couple-hundred meter to several kilometer size range are not monolithic, but rather rubble piles. Support for this conclusion derives from the important work (mentioned earlier today by Brian Warner) that many amateurs do in asteroid photometry that allows NEA rotation rates to be determined.
In 2000, Pravec and Harris reported that out of over a hundred asteroids studied larger than 200 meters, none was found to be rotating with a period shorter than 2.27 hours. This period roughly corresponds to the critical breakup spin rate for a strengthless body with a bulk density of ~3 g/cm^3. The YORP effect offers an additional explanation for and/or contributing factor to some of the bizarre bimodal shapes, suspected contact binaries, and true binaries that our known. Consider dogbone-shaped (and likely metallic) 216 Kleopatra, or dolphin-shaped 1620 Geographos: <http://echo.jpl.nasa.gov/asteroids/1620_Geographos/pics/f_8_rev.gif> While tidal disruption of a rubble-pile following a Roche-limit pass of earth, Venus or Jupiter is the most likely explanation for the shapes of these two objects and others (e.g. 12 Victoria, 433 Eros, 2963 Bacchus, 4179 Toutatis, 4769 Castalia), the YORP effect may further mold their morphology and evolution. Perhaps some of the binary asteroids known today evolved via YORP from what were once contact binaries or strongly-lobed singlets. --Rob ______________________________________________ Meteorite-list mailing list [email protected] http://six.pairlist.net/mailman/listinfo/meteorite-list
