[meteorite-list] Photometry, YORP and asteroid morphology

[Matson, Robert]

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