760 Encyclopedia of the Solar System
FIGURE 21 Spin state of asteroid 4179
Toutatis derived from radar. The axes
with no arrow tips are the asteroid’s
principal axes of inertia and the vertical
arrow is its angular momentum vector.
The direction of the spin vector (yellow
arrow) relative to the principal axes is a
(5.41-day) periodic function. A
flashlamp attached to the short axis of
inertia and flashed every 15 minutes for
20 days would trace out the intricate
path indicated by the small spheres
stacked end to end; the path never
repeats. Toutatis’ spin state differs
radically from those of the vast majority
of solar system bodies that have been
studied, which are in principal-axis spin
states. For those objects, the spin vector
and angular momentum vector point in
the same direction, and the flashlamp’s
path would be a circle.binaries 1990 OS and 2003 YT1 may be relatively young
systems.
FIGURE 22 Arecibo 13-cm, OC radar image of 53319 (1999
JM8). Radar illumination is from the top. The vertical resolution
is 15 m. The horizontal resolution depends on the asteroid’s NPA
spin state, which is not yet known. [From Benner, L. A. M.,
et al., 2002,Meteoritics Planet. Sci. 37 , 779–792.)
3.12.4 COLLISION PREDICTION AND PREVENTION
The NEA collision hazard has emerged as a primary driv-
ing issue in asteroid science. Radar provides very precise
astrometric positions and leads to more accurate trajectory
predictions than optical data alone. On average, radar has
added a third of a millennium to the window of accurate
future predictions of PHA close Earth approaches. When
radar astrometry is excluded from single-apparition PHA
radar+optical orbit solutions, some 40% cannot have their
next close approach predicted within the adopted confi-
dence level using only the single apparition of optical data.
The net effect of radar for multiapparition cases is to im-
prove the orbit’s accuracy. For example, integrations of the
radar-refined orbit of 29075 (1950 DA) revealed that in
2880 there could be a potentially hazardous approach that
had not been indicated in the half-century arc of preradar
optical data. The dominant source of uncertainty in the col-
lision probability involves the Yarkovsky effect, which is the
nongravitational “recoil” acceleration of a rotating object
due to its anisotropic thermal emission of absorbed sun-
light, and which depends on the asteroid’s size, shape, mass,
rotation, and optical and thermal characteristics.