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TABLE II.-Apollo and Amor Objects (qs 1.15 AU)”

Objects q, AU Q, AU P. yr B(1,0) Meteors Light
variation

1953 EA 1.03 4.0 0.59 4.0 21° 19.6
887 Alinda 1.15 3.9 0.54 4.0 9 16.3 Small
1948 EA 0.89. 3.6 0.60 3.4 18, 16.6
1960 UA 1.05 3.5 0.54 3.4 4 18.1
1936 CA Adonis 0.44 3.3 0.76 2.6 1 18.6 Yes
1580 Betulia 1.11 3.3 0.49 3.2 52 15.7
1968 AA 1.06 3.2 0.51 3.2 24 16.6 Maybe Large
1221 Amor 1.08 2.8 0.44 2.7 12 19.2
1937 UB Hermes 0.62 2.7 0.62 2.1 6 18.1 Maybe
1627 Ivar 1.12 2.6 0.40 2.6 8 14.2
1950 DA 0.84 2.5 0.50 2.2 12 16.1
1971 FA 0.56 2.4 0.62 1.8 22 16.3 Large
1932 HA Apollo 0.65 2.3 0.56 1.8 6 15.6 Maybe
1950 LA 1.08 2.3 0.36 2.2 26 14
1566 Icarus 0.19 2.0 0.83 1.1 23 17.6 Maybe Small
1685 Toro 0.77 2.0 0.44 1.6 9 16.3
1959 LM 0.83 1.9 0.38 1.6 3 14
433 Eros 1.13 1.8 0.22 1.8 11 12.4 Large
1620 Geographos 0.83 1.7 0.34 1.4 13 16.0 Large

*See table I of Roemer (p. 644) for current observational status. cometary origin. This meteorite, a crystalline chondrite (Tuček, 1961), passed only 1.3 AU from Jupiter 6 yr before it collided with Earth, and similar approaches occurred previously at intervals of about 70 yr.

We cannot exclude the possibility of cometary origin for some of the large-Q objects listed in table I. The three librating objects, for example, could be ex-comets that were trapped in libration when the nongravitational forces ceased. (This same explanation is not likely for the Hildas, Thule, and the Trojans because their perihelion distances are too large for the nongravitational effects to have been significant.) Photometric and other physical studies are most desirable, particularly for these three and the objects with q significantly smaller than 2 AU (719 Albert is lost, unfortunately; but we may add the single-apparition object 1963 UA, which has q = 1.2 AU and Q = 4.0 AU, and should be recoverable in 1976). If we decrease the limiting Q to 3.6 AU, the following interesting objects may be included: 132 Aethra (q = 1.6, Q = 3.6), 475 Ocllo (1.6, 3.6), 699 Hela (1.5, 3.7), 898 Hildegard (1.7, 3.7), and 1009 Sirene (1.4, 3.8; lost).

REFERENCES

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A CORE-MANTLE MODEL FOR COMETARY NUCLE AND ASTEROIDS OF POSSIBLE COMETARY ORIGIN

ZDEWEK SEKAW/WA
Smithsonian Astrophysical Observatory

Arguments for a long time have been brought forward in support of the idea that the minor planets with orbits approaching Earth's orbit might be of cometary origin. Our feeling is that before such hypotheses are considered for any particular object, it is necessary to prove that differences in physical appearance and dynamical behavior between a typical asteroid of the Apollo or Albert types and a typical short-period comet can be interpreted in terms of cometary evolution.

In this paper, we discuss models of cometary nuclei, transition of an object from cometary phase into asteroidal phase, and specific asteroidal objects that may be of cometary origin.

PHYSICAL MODEL

Nongravitational (NG) activity in a comet essentially measures the rate of mass output from the nucleus in units of the nuclear mass. An obvious method of studying the NG effects, therefore, is to test various models of mass transfer in the nucleus and the related mass loss rate from the nucleus.

The most obvious possibility is to consider a cometary nucleus composed of freely deposited ices (upper part of fig. 1), which gradually shrinks at a constant rate. The NG effects increase in proportion to the decreasing nuclear dimensions as the comet passes through the early phase (E) into a fading phase (F). The free ice model finally sublimates out completely, leaving no compact debris whatsoever.

Dynamical calculations (e.g., Marsden, 1969, 1970) show that, by contrast, for most comets the NG activity tends to decrease rather than increase with time on a secular scale. Such behavior can be explained in terms of a core-mantle model. This model is composed of a porous core of nonvolatile materials surrounded by an icy envelope. The envelope may be contaminated by loosely distributed solid grains. Secular variations in the NG activity of a core-mantle comet is schematically represented in the lower part of figure 1. The icy shell, of considerable thickness in the early phase, gradually sublimates under the effects of solar radiation; the diameter of the nucleus decreases as

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