758 Encyclopedia of the Solar System
FIGURE 19 High-resolution
Goldstone 3.5-cm, OC,
delay–Doppler images from each of
three observation dates in 1996 and
the corresponding plane-of-sky (POS)
appearance of the radar model. The
crosshairs are 5 km long and centered
on Toutatis’ center of mass (COM). In
the radar images, time delay (range)
increases from top to bottom, and
Doppler frequency (radial velocity)
increases from left to right. The
model is rendered with a Lambertian
scattering law, with the viewer
co-located with the illumination
source. The crosshairs are aligned
north–south and east–west on the
plane of the sky. In each POS frame,
the arrow radiating from the COM
shows the POS projection of the
instantaneous spin vector.configuration involving monolithic fragments with various
sizes and shapes, presumably due to collisions in various en-
ergy regimes. Toutatis might be an impact-sculpted, single,
coherent body, or it might consist of two separate objects
that came together in a gentle collision; the difference in
the two lobes’ gravitational slopes supports the latter idea
(Fig. 20).
Toutatis is rotating in a long-axis non-principal-axis
(NPA) spin state (see Fig. 21) characterized by periods of
5.4 days (rotation about the long axis) and 7.4 days (aver-
age for long-axis precession about the angular momentum
vector). The asteroid’s principal moments of inertia are in
ratios within 1% of 3.22 and 3.09, and the inertia matrix
is indistinguishable from that of a homogeneous body.
Such information has yet to be determined for any other
small body except theNEAR-Shoemakertarget 433 Eros
and probably is impossible to acquire in a fast spacecraft
flyby.
Images of another NPA rotator, 53319 (1999 JM8), reveal
an asymmetric, irregularly shaped, 7-km object (Fig. 22).
The asteroid’s rotation has a dominant 7-day period and is
not far from uniform rotation. 1999 JM8 has pronounced
topographic relief, prominent facets several kilometers in
extent, and numerous crater-like features between∼100 m
and 1.5 km in diameter.
Radar images of 6489 Golevka (1991 JX) reveal a half-
kilometer object whose shape is extraordinarily angular,
with flat sides, sharp edges and corners, and peculiar con-
cavities. Extremely large gravitational slopes in some areas
of the radar-derived model indicate the presence of ex-
posed, solid, monolithic rock (Fig. 20). This asteroid, the
first sub-kilometer object studied in this much detail, prob-
ably is a monolithic collision fragment rather than a rub-
ble pile. Golevka was the target of the first intercontinental
radar observations, in June 1995, when Goldstone provided
a transmission and echoes were received by the Russian
70-m Evpatoria antenna and also by the Japanese 34-m
Kashima antenna. The asteroid’s name is made from lead-
ing letters of those antennas’ names.
Radar has revealed numerous NEAs to have nearly cir-
cular pole-on silhouettes [e.g., 1999 RQ36, 7822 (1991
CS), 2100 Ra-Shalom, 1998 ML14 and 1998 FH12]. 1998
ML14 has isolated, several-hundred-meter protrusions on
one side, while 1999 RQ36 has no noticeable features any-
where. At the opposite extreme, several NEAs, including
22771 (1999 CU3) and 2003 MS2 have elongated shapes
with curious irregularities. Ironically, the dogbone shape
of the 235-km-long main-belt object Kleopatra is the most
exotic yet discerned by radar (Fig. 20).
Asteroids with visual absolute magnitudeHV>21 (di-
ameters 0.2 km or less) constitute about one-fourth of radar-
detected NEAs; the smallest are comparable in size to
boulders seen on the surface of Eros. Most of them have
rotation periods no longer than an hour and in some cases
only a few minutes, but at least two, 2001 EC16 and 2004
XP14, are very slow rotators.3.12.3 BINARY SYSTEMS
Radar obtained the first undeniable evidence for NEA
binary systems and has now imaged 20 of them. Cur-
rent detection statistics, including evidence from optical