764 Encyclopedia of the Solar System
radar sounder to probe many kilometers below that object’s
fractured crust using meter- to several-decameter wave-
lengths. Anather possibility is to use radar reflection tomog-
raphy to construct a three-dimensional image of the inte-
rior of an asteroid or comet. Meanwhile, the Mars Advanced
Radar for Subsurface and Ionospheric Sounding (MARSIS)
on the European Space Agency’sMars Expressspacecraft
has probed the depths of Mars’ north and south polar de-
posits, and the Shallow Subsurface Radar (SHARAD) is
about to start searching for subsurface water on Mars from
NASA’sMars Reconnaissance Orbiter.
Reconnaissance of near-Earth asteroids will occupy
ground-based radar astronomy indefinitely. Most of the op-
tically discoverable NEAs traverse the detectability win-
dows of Arecibo and/or Goldstone at least once every few
decades, and efforts are under way to increase the NEA
discovery rate by more than an order of magnitude. The
power of radar observations for orbit refinement and phys-
ical characterization motivates radar observations of newly
discovered NEAs whenever possible. Eventually the initial
radar detection of a new NEA could become an almost daily
opportunity.
Bibliography
Black, G. J., Campbell, D. B., and Nicholson, P. D. (2001).
Icy Galilean satellites: Modeling radar reflectivities as a coherent
backscatter effect.Icarus 151 , 167–180.
Butrica, A. J. (1996). “To See the Unseen: A History of Plane-
tary Radar Astronomy,” NASA History Series No. SP-4218. NASA,
Houston.
Harmon, J. K., Nolan, M. C., Ostro, S. J., and Campbell, D. B.
(2004). Radar studies of comet nuclei and grain comae. In “Comets
II” (M. Festou, U. Keller, and H. Weaver, eds.), pp. 265–279. Univ.
Arizona Press, Tucson.
Magri, C., Ostro, S. J., Rosema, K. D., Thomas, M. L., Mitchell,
D. L., Campbell, D. B., Chandler, J. F., Shapiro, I. I., Giorgini,
J. D., and Yeomans, D. K. (1999). Mainbelt asteroids: Results of
Arecibo and Goldstone radar observations of 37 objects during
1980–1995.Icarus 140 , 379–407.
Ostro, S. J. (1993). Planetary radar astronomy.Rev. Modern
Physics 65 , 1235–1279.
Ostro, S. J., and Giorgini, J. D. (2004). The role of radar in pre-
dicting and preventing asteroid and comet collisions with Earth. In
“Mitigation of Hazardous Comets and Asteroids” (M. J. S. Belton,
D. K. Yeomans, and T. H. Morgan, eds.), pp. 38–65. Cambridge
Univ. Press, Cambridge, England.
Ostro, S. J., Hudson, R. S., Benner, L. A. M., Giorgini,
J. D., Magri, C., Margot, J.-L., and Nolan, M. C. (2002). Aster-
oid radar astronomy. In “Asteroids III” (W. Bottke, A. Cellino,
P. Paolicchi, and R. P. Binzel, eds.), pp. 151–168. Univ. Arizona
Press, Tucson.
Pettengill, G. H., Ford, P. G., Johnson, W. T. K., Raney, R. K.,
and Soderblom, L. A. (1991). Magellan: Radar performance and
data products.Science 252 , 260–265.
Shapiro, I. I., Chandler, J. F., Campbell, D. B., Hine, A. A., and
Stacy, N. J. S. (1990). The spin vector of Venus.Astron. J. 100 ,
1363–1368.
Tyler, G. L., Ford, P. G., Campbell, D. B., Elachi, C., Pettengill,
G. H., and Simpson, R. A. (1991). Magellan: Electrical and physical
properties of Venus’ surface.Science 252 , 265–270.