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### Some highlights in radar astronomy (and a little bit of others...)

This series of radar images of the asteroid 4769 Castalia were taken at approximately 2 minute intervals on Aug. 22, 1989, at the Arecibo Observatory. The horizontal axis is Doppler frequency, and the vertical axis is the time the echo was received (delay). These two scales map differently onto distance, which has been approximately compensated by replicating the pixels in the vertical dimension, yielding a resolution of approximately 150 m (horizontal) by 300 m (vertical). The images are fainter at the beginning and the end of the sequence because the antenna gain was a fairly strong function of zenith angle when these images were taken. Each image is the sum of 27 separately reduced (Fourier transformed) data sets, or "looks". The sampling frequency was 16 ms, and there are 64 complex samples in each look, giving a resolution of 0.95 Hz (corresponding 150m), and allowing 27 looks to be summed. The total time of each observation is about 30 s. (From "http://naic.edu/~nolan/radar/Castalia.html")

Goldstone recording of the asteroid Toutatis on Dec. 8, 9,10 and 13, 1992 (on the left). All eight days of Toutatis observations (on the right). A 400 kW coded radio transmission was beamed at Toutatis from the Goldstone main 70-meter antenna when the asteroid was about 4 million km from the Earth. The echoes, which took 24 seconds to travel to Toutatis and back, were received by the new 34-meter antenna and relayed back to the 70-meter station, where they were decoded and processed into images. The time required to obtain the images were 55, 14, 37, and 85 minutes, respectively. In these images the radar illumination comes from the top of the page, so parts of each component facing towards the bottom are not seen. The two irregularly shaped, cratered objects are about 4 and 2.5 km in average diameter and they are probably in contact with each other. The large grater seen in the Dec. 9 image is about 700 m in diameter. (From "http://128.165.1.1/solarsys/toutatis.htm" and Public Information Office, JPL)

Asteroid Geographos viewed from above its north pole. The radar images were obtained with the Deep Space Network's facility at Goldstone on August 30, 1994, when the asteroid was 7.2 million km from Earth. The tick marks on the borders are 1 kilometer apart. The central white pixel locates the asteroid's pole. The gray scale is arbitrary and no meaning is attached to brightness variations inside the silhouette. (Dr. S. Ostro, JPL/NASA, "http://128.165.1.1/solarsys/geograph.htm")

The first radar detection of Comet Hyakutake. The Goldstone X-band radar operates at a frequency of 8510 MHz ($\lambda$=3.5 cm). One Hertz is equivalent to a radial velocity of one half wavelength per second. (From "http://www.jpl.nasa.gov/comet/hyakutake/news35.html")

And finally, for future target determination :-), the Hubble Space Telescope view of the asteroid Vesta. The images show the full 5.34-hour rotation of the 525 km diameter asteroid. Hubble resolves features as small as 80 km across, allowing astronomers to map Vesta's geologically diverse terrain. Features include ancient lava flows and a gigantic impact basin. This sequence was taken with Hubble's Wide Field Planetary Camera 2 between November 28 and December 1, 1994, when Vesta was at a distance of 252 million km (1.7 AU) from Earth. When combined with ground-based data, astronomers will be able to make the first geochemical map of Vesta's surface. (Photo release no. STScI-PRC95-20A)

### References

• Bond P., 1992, "Radar explorers of the Solar System", New Scientist, 5 December.
• Goldstein R.M., 1971, "Asteroid characteristics by radar", in Gehrels T. (ed.), Physical studies of minor planets, NASA SP-267.
• Evans J.V. and Hagfors T. (eds.), 1968, Radar Astronomy, McGraw-Hill Book Comp.
• Hapke B., 1990, "Coherent Backscatter and the Radar Characteristics of Outer Planet Satellites", Icarus, 88, 407.
• Harmon J.K. and Slade M.A., 1992, "Radar Mapping of Mercury: Full-Disk Images and Polar Anomalies, Science, 258, 640.
• Harmon J.K., Slade M.A., Velez R.A., et al., 1994, "Radar Mapping of Mercury's Polar Anomalies", Nature, 369, 213.
• Hudson R.S., and Ostro S.J., 1994, "Shape of Asteroid 4769 Castalia (1989 PB) from Inversion and Radar Images", Science, 263, 940.
• Karttunen H., Kroger P., Oja H., et al. (eds.), 1987, Fundamental astronomy, Springer Verlag.
• Mitchell D.L., Ostro S.J., Rosema K.D. et al., 1995, "Radar Observations of Asteroids 7 Iris, 9 Metis, 12 Victoria, 216 Kleopatra, and 654 Zelinda", Icarus, 118, 105.
• Ostro S.J., Campbell D.B., Pettengill G.H., and Shapiro I.I., 1980, "Radar Detection of Vesta", Icarus, 43, 169.
• Ostro S.J., Campbell D.B., and Shapiro I.I., 1983, "Radar Observations of Asteroid 1685 Toro", Astron. J., 88, 565.
• Ostro S.J., Campbell D.B., and Shapiro I.I., 1985, "Mainbelt Asteroids: Dual-Polarization Radar Observations", Science, 229, 442.
• Ostro S.J., Campbell D.B., Hine A.A., et al., 1990a, "Radar Images of Asteroid 1627 Ivar", Astron. J., 99, 2012.
• Ostro S.J., Chandler J.F., Hine A.A. et al., 1990b, "Radar Images of Asteroid 1989 PB", Science, 248, 1523.
• Ostro S.J., Campbell D.B., Chandler J.F., et al., 1991a, "Asteroid Radar Astrometry", Astron. J., 102, 1490.
• Ostro S.J., Campbell D.B., Chandler J.F., et al., 1991b, "Asteroid 1986 DA: Radar Evidence for a Metallic Composition", Science, 252, 1399.
• Slade M.A., Butler B.J., and Muhleman D.O., 1992, "Mercury Radar Imaging: Evidence of Polar Ice", Science, 258, 635.
• Wehner D.R., 1987, High resolution radar, Artech House.

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