SMITHSONIAN

CONTRIBUTIONS

to

ASTROPHYSICS

Smithsonian Institution

Astrophysical Observatory

Number 14

Discrete Levels off

Beginning Height off

Meteors in Streams

By A. F. Cook

Number 15

Yet Another Stream

Search Among 2401

Photographic Meteors

By A. F. Cook, B.-A. Lindblad,

B. G. Marsden, R. E. McCrosky,

and A. Posen

Smithsonian Institution Press

SMITHSONIAN CONTRIBUTIONS TO

ASTROPHYSICS

NUMBER 14

A. F. cook Discrete Levels of

Beginning Height of

Meteors in Streams

SMITHSONIAN INSTITUTION PRESS

CITY OF WASHINGTON

1973
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Library of Congress Cataloging in Publication Data

Cook, Allan F.

Discrete levels of beginning height of meteors in streams.

(Smithsonian contributions to astrophysics, no. 14.)

Bibliography: p.

1. Meteors. I. Title. II. Series.

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A. F. cook Discrete Levels of

Beginning Height of

Meteors in Streams1

Introduction

Ceplecha (1968) plotted the photographed meteors reduced by McCrosky and Posen (1961) and

not identified as members of streams, with beginning height as ordinate and velocity outside the

atmosphere as abscissa. His Figure 1 exhibits the

basic result. He found three ridges of maximum density of points. The lowest of these he designated as

Class A, the highest as Class C, and an intermediate

maximum apparent only from 27.5 to 43.7 km sec"1

as Class B. His Class C showed two peaks, one below 41.8 km sec"1

 and the other above it, and he

designated these regions as Classes Ci and C2,

respectively.

Ceplecha favored the interpretation that the differences in beginning height were due solely to variations in density of the meteoroid. Subsequent

studies by McCrosky and Ceplecha (1970) and

by the author (in preparation) provide plots of

log (I/Km) versus log V.,, where Km is a coefficient

in the deceleration equation for meteors and V. is

the velocity of the meteoroid outside the atmosphere. The three classes appear in these plots too.

However, the separation is not enough to explain

fully the separations of Ceplecha's discrete levels.

He also found that meteors of Class A had trajectories shorter than those of Class C, which implies

that they have larger values of Jacchia's (1955)

fragmentation index x- This provides the explanation for the remainder of the separation of beginning

heights.

It appears, therefore, that we may be able to use

A. F, Cook, Smithsonian Institution Astrophysical Observatory,

Cambridge, Massachusetts 02138.

1

 Published 1970 as Smithsonian Astrophysical Observatory

Special Report No. 884.

Ceplecha's classes to separate streams of meteors

according to the density of the meteoroids. Ceplecha (1968) has already done this for some streams.

Lindblad (1971b) subsequently analyzed the meteors of McCrosky and Posen (1961) statistically

to identify several new streams. Identifications of

streams from more restricted groups of meteors had

previously been made by Whipple (1954), Jacchia

and Whipple (1961), Southworth and Hawkins

(1963), and Lindblad (1971a). McCrosky and Posen

(1959) identified streams from the same sample as

did Lindblad (1971b). The present paper considers

these additional streams as well as the better known

ones classified by Ceplecha.

Criteria for Reality of Streams

We accept as real only those streams that show

four meteors, or three meteors tind an associated

comet, in McCrosky and Posen's (1961) list or those

that are already well known. This is almost the criterion recommended by Lindblad (1971b). Inasmuch as most of the observations used by McCrosky

and Posen were obtained from 1952 to 1954, there

is little assurance that all these streams recur annually. Incidentally, for the purposes of this paper, the

Cyclids (Southworth and Hawkins, 1963) are regarded as the product of observational selection,

caused by the earth's enhanced collisional cross section, for meteoroids in orbits nearly the same as that

of the earth; they are not counted as a stream. Application of these criteria yields 39 optical streams.

Of these, 1 does not appear in the observations, 12

do not show enough meteors to permit classification

(this point will be discussed further below), and 11

exhibit such extreme character that some remark

about their densities can be made in spite of the
SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 1.—The three Taurid streams of Lindblad {1971b)

Name Lindblad's duration

Radiant (Eq. 1950)

VG NO. of

RA Dec. (km sec"1) meteors

N. i Aquarids*.

Piscids

Taurids

21 Aug.-20 Sept.

25 Sept.-19 Oct.

19 Sept.-21 Nov.

0.33

0.40

0.33

2.00

2.06

1.99

0.83

0.80

0.83

4°0

3.4

3.3

300°

291

119

161°

199

29

101°

130

148

354C

26

40

+ 1

+ 14

+ 13

31

29

31

3

9

91

* McCrosky and Posen (1961) identify two additional members on 27 July and 18 August, which would raise the number of meteors to five and

extend the duration back to 27 July. The author concurs in these identifications.

small number of meteors involved. We are left with

15 streams that are sufficiently abundant to be

unambiguously classified.

Classification of Streams

The logical method of classification is to begin

with the most abundant stream and continue with

decreasing numbers until it is evident that we are

approaching uncertain ground.

1. The most abundant stream is the extended one

of the Taurids, which are 105 in number and are

usually divided into a northern and a southern component. Lindblad's search does not make this division but instead subdivides the stream into three

overlapping parts according to the sun's longitude

Lo. The elements q, a, i, « remain constant as LG

increases, while ft and tr progress steadily from part

to part of the stream (here q is the distance at perihelion, a the semimajor axis, i the inclination, « the

argument of perihelion, ii the longitude of the ascending node, and x the longitude of perihelion).

There is a reversal of nodes as Lindblad switches

from predominance of the northern branch to that

of the southern branch. Table 1 presents the details

of the Taurid streams found by Lindblad. It is apparent that the total activity extends over almost

4 months, growing steadily until the maximum at

about 1 November given by McKinley (1961). Figure 1 exhibits the distribution of beginning heights

of this stream.

The heights for Ceplecha's Classes Ci, B, and A

are marked in Figure 1. The coincidence of the maximum with Class Ci is evident, as are also the rather

abrupt decline in numbers at greater beginning

heights and the straggling tail to lower beginning

heights. If the distribution were symmetrical, we

might hope that as few as 32 = 9 meteors would suffice to classify a stream. The process that causes

110

84

10 20

NUMBER OF METEORS

30

FIQUBE 1.—Distribution of beginning heights for the

extended stream of the Taurids.

NUMBER OF METEORS

FIGURE 2.—Distribution of beginning heights for the

Piscids.
NUMBER XIV

skewing toward the lower heights implies, however,

that 9 meteors will be insufficient. Inasmuch as

Lindblad has segregated 9 of these Taurids as

Piscids, we can examine their distribution as shown

in Figure 2. They appear to suggest Class B, but

since it is evident that three mavericks at lower

beginning heights could account for this appearance,

we conclude that more than 9 meteors are required

to classify a stream unambiguously.

2. The Geminids are 77 in number and are displayed in Figure 3. This shower is plainly of Class

B. Again we see the skew tail to lower beginning

heights.

3. The Orionids, displayed in Figure 4, are 49 in

number. They are plainly of Class Ct and do not

show the skew distribution.

4. The Perseids with 45 meteors are shown in

Figure 5. Again these are clearly of Class C2 and the

distribution is symmetric.

5. The Andromedids (which from the current direction of the radiant have been called e Piscids)

number 33 meteors. The distribution appears in

Figure 6 and is clearly bimodal with peaks at

108

100

90

82

10 20

NUMBER OF METEORS

E

E

to

UJ

I

o

to

•<

o

30

122

120

94

5 10

NUMBER OF METEORS

15

FIGURE 5.—Distribution of beginning heights for the

Perseids.

FIGURE 3.—Distribution of beginning heights for the

Geminids.

5 10

NUMBER OF METEORS

15

A AT 100

TO 102 km

106

- 100

to

Z 90

80

UJ

El

IE 5

FIGURE 4.—Distribution of beginning heights for the

Orionids.

0 5 10

NUMBER OF METEORS

FIGURE 6.—Distribution of beginning heights for the

Andromedids.
SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Classes Ci and A or lower. These peaks are delineated well enough for us to conclude that 16 meteors

are more than sufficient to classify a stream with

one peak, whereas we have already seen that 9

meteors are apparently insufficient.

6. The 8 Leonids, newly discovered by Lindblad

(1971b), contribute 24 meteors to our sample. Figure 7 exhibits a bimodal distribution with peaks at

Classes Ci and A. The peaks are well denned so that

we conclude that 12 meteors are sufficient to classify

a stream with one peak. Since 9 meteors appear to

be insufficient, we adopt 10 meteors as a working

lower limit for well-defined classification unless we

see what appears to be a bimodal distribution; in

108

E

co 100

2

i

1

= 90

82

0 5 »

NUMBER OF METEORS

FIGURE 7.—Distribution of beginning heights for the 5

Leonids.

Jwr

10

NUMBER OF METEORS

FIGURE 8.—Distribution of beginning heights for the 3

Aquarids.

such a case the working least number of meteors will

be 20.

7. The 5 Aquarids number 22 meteors. Figure 8

exhibits the distribution of beginning heights. Noise

in the distribution is noticeable, but it seems clear

that the stream is of Class B.

8. The a Capricornids with 21 meteors are shown

in Figure 9. The Class is Ci (although B cannot be

104

100

90

80

'////////////A

UJ

E CO

<

0 5 10

NUMBER OF METEORS

FIGURE 9.—Distribution of beginning heights for the a

Capricornids.

0 9

NUMBER Of METEORS

FIGURE 10.—Distribution of beginning heights for the «•

Leonids.
NUMBER XIV

excluded), with a skew tail on the distribution to

lower heights.

9. The a Leonids with 19 meteors are exhibited in

Figure 10. This stream was discovered by Southworth and Hawkins (1963). It appears to be Class B

(although Class A cannot be entirely ruled out). We

note two very low heights observed, which suggest

skewness to lower heights in this case.

10. The Quadrantids with 17 meteors are shown

in Figure 11. This stream is clearly Class B.

11. The T Herculids number 15 meteors. Figure

12 shows their distribution; the stream is plainly of

Class A or lower with a suggestion of a skewness to

greater heights. The stream was discovered by

South worth and Hawkins (1963).

12. The 5 Arietids with 14 meteors were discovered by McCrosky and Posen (1959). Figure 13 exhibits the distribution. The stream is of Class A

(or possibly B), with considerable noise in the

distribution.

13. The Southern t Aquarids with 13 meteors are

exhibited in Figure 14. They are plainly of Class A.

It should be noted that the orbit of the Southern

t Aquarids is very different (a much larger semimajor axis and a somewhat smaller distance at perihelion) from the Taurid-like orbit of the Northern

t Aquarids; it also produces a geocentric velocity

100

CLASS A AT 94

TO 96 km

o NUMBER OF METEORS

0

FIGURE 11.—Distribution of beginning heights for the

Quadrantids.

76

E

V///7////77A

0 9

NUMBER OF METEORS

FIGUR E 13.—Distribution of beginning heights for the i

Arietids.

90

e

<o

UJ

0 5 10

NUMBER OF METEORS

100

3

m 84

• F

0

NUMBER OF

IE

1

5

METEORS

-ECHA'S

ASSES

CLASS

104 TO

C, AT

106 km

FIGURE 12.—Distribution of beginning heights for the r FIGURE 14.—Distribution of beginning heights for the

Herculids. Southern t Aquarids.
SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

some 5 to 10 km sec"1 higher for the Southern i

Aquarids than for the Northern i Aquarids.

14. The x Orionids with 12 meteors are shown in

Figure 15. They are of Class Ci. The two very low

meteors have abrupt beginnings and thus are not

really comparable to the others. The x Orionids

were first reported by Whipple (1954).

106

100

90

82

V////////A

o 5

NUMBER OF METEORS

FIGURE 15.—Distribution of beginning heights for the x

Orionids.

15. The x Scorpiids with 11 meteors are newly

discovered by Lindblad (1971b). Figure 16 exhibits

the distribution of beginning heights. It appears to

fit Class B with a broad distribution. The isolated

meteor at great height may be a badly reduced

interloper.

16. The K Cygnids number 10 meteors and the

distribution of heights is shown in Figure 17. While

we might classify this stream as Ci, there is a suggestion of a double peak, i.e., Ci and A. The meteors

are too few to delineate a bimodal distribution, so

that we must treat this stream as unclassifiable.

One stream, the n Sagittariids with four meteors,

is so extreme in beginning heights that we can classify it as Class A or lower. This stream is newly discovered by Lindblad (1971b). Two other streams,

the October Draconids with two meteors and the

Monocerotids with three, are so extreme that we

can classify them as higher than Class d . The latter

stream was first reported by Whipple (1954). Finally, the Leonids with five meteors are so extreme

that they can be classified as higher than C2.

Seven streams are sufficiently extreme that Class

A can be ruled out: Lyrids with five meteors, rj

Aquarids with seven, o Serpentids with four, K Aquarids with five, e Geminids with seven, Leo Minorids with three, and a Hydrids with six. Of these

streams, the 0 Serpentids are newly discovered by

Lindblad (1971b), the #c Aquarids by Lindblad

(1971a); the c Geminids and Leo Minorids were discovered by McCrosky and Posen (1959), and the a

Hydrids by Jacchia and Whipple (1961).

M 6 r

no

x 100

o

90

84

IE-;

El

0 5

NUMBER OF METEORS

FIGURE 16.—Distribution of beginning heights for the

Scorpiids.

108 r

0 1 2 3

NUMBER OF METEORS

FIGURE 17.—Distribution of beginning heights for the K

Cygnids.
NUMBER XIV

The 11 streams that remain are too poorly represented in our sample to be classified: 5 Cancrids

(7 meteors), Coma Berenicids (7 meteors), Northern

Virginids (including Northern X Virginids) (6 meteors), K Serpentids (4 meteors), n Virginids (7 meteors),

a Scorpiids (5 meteors), a Bootids (8 meteors), <j>

Bootids (6 meteors), 0 Ophiuchids (4 meteors), a

Triangulids (4 meteors), and Pegasids (5 meteors).

Of these streams, the 5 Cancrids, n Virginids, a

Scorpiids, and a Triangulids are newly discovered

by Lindblad (1971b); the a Bootids, <f> Bootids, and

0 Ophiuchids were discovered by Southworth and

Hawkins (1963), and the Coma Berenicids, K

Serpentids, and Pegasids by McCrosky and Posen

(1959).

Finally, one optical stream is well known and has

not been observed in the list of McCrosky and

Posen (1961): the Corona Australids. It is a Southern Hemisphere shower.

Table 2 summarizes the foregoing results of

classification.

Associated Comets

Table 2 includes in its last column the currently

preferred identity of the parent comet of each

stream. If we confine our attention to comets now

accessible to observation, we find two streams above

Ci, one above C2, two Ci, two C2, one not A, and

one A. P/Comet Halley contributes two streams,

so that the classification of the rj Aquarids as not A

is redundant and should be preempted by the classification of the Orionids as C2. If we consider the

one comet known to have disappeared (P/Comet

Biela), we find its stream exhibits Classes A and Ci

together. If we consider the one comet no longer

in an orbit accessible to observation (17701 Lexell),

we find its stream to be of Class A or lower.

One comet's stream cannot be classified (1819 IV,

Blanplain).

Discussion

Four salient points command attention:

1. Meteoroids of Ceplecha's Class A have a density of about 1.2 g cm"8

, approaching that of Type I

carbonaceous chondrite meteorites (2 g cm"3) (McCrosky and Ceplecha, 1970; Cook, in preparation).

2. Whipple and Stefanik's (1966) model for the

redistribution of ices within the nuclei of comets by

radioactive heating might lead naturally to gravitational compaction of the less volatile material in

the interior, a natural explanation of Ceplecha's

Class A and of carbonaceous chondrites of Type I

(although rather large nuclei would be required in

the latter case). Meteoroids of Classes Ci and C2

would then be the low-density residual framework

left after evaporation of the volatile ices from the

outer shell.

3. Millman, Cook, and Hemenway (1971) find

that faint Perseid meteors exhibit spectra ranging

from the traditional atomic-line spectra of vaporized meteoroids to almost entirely atmospheric

radiations or an extended continuum or both followed by an interval of radiation from atomic lines

of meteoric elements lower along the trajectory.

This type of behavior seems the most probable

cause for the skew distribution toward lower heights

because the Super-Schmidt cameras with X-ray

film do not photograph the nonblue part of the

atmospheric radiations or the continuum, but do

respond to those in blue to ultraviolet from the

meteoric vapors.

4. Whipple and Stefanik pointed out that many

smaller comets that separate from large comets are

short lived, as would be expected if only a piece of

the ice-impregnated surface peeled off. If a comet

cracks all the way through, then some core is left to

hold the ice-impregnated surfaces together, which

matches the behavior of P/Comet Biela. To complete the picture for that comet, we need only conjecture that the icy surface layers either peeled or

were exhausted at the last observed return.

These considerations lead naturally to several

ideas. Two inert pieces disguised as small asteroids

may yet remain. The exposure of both core and impregnated surface layers would explain the observed

bimodal A and Ci distribution. A similar explanation would follow for the 8 Leonids, although no

comet has ever been observed in that orbit.

Furthermore, the presence of some ice in an old

cometary core is to be expected because, of the

ice sublimed by the sun, some would evaporate

away but some would diffuse inward and redeposit

as frost on the cold interior. In this context, we

may understand the A classification of the meteors

from the very faint Comet 1930 VI, SchwassmannWachmann 3. Recovery of this comet is desirable.

B. G. Marsden (private communication) indicates

that the next opportunity will come in 1979. Finally, the lower density of Class B streams com-
SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 2.—Optical meteor streams classified according to Ceplecha's (1968) system

Name

Radiant (Eq. 1950)

Duration RA Dec.

Number

VG of

(km sec"1) meteors

Ceplecha's

class Comet

Quadrantids

* Cancrids

Coma Berenicids...

N. Virginids

N. X Virginids

8 Leonids

Corona Australids..

a Leonids

K Serpentids

ft Virginids

a Scorpiids

a Bootids

4> Bootids

Lyrids

7 Aquarids

r Herculids

X Scorpiids

$ Ophiuchids

0 Serpentids

it Sagittariids

a Capricornids..

S. t Aquarids

S. 8 Aquarids

N. b Aquarids

Perseids

K Cygnids

N. t Aquarids

Piscids

8. Taurids)

N. TauridsJ

Andromedids

* Aquarids

October Draconids.

Orionids

« Geminids

Leo Minorids

Pegasids

a Triangulids

Leonids

N. x Orionids

S. x Orionids

9 Hydrida

Geminids

1 Arietids

Monocerotids

2-3 January

13-21 January

13-23 January

3 Feb.-12 March

4-15 April

5-19 February

14-18 March

21 March-13 May

1-7 April

I April-12 May

II April-5 May

9-12 May

14 April-12 May

16 April-12 May

21-22 April

3-12 May

19 May-14 June

27 May-20 June

4-16 June

9-25 June

22 June-6 July

15 July-10 Aug.

4-9 August

19 July-6 Aug.

5-22 August

21 July-8 Aug.

5-25 August

8-15 August

10 Aug.-6 Oct.

27 July-20 Sept.

25 Sept.-19 Oct.

19 Sept.-21 Nov.

31 Aug.-2 Nov.

11-28 September

9 October

14 Oct.-7 Nov.

16-27 October

22-24 October

29 Oct.-12 Nov.

7-12 November

15-20 November

4-13 December

7-14 December

4-15 December

4-16 December

8 Dec.-2 Jan.

10-17 December

299°

126

187

173

210

159

245

195

230

221

235

247

218

240

271

340

228

246

266

274

268

304

317

320

348

340

347

46

273

354

26

40

10

338

276

95

102

162

344

22

152

83

85

128

111

54

104

+49°

+20

+19

+ 5

-10

+19

-48

- 5

+18

- 5

-21

-24

+19

+51

+34

- 2

+40

-12

-28

-11

-15

-10

-17

-15

-10

-16

+ 1

+57

+61

+ 1

+14

+13

+ 6

- 5

+49

+ 16

+27

+37

+19

+30

+23

+26

+16

+ 2

+32

+25

+10

42

28

64

36)

32 J

23

20

45

29

34]

35j

23

16

47

67

18

23

30

30

23

251

28 j

351

41 j

43]

40 J

60

22

31

29

31

27

20

21

67

70

62

16

21

71

28

28

58

37

17

42

17

7

7

24

0

19

4

7

8

6

5

7

14

11

4

4

4

21

12

91

105

12

A+C,

B*

not A

not A

A or lower

B

not A

A or lower

Ci

A

B

c,

1861 I, Thatcher

P/Halley

1930 VI,

SchwassmannWachmann 3

1770 I, Lexell

1954 III, HondaMrkosPajduSakova

1862 III, SwiftTuttle

P/Encke

A+C,

not A

above Ci

c,

not A

not A

—

above C*

not A

B

A**

above C,

P/Biela

P/GiacobiniZinner

P/Halley

1739, Zanotti

1819 IV,

Blanplain

P/Tempel-Tuttle

1917 I, Mellish

* Class A cannot be entirely ruled out. •• Class B cannot be entirely ruled out. 112 northern, 2 southern.
NUMBER XIV

pared with that of Class A may be due to comets

so small that their cores were gravitationally compacted to a density of only about 0.6 g cm"1

. We

then assume that their ice-impregnated low-density

surfaces have been completely removed. Again, the

cores would have accumulated some frost from ice

that sublimed and diffused inward. These comets

would now be very faint or would be disguised as

asteroids. It is perhaps not surprising that no comets have been detected in association with streams

of Class B.

In this context, it is difficult to believe that a

stream as rich as the Geminids does not still have a

parent body. The configuration and shortness of

period of the orbit make it the only reasonable candidate for a search for such a body among the

streams of Class B.

In conclusion, it is desirable to review the classifications for reliability and the individual streams

for certainty of their existence. The adopted classifications appear to the author to be unassailable for

10 streams (Taurids, Geminids, Orionids, Perseids,

Andromedids, 8 Leonids, Quadrantids, r Herculids,

Southern i Aquarids, and x Orionids). These are

distributed two each over the possible classifications—A (including A or lower), B, Ci,andCj. There

is room for the possibility that the A+Ci distributions may be uniform and broad without a minimum at B. Nevertheless, all the arguments made

above can be put forward on the'basis of these

streams alone.

It is therefore desirable to classify as many other

streams as possible. Another stream (5 Aquarids)

is almost as securely classified as the 10. Three

more streams have somewhat uncertain classifications (a Capricornids, a Leonids, and 8 Arietids).

It might be tempting but not very good practice to

take note of Ceplecha's failure to find the ridge at

Class B down to lower velocities among the sporadic

meteors and move the a Leonids to Class A. Similarly, the possibility of Class B would then be ruled

out for the 8 Arietids. One stream is of very uncertain classification (x Scorpiids). If the Class

A+Ci has in fact a broad uniform distribution

without a minimum at B, the x Scorpiids would

become unclassifiable.

It does not seem important to comment in this

vein on the classification of the extreme cases among

the less numerous streams except to draw attention

to the three streams whose existence is most open

to doubt: the 6 Ophiuchids, o Serpentids, and M Sagit tariids. AH these radiants come from a diffuse

region of general activity in June and early July

in Scorpius, Sagittarius, Ophiuchus, and Serpens.

While more than one stream must contribute to this

activity, the computer programs of Hawkins and

South worth and of Lindblad may have arbitrarily

grouped these meteors to yield the three listed

streams of only four meteors each.

Special suspicion attaches to a stream associated

with Comet 1770 I, Lexell, because it only came

close to the sun twice (in 1770 and 1776), having

passed close to Jupiter in 1767 and 1779. The existence of a detectable stream almost 2 centuries

later is rather dubious.

It would be convenient to be rid of these streams

since the n Sagittariids are Class A or lower and

Comet Lexell was an embarrassing 5 magnitudes

brighter than Comet 1930 VI (Schwassmann-Wachmann 3) and thus not a good candidate for a nearly

exhausted comet. Evidently, further study of individual radiants and velocities of meteors in June

and early July in Scorpius, Sagittarius, Ophiuchus,

and Serpens is desirable.

Acknowledgments

It is a pleasure to acknowledge discussions with

Prof. Fred L. Whipple, Dr. Richard E. McCrosky,

and Prof. Bertil-Anders Lindblad of Lund Observatory, who have read this paper. Particular thanks

for a very detailed reading and extensive discussions

are due Dr. Brian G. Marsden.

This work was supported in part by contract

NSR 09-015-033 and grant NGR 09-015-004 from

the National Aeronautics and Space Administration.

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Abstract

Of the 26 streams of meteors classified according to Ceplecha's discrete levels of beginning

height, 13 are associated with known comets. Comet Biela produced in the Andromedids a

double-peaked distribution (Classes A and Ci). Apparently no known comets produce a stream

of Class B. Consideration of Whipple and Stefanik's model of an icy conglomerate nucleus with

radioactive heating and redistribution of ice leads to association of Ceplecha's Class C with the

residue of the ice-impregnated surface of a cometary nucleus after sublimation of the ices, and

of Ceplecha's Class A with the core of a cometary nucleus. Class B meteoroids are then to be

associated with less dense cores of smaller cometary nuclei that have lost their surfaces and are

too small to have been observed. Furthermore, the density of Class A meteoroids (1.2 g cm"1)

is so close to that of Type I carbonaceous chondrites (2 g cm"1) as to suggest that the latter

come from old cores of very large nuclei of comets, an idea originally proposed by McCrosky

and Ceplecha.

It is suggested that two inert objects that look like asteroids may yet remain from the two

pieces observed at the last return of P/Comet Biela. The recovery of Comet 1930 VI, Schwassmann-Wachmann 3, at its return in 1979 is urged since it is the only available comet producing

a shower (T Herculids) of Class A. A search for an asteroidal object or a very small comet in the

orbit of the Geminids is also urged as the best chance of finding an object that produces meteoroids of Class B. Further study of the distribution of radiants and velocities of meteors in June

and early July in Scorpius, Sagittarius, Ophiuchus, and Serpens is required to sort out the true

structure there, if indeed one exists.