756 Encyclopedia of the Solar System
C DEFIGURE 17 (Continued)XP14, indicate extreme near-surface structural complexity,
but we cannot distinguish between multiple scattering from
subsurface heterogeneities (see Section 3.7) and single scat-
tering from complex structure on the surface.
3.12.2 IMAGING AND SHAPE RECONSTRUCTION
During the past decade, delay–Doppler imaging of aster-
oids has produced spatial resolution as fine as a decameter.
The images generally can be “north–south” ambiguous, that
is, they constitute a two-to-one (or even many-to-one) map-
ping from the surface to the image. However, if the radar is
not in the target’s equatorial plane, then each surface point
has a unique delay–Doppler trajectory as the target rotates.
Hence images that provide adequate orientational coverage
can be inverted, and in principle one can reconstruct the
target’s three-dimensional shape as well as its spin state.
The first asteroid radar data set suitable for recon-
struction of the target’s shape was a 2.5-hour sequence
of 64 delay–Doppler images of 4769 Castalia, obtained
2 weeks after its August 1989 discovery. The images, which
were taken at a subradar latitude of about 35◦, show a
bimodal distribution of echo power over the full range
of sampled rotation phases, and least-squares estimation
of Castalia’s three-dimensional shape reveals it to consist of
two kilometer-sized lobes in contact. Castalia was the first
of several “contact binaries” revealed by radar.
If the radar view is equatorial, unique reconstruction
of the asteroid’s three-dimensional shape is ruled out, but
a sequence of images that thoroughly samples rotation
phase can allow unambiguous reconstruction of the aster-
oid’s pole-on silhouette. For example, observations of 1620
Geographos yield several hundred images with∼100-m res-
olution. The pole-on silhouette’s extreme dimensions are in
a ratio, 2.76+0.21, that establishes Geographos as the most
elongated solar system object imaged so far (see Fig. 7b).
Delay–Doppler imaging of 4179 Toutatis in 1992 and
1996 achieved resolutions as fine as 125 ns (19 m in range)