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Figure 4.—Set of spectrograms taken in the depolarized mode. The upper curve is from Venus, the lower from Mercury. The individuality of the planets shows strongly here.

a factor of over 20, showing that most of the echo power had originated from the relatively smooth areas near the sub-Earth point. It is for this reason that the depolarized mode of radar observation has not yet been attempted for an asteroid. The power from Mercury, on the other hand, dropped only by a factor of 10, showing that Mercury is rougher (to the scale of a wavelength) than Venus. Two very interesting features appear in the Venus data that do not appear in the Mercury data. These features were the first evidence that the surface of Venus, hidden under its cover of clouds, is not homogeneous. On the contrary, large topographic features exist on the surface, rotate with the planet, and appear year after year to radar view. These features have high radar contrast to the surrounding areas, and have a much stronger ability to depolarize radar waves. It is not known whether they are mountains or craters or some other formation such as lava flows. Their existence, however, permits the rotation of Venus to be determined to very high precision. By tracking the Doppler frequency shift of these objects, a period of rotation for Venus of 243.0 days, retrograde, has been deduced. The direction of the spin axis is (or almost is) perpendicular to the orbital plane. At first glance, the Mercury spectrum of figure 4 contains no significant features. However, a left-right (east-west) asymmetry can be seen, and the motion of this lesser feature has been used to determine the rotation period of Mercury to an accuracy of 0.5 percent. The important thing is that, to radar, Venus is different from Mercury and, presumably, both are different from asteroids. The study of these differences can add to our knowledge of asteroids. o We return now to the problem mentioned earlier, the extreme weakness of echoes from an asteroid. The radar equation shows that the received power is proportional to R*A-* where R is the radius of asteroid and A is the geocentric distance. Thus the small size of an asteroid compared to a planet reduces the received power by a factor of 10°.

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When considering the SNR, account must also be taken of the bandwidth of the echo and of the integration time to

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where v is, the perpendicular component of the velocity of rotation. For a calibration point, figure 5 is a spectrogram from Icarus, taken in June 1968 when the asteroid was 6.5 x 10° km from Earth. The spectrogram required 17 hr of integration. As can be seen, this very noisy spectrogram cannot support much analysis. The edges of the spectrum are not distinguishable, so that it cannot be used to determine the rotation. The rotation, however, was measured with great precision optically (Gehrels et al., 1970), and the combination of the two data types is useful. A lower limit (0.5 km) was set to the radius of Icarus and an upper limit to the reflectivity. A surface model based on Mercury or Venus would not fit the data. The close approach of Icarus was a rare opportunity for radar asteroid astronomy. The next weaker targets are the Jovian satellites Ganymede and Callisto, weaker than Icarus by a factor of 40. Next come the asteroids Vesta and Juno, down by an additional factor of 2. Another factor of 2 brings in Ceres and Pallas. Radar capability continues to grow. The Goldstone radar is 5000 times stronger than when Venus was first detected in 1961. It is stronger by a factor of 6 than when the Icarus experiment was performed. Perhaps when Toro swings by in 1972 (an opportunity comparable to that of Icarus), much more of the potential of radar asteroid astronomy will be realized.

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Figure 5.—Spectrogram of echoes from Icarus taken during closest approach of 1968.

REFERENCES

Gehrels, T., Roemer, E., Taylor, R. C., and Zellner, B. H. 1970, Asteroid (1566) Icarus.
Astron. J. 75, 186-195.
Goldstein, R. M. 1964, Venus Characteristics by Earth Based Radar. Astron. J. 69, 12-18.
Goldstein, R. M. 1969, Radar Observations of Icarus. Icarus 10,430.
Goldstein, R. M. 1970, Radio and Radar Studies of Venus and Mercury. Radio Science
5(2), 391.
Pettengill, G. H., Shapiro, I. I., Ash, M. E., Ingalls, R. P., Rainville, L.P., Smith, W. B., and
Stone, M. L. 1969, Radar Observations of Icarus. Icarus 10,432-435.
Shapiro, I. I. 1968, Planetary Radar Astronomy. IEEE Spectrum 5, 70.

DISCUSSION

CHAPMAN: After the anticipated improvement in the Arecibo dish, do you expect the larger asteroids such as Ceres to be detectable by radar?

GOLDSTEIN: The largest asteroids may just marginally be detected with the Goldstone (64 m) dish now, and they certainly should be detectable when Arecibo resurfaces within a few years.

DESCRIPTIVE SURVEY OF FAMILIES, TROJANS,
AND JETSTREAMS

C. J. VAW HOUTEM
Sterrewacht te Leiden
Wetherlands

The word “group” is so general that I would like to suggest that here it is used in its most general way: a group of asteroids is a collection of minor planets that have some feature in common. If we agree on this use of the word “group,” then we can discern the following asteroidal groups:

(1) Groups that have a dynamical cause:
(a) Trojans
(b) Commensurability groups:
(i) Hungaria group
(ii) Hilda group
(iii) Thule
(2) Groups that probably have no dynamical cause:
(a) Hirayama families
(b) Brouwer groups
(c) Jetstreams

Because the topic of this colloquium is the physical studies of the asteroids, I shall here briefly review the physical studies made on these groups.

TROJANS

These are asteroids moving near the lagrangian points L4 and L5 of the Sun-Jupiter system. There are 15 numbered Trojans, but at fainter magnitudes than the limit of the ephemeris they are very numerous; it was shown that around L5 there are 700 Trojans brighter than B(a,0) = 21.0 (van Houten, van Houten-Groeneveld, and Gehrels, 1970). Their distribution as a function of absolute magnitude is similar to the normal asteroids. Rotation lightcurves of three Trojans were obtained (Gehrels, 1970) and they show relatively large amplitudes. The color measurements indicate a small ultraviolet excess (Gehrels, 1970), but the number of Trojans observed (2) is too small to be certain on this point. (U-B = 0.22 of two Trojans against s 0.40 of field asteroids.) The phase function of the Trojans will be discussed in my next

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