Prospects for meteor shower activity in the venusian atmosphere

Icarus 168 (2004) 23–33 www.elsevier.com/locate/icarus

Prospects for meteor shower activity in the venusian atmosphere Apostolos A. Christou Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK Received 4 July 2003; revised 5 November 2003

Abstract We investigate the possibility of detectable meteor shower activity in the atmosphere of Venus. We compare the Venus-approaching population of known periodic comets, suspected cometary asteroids and meteor streams with that of the Earth. We find that a similar number of Halley-type comets but a substantially lesser population of Jupiter family comets approach Venus. Parent bodies of prominent meteor showers that might occur at Venus have been determined based on minimum orbital distance. These are: Comets 1P/Halley, parent of the η Aquarid and Orionid streams at the Earth; 45P/Honda–Mrkos–Pajdusakova which currently approaches the venusian orbit to 0.0016 AU; three Halley-type comets (12P/Pons–Brooks, 27P/Crommelin and 122P/de Vico), all intercepting the planet’s orbit within a 5-day arc in solar longitude; and Asteroid (3200) Phaethon, parent of the December Geminids at the Earth. In addition, several minor streams and a number of cometary asteroid orbits are found to approach the orbit of Venus sufficiently close to raise the possibility of some activity at that planet. Using an analytical approach described in Adolfsson et al. (Icarus 119 (1996) 144) we show that venusian meteors would be as bright or up to 2 magnitudes brighter than their Earth counterparts and reach maximum luminosity at an altitude range of 100–120, 20–30 km higher than at the Earth, in a predominantly clear region of the atmosphere. We discuss the feasibility of observing venusian showers based on current capabilities and conclude that a downward-looking Venus-orbiting meteor detector would be more suitable for these purposes than Earth-based monitoring. The former would detect a shower of an equivalent Zenithal Hourly Rate of at least several tens of meteors.  2004 Elsevier Inc. All rights reserved. Keywords: Meteors; Meteoroids; Venus, atmosphere

1. Introduction The meteor-producing meteoroid population is divided into the sporadic background and meteoroid streams, the latter giving rise to annual, as well as occasional, meteor showers. The stream component is thought to be the main contributor of meteoroids of this size range in the Solar System (Whipple, 1967; Hughes, 1975). Using the Earth’s atmosphere as an area detector for these meteoroids can, however, only yield information at 1 AU. Observations of meteors and meteor showers in the atmospheres of other planets will enable studies of the relative abundance and spatial distribution of meteor-producing meteoroids, typically in the 10−6 –102 g mass range, at different heliocentric distances. It will allow detection of showers that are either entirely unobservable at the Earth or observable at more than one planet. In the former case, it will be possible to study the parent objects of those streams by proxy, if E-mail address: [email protected]. 0019-1035/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2003.12.006

identified. In the latter case, observations will help confirm or reject suspected stream-parent associations. Additionally, the spatial structure of these streams, both in orbital longitude and in cross-section, will be mapped more efficiently since encounter conditions at each planet will generally be different. Despite the outer planets’ huge atmospheric area which would serve these purposes well, the terrestrial planets, and in particular Venus and Mars, are significantly more accessible by spacecraft. Mars, for example, combines a predominantly clear atmosphere and a solid surface to serve as a stable observation platform. It is also the target of a vigorous international exploration program. It is not surprising, therefore, that studies have been carried out to investigate the possibility of observing meteors in the atmosphere of that planet. Adolfsson et al. (1996) found that so-called photographic meteors, with typical absolute magnitudes ranging from −1m to +4m , would be as intrinsically bright at Mars as they are at the Earth for speeds above 30 km sec−1 (absolute visual magnitude of a meteor being its apparent magnitude if it was placed

24

A.A. Christou / Icarus 168 (2004) 23–33

at the zenith and at a height of 100 km). Slower meteors would be significantly fainter. The meteors would be at their maximum brightness at altitudes typically ranging from 90 to 50 km in the martian atmosphere. Christou and Beurle (1999) and subsequently Treiman and Treiman (2000) and Larson (2001) searched through known comet, asteroid, and meteor stream orbit databases for potential parent bodies of martian meteor showers and concluded that several such objects, such as Comets 1P/Halley and 13P/Olbers, do exist. Venus, on the other hand, may not appear as appealing. Spacecraft that have operated on its surface to date have only survived for a few hours at most. In any case, the venusian sky is perpetually hidden from the surface owing to the presence of perennial, planet-encircling, cloud layers (Esposito et al., 1983). This still leaves the possibility of observing meteors from above the atmosphere, a technique that was employed recently in the monitoring of meteor showers at the Earth (Jenniskens et al., 2000). This paper investigates the potential of using the venusian atmosphere as a meteor shower detector. This is done as follows: in Section 2 we characterise the expected meteor shower parent body population at Venus by comparing the orbits of known periodic comets, asteroids, and meteor streams with those of Venus and the Earth. We quantify the relative statistics, identify particularly promising candidates for such meteor activity and provide detailed information to aid observational searches. Where appropriate, we compare our results with those of earlier works, most notably Beech (1998). In Section 3 we utilise an analytical method used in Adolfsson et al. (1996) (hereafter AGM96) to show that a given meteoroid ablating in the atmosphere of Venus would give rise to a meteor which

2. Stream predictions at Venus

upper limit for this quantity constitutes a necessary, but not sufficient, condition for deciding whether a particular object can give rise to a meteor shower at the planet. In previous work, Beech (1998) (hereafter B98) searched for known meteor streams with heliocentric nodal distances less than 0.1 AU from those of Earth and Venus. A search was also conducted in that paper for comets and asteroids with a heliocentric nodal distance within 0.1 AU to that of Venus. As in Beech and Brown (1995), emphasis in that paper was given to characterising the large body meteoroid population (> 107 g) while we are concerned with a different part of the mass spectrum, typically 10−6 –102 g; one of Beech’s objects could have been responsible for a transient event observed by the OUVS instument aboard the Pioneer Venus Orbiter on February 17, 1979 (Huestis and Slanger, 1993). Nevertheless, his search concerning proximity of meteor stream, comet and asteroid orbits to those of the Earth and Venus is relevant here; we shall comment on his results and how they compare to ours as appropriate throughout this section of the paper. The primary tool employed in this search is a program written by the author and utilising the computer algebra package Mathematica (Wolfram, 1999). It calculates the value of ∆ between two given Keplerian ellipses in threedimensional space. In this respect, our investigation is different than B98 who used heliocentric nodal distances. The latter can significantly overestimate distances between threedimensional orbits at low relative inclination and, as a result, promising parent body candidates could be missed. The formulation of the algorithm can be found in Christou and Beurle (1999). It is numerically implemented by providing four initial values of the true anomaly for each of the two orbits, spaced by 90◦ . The output consists of a number of local orbit-to-orbit distance minima and the corresponding critical true anomalies of the respective orbits. To test the algorithm we have used the search tool available at the University of Pisa’s NeoDys site (http://newton. dm.unipi.it/cgi-bin/neodys/neoibo) to identify, as of May 12, 2003, high-eccentricity asteroids (e
0.7) whose orbits approach that of the Earth closer than 0.01 AU. For the 27 objects involved, the absolute differences between the NeoDys Minimum Orbit Intersection Distances (MOIDs) with our calculated ∆ estimates have a median of 7 × 10−5 AU. The median of the MOIDs themselves is 2.5 × 10−3 AU. Therefore, we are confident that our ∆ estimates are sufficiently precise for the purposes of this paper.

2.1. Method

2.2. Comets

In this section we quantify expectations on the population of meteor streams at Venus relative to the Earth. We tackle this by conducting a census of known comet, asteroid, and meteor stream orbits that approach the two planets. The main sample discriminator employed here is the minimum object–orbit-to-planet–orbit distance ∆. Choosing an

Gas-induced ejection of particles from a cometary nucleus is an established mechanism for creating meteor streams (Kirkwood, 1861; Whipple, 1950). Although there is no infallible rule to determine which comets generate prominent meteor showers (“prominent” here means activity detectable with current instrumentation as exemplified

(a) is intrinsically as bright or brighter than in the atmosphere of the Earth or Mars, (b) reach maximum brightness well above the main cloud decks and the haze layer which exists at higher altitudes. In Section 4 we examine the feasibility of observing meteors under these conditions. We address both Earth-based and in situ observational searches, as a background luminosity increase on the night side of Venus and by placing meteor-detecting instruments aboard Venus orbiters, respectively.

Meteors in the venusian atmosphere

in Section 4), annually recurring activity is typically associated with Earth-approaching Halley-type comet (HTC) orbits (Jenniskens, 1994), defined here as those comets with periods between 20 and 200 yr. On the other hand, Jupiter family comets (JFCs)—with periods less than 20 yr—yield prominent activity mainly in the form of outbursts (Jenniskens, 1995). Using a known comet database should, therefore, generate statistically significant results on the venusian stream population as well as pick out individual comets that come exceptionally close to the orbit of Venus and, as such, make outstanding stream parent candidates. Here we have used the JPL DASTCOM comet orbital element database as of January 2003. We have excluded comets with “C/” designations which are single-apparition comets with periods greater than 30 yr (Marsden, MPC 35155, 1999) but we have kept “defunct” or “D/”-prefixed comets. The resulting sample consists of 149 secure (numbered) periodic comets, 73 “P/” designated comets, which are singleapparition (unnumbered) periodic comets with periods less than 30 yr and 9 objects listed as defunct. The inevitable consequence of this choice is that we do not use the total number of HTCs within DASTCOM in our analysis as to avoid objects for which their energy state is uncertain. Rather, we put the emphasis on those comets which are “securely periodic.” In any case, inspection of the database reveals that it contains only 3 single apparition HTCs with perihelia inside 1.2 AU so our statistics for this subpopulation will not be significantly affected. The orbits of the Earth and Venus were held fixed with osculating elements taken from the HORIZONS ephemeris service (Giorgini et al., 1996) for the J2000 epoch. The program is run for these objects, searching for local minima where the orbit of the comet and the planet come within 0.2 AU. This should exclude comets with no significant potential for meteor showers at Venus while at the same time allow the compilation of meaningful speed statistics. Note that there can be more than one occasion that this happens for a given pair of orbits and these are treated as distinct cases. As a result of this procedure, we find 33 cases for the Earth and 14 for Venus. For these we also calculate the associated relative velocities (both v∞ and impact speed v), the radiant in the sky in the J2000 Earth-Equator–Equinox reference frame and the solar longitude from the planet at the minimum distance. Population statistics are presented in Table 1 with the speed statistics illustrated separately in Fig. 1. Note than the HTC sample also contains a Venus-approaching longperiod comet, 153P/Ikeya–Zhang. Overall, there are approximately three times more Earth-approaching comets than Venus-approaching ones. Although the numbers of HTCs are similar, there is a 4 : 1 ratio of JFC cases approaching the two planets corresponding to a 6 : 1 ratio of individual cometary orbits.

25

Table 1 Statistics of comets approaching the orbits of Earth or Venus to within 0.2 AUa Ntotal Earth 35 (29) Venus 13 (11)

NHTC 7 (6) 7 (7)

NJFC

v (v∞ ) v (v∞ ) v (v∞ ) (km sec−1 ) (km sec−1 ) (km sec−1 )

28 (23) 18.0 (14.3) 6 (4) 32.8 (31.2)

60.4 (59.4) 45.8 (44.7)

17.0 (13.0) 25.6 (23.5)

a The first three columns give the number of instances (“cases”) where a comet’s orbit satisfies the distance criterion for the entire sample, Halleytype comets only and Jupiter family comets only. The actual number of distinct comets is given in brackets. The remaining three columns give the median atmospheric impact speed v, in the same order, at the two planets and at a height of 190 km. The corresponding median geocentric speed v∞ is given in brackets.

Fig. 1. Histogram of impact speeds for comets in our sample that approach the Earth (a) and Venus (b) to within 0.2 AU. Bin size is 10 km sec−1 .

Looking at the overall speed statistics, impact speeds at Venus are almost twice as large as Earth-encounter speeds. with the JFC and HTC regimes being substantially different. HTCs encounter Earth 30% faster on average than they do Venus. A closer inspection of the sample reveals this to be due to a combination of more retrograde comets in the Earth sample and small number statistics. JFCs, on the other hand, encounter Venus 50% faster than they do the Earth. The JFC impact speed median estimate for the Earth, in particular, is somewhat lower than that reported recently by Hughes and Williams (2000) partly because, in that study, the authors only considered comets with perihelia inside 1 AU. To select candidate meteor shower parent bodies within the comet sample we now further reduce our maximum ∆ criterion to 0.08 AU. This value is close to the 0.1 AU “rule of thumb” for determining which comets can produce meteor showers at the Earth (see, for example, McIntosh, 1991) and is discussed in Jenniskens (1994) in the context of a stream’s effective cross-section. One would also expect streams to encounter Venus nearer to their perihelia than the Earth and so to be more concentrated in terms of their crosssectional area. No correction has been applied for this effect.

26

A.A. Christou / Icarus 168 (2004) 23–33

Table 2 Encounter characteristics of Halley-type comet orbits (20 yr < P < 200 yr) that approach Venus (V) to within 0.08 AUa Designation 1P/Halley 1P/Halley 12P/Pons–Brooks 27P/Crommelin 122P/de Vico

Planet

∆ (AU)

v (km sec−1 )

E V V V V

0.0662 0.0487 0.0731 0.0255 0.0530

67.4 80.0 53.1 28.4 58.7

α (deg)

δ (deg)

λ (deg)

338.3 1.4 47.7 354.5 4.9 70.0 198.3 66.2 256.1 319.7 67.17 252.1 135.3 −44.0 259.6

a Minimum orbit distance ∆, impact speed v, right ascension α and dec-

lination δ of the radiant and solar longitude λ are given. Occurences at which the same cometary orbits approach the Earth’s (E) within the same distance are also listed. Note the similar solar longitudes for the three comets (apart from comet Halley) that approach Venus.

This criterion is satisfied by 17 cases in our sample for the Earth and 8 cases for Venus. The encounter parameters for the two planets are given in Tables 2 and 3 for HTCs and JFCs, respectively. Our results overlap with those of B98 for four of his eleven comets. The DASTCOM database we used does not contain Comets D/1770 L1 Lexell, D/1917 F1 Mellish and D/1937 D1 Wilk. Furthermore, B98 has investigated the different orbits of a given comet during several perihelion returns and found that 7P/Pons–Winnecke, 26P/Grigg–Skjellerup, and 35P/Herschel–Rigollet satisfied his nodal proximity criterion until 1869, 1902, and 1939, respectively. On the other hand, he does not identify Comet 45P/Honda–Mrkos–Pajdusakova, presumably because this comet’s low inclination causes the nodes to be situated far from Venus whereas the orbit itself approaches that of Venus to within 0.01 AU. As in the 0.2 AU sample, the population ratio of HTCs for the two planets is similar. Note that all the Earthapproaching comets found in our study are associated with significant meteor activity at the Earth (1P/Halley— η Aquarids and Orionids, 55P/Tempel–Tuttle—Leonids, 109P/Swift–Tuttle—Perseids). A reasonable inference here is that one would expect a similar level of HTC-related meteor activity at Venus. 1P/Halley, in particular, approaches both planets and also Mars (Christou and Beurle, 1999) while the other three Venus-encountering orbits appear to do so within a short inverval in solar longitude (7.5◦) and hence time (4.6 days). This would be a prime monitoring period for venusian meteor shower activity.

The statistics for close-approaching JFC orbits again reflect our initial findings for the 0.2 AU sample. Earth is approached on 14 occasions as opposed to only 4 for Venus. These correspond to 12 and 3 comets respectively, a 4 : 1 ratio. For some of the comets in the Earth-approaching sample, fairly strong cases for comet-stream association have been made. Comet 8P/Tuttle is thus related to the December Ursids, known for their far-comet type outbursts (Denning, 1923; Larsen, 1994; Jenniskens et al., 2002); 21P/Giacobini–Zinner is the parent of the October Draconids (Denning, 1915); the relatively new π Puppid shower in April is linked to 26P/Grigg–Skjellerup (Baggaley, 1973); 45P/Honda–Mrkos–Pajdusakova is related to the α Capricornids in August; and 73P/Schwassmann–Wachmann 3 has been associated with the τ Herculids which produced an outburst in 1930 (Nakamura, 1930). On the basis of the statistics alone, one would expect 1–2 of the Venus-approaching comets in Table 3 to be associated with occasional, prominent meteor activity at Venus. The case of 45P/Honda–Mrkos–Pajdusakova is exceptional and is here discussed in more detail. This comet’s orbit currently makes two close approaches to that of Venus, at 0.0016 and 0.01 AU, respectively. At Earth, this comet has been associated with the α Capricornid stream (Hasegawa, 1990) on the basis of its radiant. Interestingly, the other two comets in our list, 72P/Denning–Fujikawa and 141PA/Machholz 2 have also been suggested as possible parents in Hasegawa (2001) along with (2101) Adonis and (9162) 1987 OA. Adonis, in particular, has been dynamically connected to a number of streams active in July with radiants in Sagittarius as well as their possible returning branches in February (Babadzhanov, 2003), but not specifically with the α Capricornids. A comparison with JFCs associated with meteor activity at the Earth is appropriate here. Such comets are 21P/Giacobini–Zinner and 26P/Grigg–Skjellerup, regarded as the parent bodies of the Draconid shower in October and the π Puppid shower in April, respectively. Both these showers are characterised by typically low to nonexistent annual activity punctuated with outbursts with ZHRs varying between a few tens (π Puppids) to several thousand meteors per hour (Draconids) (Jenniskens, 1995). This erratic behaviour is due to the fact that these comets undergo significant orbit-changing encounters with Jupiter typically every cen-

Table 3 Encounter characteristics (as in Table 2) of Jupiter family comet orbits (P < 20 yr) that approach Venus to within 0.08 AU Designation 45P/Honda–Mrkos–Pajdusakova 45P/Honda–Mrkos–Pajdusakova 72P/Denning–Fujikawa 45P/Honda–Mrkos–Pajdusakova 45P/Honda–Mrkos–Pajdusakova 72P/Denning–Fujikawa 141P-A/Machholz 2

Planet

∆ (AU)

v (km sec−1 )

α (deg)

δ (deg)

λ (deg)

E E E V V V V

0.0599 0.0619 0.0745 0.0016 0.0100 0.0551 0.0644

27.0 27.0 21.8 25.6 25.6 15.3 19.1

325.5 330.0 271.4 317.9 338.0 286.4 279.6

−14.2 −19.0 −42.3 −21.2 −12.0 −49.5 22.1

141.7 326.5 250.3 300.7 168.4 197.3 238.6

Occurences at which the same cometary orbits approach the Earth’s within that distance are also listed.

Meteors in the venusian atmosphere

tury. This timescale is comparable to the time it takes meteoroids freshly ejected from the comet to disperse around the comet’s orbit (e.g., McIntosh, 1991). As a result, significant activity is likely (but not certain) when the comet itself is close to the Earth, with ∆ typically no more than a few times 10−2 AU, and the latter is at the critical solar longitude. This mechanism gives rise to near-comet outbursts as defined in Jenniskens (1995). Using the above as a guiding principle to Comet 45P/ Honda–Mrkos–Pajdusakova, we note that the comet arrived in its present orbit in 1983 after a 0.11 AU encounter with Jupiter (Carusi et al., 1985). Meteoroids ejected from the nucleus in its current orbit would therefore be close to the cometary longitude and thus prominent meteor activity in the venusian atmosphere can be expected only when the comet itself is near Venus. On future perihelion returns in the next 20 years, the comet approaches Venus on two instances, in June 2006 at 0.085 AU and in January 2017 at 0.18 AU. For the first instance in particular, Venus passes through the first critical longitude (∆ = 0.01 AU) on June 9th, five days after the comet and the second (∆ = 0.0016 AU) on August 31st, 38 days after the comet. Both these instances should be regarded as opportunities for observing prominent meteor shower activity in the atmosphere of another planet. In addition, opportunities exist to observe meteor activity related to Comets 72P/Denning–Fujikawa and 141P/Machholz 2 (fragment A) on April 4th and 30th, 2005, respectively, when these comets are near perihelion. As a final note, all but one of the HTC and JFC radiants in Tables 2 and 3 have solar elongations in excess of 45◦ and thus should be detectable by optical as well as radio/radar methods. The one exception corresponds to the 0.0016 AU approach of the orbit of 45P/Honda–Mrkos–Pajdusakova. Detection of meteors associated with this, otherwise promising, candidate is likely to rely heavily on radar or radio techniques. However, optical methods could take advantage of the significant gravitational focusing of the meteoroid trajectories for low to moderate encounter speeds and may still generate useful results. 2.3. Asteroids The existence of a meteor stream associated with a comet can be predicted with a significant probability of success from its orbit proximity to a planet’s and the absence of strong planetary perturbations at least in the comet’s recent dynamical past. The situation regarding asteroids is less clear, both on the issue of formation mechanism and the establishment of parent-stream associations, the latter owing to the dearth of observations (see Jopek et al., 2002, for a review). On the other hand, a comet can cease to be active and become “asteroidal” either by exhausting its volatiles or developing a crust which prevents outgassing (Weissman et al., 1989, 2002; Rickman et al., 1990; Coradini et al., 1997). Observationally, the strongest case for an association

27

between an asteroid and a prominent meteor stream is the object (3200) Phaethon and the December Geminids (Whipple, 1983; Williams and Wu, 1993). Apart from its stream association, Phaethon does not exhibit any other cometary characteristics such as outgassing (Chamberlin et al., 1996). Its spectral type, BF, is consistent with both a bona-fide asteroidal or cometary nature. Hence, the nature of Phaethon is still a matter of debate. Several minor streams have also been linked to near-Earth asteroids (Drummond, 1982; Olsson-Steel, 1988) by virtue of orbital similarity. Establishing such associations can provide valuable clues to the nature of objects suspected to originate from comet source regions and it is under this premise that the following search is carried out. Several authors have compiled lists of objects suspected to be inactive comets owing to their dynamical or physical properties. Bottke et al. (2002) considered the possible source regions of near-Earth objects (NEOs) and, through extensive numerical integrations, assigned probabilities to every source region for individual objects. As part of their study, they identified objects that are likely to have originated from the Jupiter family of comets on dynamical grounds. These are found in Table 1 of Weissman et al. (2002). Here we consider the 20 objects with PJFC of more than 50%. To this set we add the objects with Tisserand parameter less than 2 listed in Table 2 of the same paper. Fourteen of these objects have 17 occasions with ∆ < 0.2 AU. Eight of those approach the Earth to within 0.08 AU on 12 occasions and within 0.01 AU on 3 occasions. The corresponding statistics for Venus are 5 objects on 6 occasions, 3 objects on 3 occasions and no objects within 0.01 AU. Notably, three of the objects that approach both Earth and Venus within 0.2 AU have similar orbits to three minor streams, namely the δ Leonids, the Virginids and the Piscids. Their encounter characteristics are given in Table 4. The orbital similarity Table 4 Earth (E) and Venus (V) encounter parameters (as in Table 2) for cometary asteroids 1a Designation δ Leonids 1999 RD32

Virginids 1998 SH2 Piscids 1984 QY1

Planet

∆ (AU)

α (deg)

δ (deg)

v (km sec−1 )

λ (deg)

E V V

0.067 0.019 0.115

168 166 142 174

+16 +10 −4 −1

23 25 23 22

336 345 122 20

E V

0.010 0.024

195 195 176

−4 −2 −8

30 22 15

4 27 100

E V

0.175 0.125

5 9 17

−1 4 4

26 35 39

177 173 185

a These objects are characterised by either (a) a 50% or larger probability to have originated in the JFC source region according to Bottke et al. (2002) or (b) Tisserand parameter value of T < 2 as listed in Weissman et al. (2002). In addition, they have been found to approach both planets to within 0.2 AU and have orbits similar to those of certain minor meteor streams. The encounter parameters of these streams at the Earth are also given.

28

A.A. Christou / Icarus 168 (2004) 23–33

case seems to be strongest for the δ Leonids and the object 1999 RD32. The Southworth–Hawkins metric D (Southworth and Hawkins, 1963) yields a value of 0.20 whereas the modified metric D by Drummond (1981) gives 0.09. In comparison the Asteroid (4450) Pan, for which a case for association with this stream has also been made (Olsson-Steel, 1988), yields D and D values of 0.20 and 0.13, respectively. The remaining two possible associations in Table 4 are less certain with values of the Drummond criterion of 0.26 and 0.12, respectively. Since the Earth and Venus encounters for these showers and their putative associations occur at different values of ∆ in each case, a systematic monitoring program operating at both planets can discriminate between genuine genetic relationships and chance associations. A last group of objects suspected to be extinct or dormant comets was given originally in Weissman et al. (1989) and also appears in Table 3 of their 2002 paper. Results for those objects that approach the orbit of Venus within 0.08 AU are given in Table 5. Also provided is information on their possible meteor shower associations found in the literature. (2101) Adonis and (2201) Oljato approach the two planets at similar distances and so meteor activity should occur at the same level. (2212) Hephaistos currently approaches Venus at 65% the distance at which it approaches Earth. Observations at the critical epoch may reveal some activity at Venus and thus provide insight on the nature of this asteroid. Finally, (3200) Phaethon, the parent body of the Geminids approaches the orbit of Venus at 0.04 AU while the stream orbit itself is at 0.06 AU. This raises the possibility that the Geminids are active on both planets. If that is the case, new Table 5 Earth and Venus encounter parameters (as in Table 2) for cometary asteroids 2a Designation α Capricornids Adonis

χ Orionids Oljato

δ Cancrids Hephaistos

Geminids Phaethon

Planet

∆ (AU)

α (deg)

δ (deg)

v (km sec−1 )

λ (deg)

E E V V

0.0122 0.0218 0.0137 0.0222

307 313 295 301 309

−10 −17 −23 −17 −24

23 27 27 28 28

127 319 105 297 128

E E V V

0.0017 0.0097 0.0065 0.0083

82 80 87 66 100

23 27 20 20 25

28 23 23 20 20

250 79 270 41 306

E E V V

0.1235 0.1994 0.0797 0.0954

130 161 142 153 154

20 7 28 13 30

28 31 31 33 33

297 172 305 155 323

E V

0.0209 0.0415

112 115 119

33 33 33

35 36 42

262 263 271

a These are objects suspected to be inactive (dormant or extinct) cometary nuclei as reported in Weissman et al. (1989) and with ∆Venus < 0.08 AU. Also given are the encounter parameters for streams for which a case of association with these objects has been made in the literature.

information about the nature of the Geminid meteoroids and its parent body would be obtained by monitoring the stream at Venus. 2.4. Known streams A characterisation of meteor activity at Venus would not be complete without a search for known streams with orbits that also approach Venus. For this purpose we have utilised the 38 meteor stream list in Rendtel et al. (1995). A list of their mean orbits was compiled using photographic orbits provided therein or radar orbits where the former were not available. Orbits for 5 showers could not be found and were thus not considered further. The orbits of the remaining 33 streams were compared to those of Earth and Venus as previously and the resulting encounter parameters tabulated. One additional stream, the Piscis Austrinids was found to approach the Earth at a distance greater than 0.2 AU and was similarly removed from the sample. The Earth-to-stream ∆ statistics for these 32 orbits are presented in Fig. 2. More than 75% of the sample is concentrated within 2 × 10−2 AU while the median has a value of 5.3 × 10−3 AU. Both these statistical descriptors are indicative of the accuracy of the stream orbits in the sample. They can thus be used to identify those streams that can reasonably be expected to intercept Venus as well. In effect, we are compensating for the low accuracy of the orbits of minor streams (partly due to the small number of accurate individual meteor orbits) by stipulating that the same part of the stream cross-section intercepts the orbits of both planets. This may not be necessarily so in reality. Setting a Venus-to-stream ∆ upper limit of 0.02 AU we find 10 streams which satisfy that criterion, three of which approach to within the median (Table 6). All these streams are characterised by low activity (ZHR  5) and slow to moderately-high velocities (20– 50 km sec−1 ). Their detection at Venus is likely to involve instrumentation optimised for meteor searches. The α Capricornids have been previously discussed in this paper in the

Fig. 2. Histogram of the minimum Earth–orbit-to-stream–orbit distance ∆Earth for 26 meteor streams listed in Rendtel et al. (1995) out to ∆Earth = 0.05 AU. Bin size is 5 × 10−3 AU.

Meteors in the venusian atmosphere

29

Table 6 Encounter characteristics at Earth and Venus for meteor streams in Rendtel et al. (1995) with ∆ < 0.02 AU at Venusa Designation Sagittarids Northern δ Aquarids α Capricornids Northern ι Aquarids (1) Northern ι Aquarids (2) Piscids Northern Taurids (1) Northern Taurids (2) χ Orionids Monocerotids

∆Earth (AU)

∆Venus (AU)

vEarth (km sec−1 )

vVenus (km sec−1 )

λEarth (deg) 

λVenus (deg) 

0.0044 0.0092 0.0028 0.0122 0.0122 0.0035 0.0039 0.0039 0.0000 0.0807

0.0061 0.0004 0.0080 0.0138 0.0149 0.0176 0.0118 0.0139 0.0009 0.0048

33 40 25 32 32 32 30 30 27 46

37 47 23 35 35 35 31 31 27 50

43 142 128 152 152 182 232 232 258 254

55 150 160 12 168 46 251 72 78 260

a Quantities given in the Table are minimum orbit distance ∆, impact speed v and solar longitude λ. The three streams with ∆ −3 AU are Venus < 5 × 10

highlighted.

context of their association to Comet 45P/Honda–Mrkos– Pajdusakova. The Sagittarids have been associated with Comet D/Lexell 1770 I (Gartrell and Elford, 1975) although this association is far from certain (Kresaková, 1980). The Taurids are associated with Comet 2P/Encke (Whipple, 1940) while the χ Orionid stream’s orbit is similar to that of minor planet (2201) Oljato, a member of the Taurid complex (Porubcan and Stohl, 1987). B98 also compiled a list of occasions where particular stream nodal distances were within 0.1 AU that of Venus. His list included the Sagittarids and the χ Orionids (on two occasions). Additionally, he identified the δ Cancrids, the Virginids (on two occasions), the Piscis Austrinids (which were removed from our sample due to their large ∆Earth ) and the α Monocerotids. In trying to explain why the Virginids are missing from our list despite their close proximity of their nodal distances to that of Venus, we note the possibility that B98 used the orbit in Gavajdová (1994) derived from fireball observations, instead of the photographic orbit in Lindblad (1971). The absence of the other two showers in our list is probably due to the low cutoff value imposed here on ∆.

3. Brightness and height of venusian meteors The detectability of venusian meteors related to the bodies considered in the previous section depends critically on their intrinsic, as well as their apparent, brightness. Our aim here is to make a broad but well-founded statement on meteor brightness at Venus. For this purpose we follow AGM96 who addressed the question of average meteor flux, height and brightness in the martian atmosphere relative to the Earth’s. They solved numerically the heat conduction, heat balance, deceleration and mass loss equations along a meteoroid’s flight path in the atmospheres of Earth and Mars on the assumption that atmospheric composition is not a significant factor affecting meteor brightness. This is supported by observations of meteor spectra in the Earth’s atmosphere (Bronshten, 1983). Their principal findings were that

(a) martian meteors reach their maximum brightness between altitudes of 90 and 50 km, (b) fast (
30 km sec−1 ), low-density (0.3 g cm−3 ) meteoroids in the two atmospheres generate meteors of similar brightness whereas slower, denser (3 g cm−3 ) particles produce significantly fainter meteors in the atmosphere of Mars. To reproduce their numerical results analytically, they adopted an intensity law Im ∼ mv 3+n /H,

(1)

where m is the meteoroid’s mass, v its speed at maximum luminosity (which we take here to be equal to the atmospheric impact speed), and H the atmospheric density scale height (Hawkins and Southworth, 1958; Verniani, 1961). The index n is related to the luminous efficiency and is taken to be 1 for slow, dense meteoroids (Whipple, 1938) and −1 for fast, low-density meteoroids (Öpik, 1958) as defined above. They concluded that this law matched their numerical results satisfactorily, due primarily to the relatively low sensitivity of their Earth-to-Mars magnitude differences to meteoroid size, density and entry path zenith angle. It can be similarly used here to estimate the relative intensity of meteors of the same mass at Venus and the Earth by substituting appropriate values for the impact speed and the scale height. It is implicitly assumed that the transition in luminous efficiency between fast and slow meteoroids would be the same at Venus as at the Earth. Eventual confirmation of this assumption will be borne out of observations at Venus. In what follows we separate three cases based on the speed statistics in Section 2.2: vE = 60 km sec−1 , vV = 46 km sec−1 , and n = −1 (Model I); vE = 17 km sec−1 , vV = 26 km sec−1 , n = −1 (Model II); and vE = 17 km sec−1 , vV = 26 km sec−1 , n = 1 (Model III). These parameter values are chosen to bracket the expected range of typical intensity ratios at the two planets. Specifically, HTC meteoroids pertain to Model I whereas JFC meteoroids are bracketed by Models II and III due to the transition in the luminous efficiency dependence on speed observed for vE between 15 and 25 km sec−1 (Öpik, 1955). Cometary aster-

30

A.A. Christou / Icarus 168 (2004) 23–33

AGM96. At the midpoint altitude of 110 km in the venusian atmosphere the density scale height is 3.5 km according to the “nighttime” atmospheric model by Seiff (1983). Using Eq. (1) for Venus and the Earth we have for a meteoroid of the same mass IV /IE = (vV /vE )3+n HE /HV .

Fig. 3. Atmospheric density-height profiles for Earth (US Standard Atmosphere 1976—dotted line), Mars (Mars Pathfinder entry profile—bold line) and Venus (“nightside” atmospheric model by Seiff (1983)—dashed line). The vertical and horizontal black lines indicate the density/height range at which meteors ablating in the atmospheres of those planets are expected to be at their maximum luminosities.

oid meteoroids cover the entire spectrum of the three models since their ablation characteristics will depend on the orbit (and origin) of the individual parent object. The scale height to be used in Eq. (1) should correspond to the altitude at which the meteor is at its most bright. AGM96 found that maximum brightness is attained at an atmospheric density between 10−9.2 and 10−7 g cm−3 . For Earth and Mars this was found to correspond to the intervals 70–100 and 50–90 km, respectively, in good agreement to what is observed at the Earth. For example, 94% of the meteor entries fainter than absolute magnitude −1 in the catalog by Betlem et al. (1998) attain maximum luminosity in that height interval while for meteors fainter than −4 this ratio is 90%. Superimposing a model of the atmosphere of Venus (Fig. 3—see caption) shows that, under the same assumptions, meteors would be at their most bright between 100 and 120 km. Incidentally this is advantageous as the venusian atmosphere, unlike that of the Earth or Mars, contains perpetual cloud layers and hazes which can attenuate meteor brightness (Esposito et al., 1983). There are three main cloud decks in the venusian atmosphere up to 60 km altitude. In addition there is a haze layer extending from 70–90 km. Since the maximum brightness interval occurs above 90 km meteors will not suffer from significant aerosol extinction to a spaceborne observer. Considering the altitude interval midpoints in all three cases, we find that the density scale height at the Earth’s atmosphere is 4.9 km at an altitude of 85 km (US Standard Atmosphere 1976) and 7.9 km for the atmosphere of Mars at an altitude of 70 km according to Mars Pathfinder atmospheric entry profile data taken at local predawn hours (available through the Planetary Data System Atmospheres Node at http://atmos.NMSU.Edu/PDS/data/mpam_0001/edl_ddr). The resulting Earth-to-Mars scale height ratio of 0.63 is somewhat smaller than the value HE /HM  0.8 adopted by

(2)

Substituting the respective values of the scale height and the impact speed in Eq. (2) we find intensity ratios of 0.82 for Model I, 3.27 for Model II, and 7.66 for Model III. Taken at face value, these results imply that Model I meteoroids would be 0.2 magnitudes fainter at Venus whereas Models II and III meteoroids would be 1.3 and 2.2 magnitudes brighter respectively. In reality, the limitations imposed by small number statistics leads one to conclude that the average HTC meteoroid will be about as bright as on Earth whereas slower JFC meteoroids will be significantly brighter, by one to two magnitudes. The same conclusions apply with respect to martian meteoroids as investigated in AGM96. The results presented in Section 2 also allow us to make predictions on individual cases of orbits which encounter both planets. The speed ratio for Comet 1P/Halley at the two planets is 1.19 (Table 2) which would lead to meteors 0.7 mag brighter at Venus than at the Earth according to Eq. (2). Meteoroids ejected from Comet 45P/Honda– Mrkos–Pajdusakova encounter Venus slightly slower than the Earth according to Table 3. Using n = 1 and n = −1 in Eq. (2) we find that the resulting meteors would, however, be slightly brighter at Venus, by 0.15–0.25 magnitudes. Simlarly, Geminid meteoroids would be 0.7–1.0 magnitudes brighter at Venus (Table 5).

4. Detectability If meteor showers do occur at Venus, observing them for scientific purposes will not be trivial. It is prudent to ask the question of whether, and what level of, activity at Venus is detectable under reasonable assumptions. Two possibilities will be examined here: Earth-based or in situ monitoring. 4.1. Earth-based observations Beech and Brown (1995) investigated the feasibility of observing bright fireballs in the venusian atmosphere from the Earth. This paper deals with an entirely different meteoroid mass regime than Beech and Brown. The question here is whether such activity is observable from Earth as a transient increase in the nightside brightness of Venus. To this effect, we assume that a Leonid-class meteor storm occurs at Venus. The atmospheric flux during the November 1999 Leonid storm was reported as 1 meteor km−2 hr−1 (Arlt et al., 1999) whereas Holman and Jenniskens (2001) give a higher estimate, 2.8 meteors km−2 hr−1 . For the order-of-magnitude calculations to follow we have

Meteors in the venusian atmosphere

adopted the latter estimate. Further, we set all these meteors to be of absolute magnitude +2. Assuming a Venus apparent disk radius of 25 arcsec, a geocentric distance of 4 × 107 km and that the search area is half the observable disk, the combined brightness of these meteors corresponds to a magnitude of +17.6 and an intensity of 2 × 10−5 ergs cm−2 sec−1 ster−1 . Airglow emission in the upper atmosphere of Venus, caused by recombination of oxygen atoms in the upper atmosphere, in the visible region of the spectrum is typically 3–5 kR (Krasnopolsky, 1983; Chanover et al., 2001). Conversion of Rayleighs into CGS units depends on wavelength but would correspond to 10−3 – 10−2 ergs cm−2 sec−1 ster−1 in the visible band. Current Earth-based detection threshold is a few tens of Rayleighs (Slanger et al., 2001). Considering the fact that what we have considered here is a best case scenario in terms of the assumed meteor activity level, we conclude that Earthbased detection of meteor activity of Venus using current capabilities is in principle possible but challenging given the observed strength and variability of venusian nightside airglow. We now turn our attention to in situ detection of a shower. 4.2. Venus-based monitoring This second possibility will generally involve a spacecraft in orbit around Venus. Determining actual meteor detection rates at Venus under these circumstances depends on the as-yet-unknown levels of activity during shower periods, detector characteristics such as sensitivity, noise, field-of-view etc., and observation geometry such as distance from the planet and the planetocentric angle between the observed area and the shower radiant. A study of such magnitude is outside the scope of this work. The feasibility of carrying out these observations can nevertheless be demonstrated by way of an example. For this purpose we can make use of the circumstances of the MSX space-based observations of the Leonid shower in November 1997 as reported in Jenniskens et al. (2000). On that occasion, the 13.1 × 10.5 degree FOV Wide Field Visible Imager (WFVI—Carbary et al., 1994) recorded 29 meteors in the Earth’s upper atmosphere by capturing 0.5 sec exposures over an effective observing period of ∼ 20 minutes. At that time the Zenithal Hourly Rate (ZHR) was approximately 70 (P. Jenniskens, personal communication), also typical of other showers such as the Perseids, the Quadrantids, and the Geminids. We note that ZHR is defined as number of shower meteors per hour an observer would see if his limiting magnitude is +6.5 and the radiant is in his zenith. The limiting apparent meteor magnitude of the camera was estimated to be +6 and, under these conditions, detection in the surveyed area was complete down to meteors of absolute magnitude −2. From Section 2.2 we expect high-speed cometary meteoroids at Venus to produce, on average, meteors of approximately the same magnitude. It should be cautioned at this

31

point that the mass index of the 1997 Leonids was relatively shallow (r = 1.7); a typical shower would yield fewer −2 magnitude meteors for a given ZHR. The result is, however, indicative of what is achievable. The probability P (n = N; t) of observing N meteors in t camera frames assuming a mean rate of ν meteors per frame is assumed to be Poissonian, given by P (n = N; t) = exp(−νt)

(νt)N . N!

(3)

The number of frames required to detect at least N meteors with a given probability P is obtained by solving a nonlinear equation in λ 1−P =e

−λ


N−1   λi i=0

i!

,

(4)

where λ = νt. Here we consider the cases N = 1 and N = 3 with P = 0.95, for which Eq. (4) yields λ  3 and λ  6.3, respectively. For ν values for the MSX case of 0.012 meteors per frame for the two planets, 520 frames would be required for the detection of at least 3 meteors in the two atmospheres. To detect 1 meteor, 250 frames are needed. The expected data volume for the three-meteor image set at Venus would correspond to ∼ 150 Mbits for WFVI detector characteristics, with 8-bit encoding and a data compression ratio of 2. Customised compression schemes could reduce this further (e.g., Trayner et al., 2000). This appears manageable with the current onboard data storage and downlink capabilities of interplanetary spacecraft (e.g., Mars Odyssey—http://grs.lpl.arizona.edu/faq/ Odyssey_FAQ.pdf). The previously mentioned venusian nightside airglow is several times more intense than at the Earth’s nightside in the WFVI passband of 400–695 nm (Krasnopolsky, 1983); we note that nightglow was present along the limb of the Earth in the MSX observations. Moreover, it is emitted from altitudes varying from 85 to 120 km. This implies that many meteors at Venus will be embedded in this layer when at their brightest. To determine whether this is an impediment to such observations, we consider the case of a meteor of apparent magnitude +6 seen from a spacecraft in an MSX-like orbit above the limb of the planet where airglow intensity is highest (typically 40–100 kR). Such a meteor would appear at an intensity of 10−2 –10−1 ergs cm−2 sec−1 ster−1 within a single WFVI pixel. This estimate takes into account that meteor residence time on the pixel is smaller (assumed 0.1 sec) than the exposure time. Airglow intensity occupies the same range of values which implies that meteors would be discernable from the background as long as exposure times are sufficiently short to avoid saturation. In conclusion, detection of meteors during prominent showers (equivalent ZHR of several tens or above) appears to be feasible under the stated assumptions.

32

A.A. Christou / Icarus 168 (2004) 23–33

5. Conclusions/future work This work has addressed the question of meteor brightness and height in the venusian atmosphere, the abundance of meteor showers at Venus relative to the Earth and the potential observability of these phenomena. It has been shown that such meteors would typically be as bright or brighter and ablate higher in the venusian atmosphere than at the Earth. Using order-of-magnitude intensity calculations it is determined that observing the indirect effects of such showers from the Earth is likely to require significant reduction in current detection thresholds, with the possible exception of a Leonid-class storm. A shower with an equivalent Zenithal Hourly Rate of several tens of meteors should be detectable from Venus orbit with no significant additional demands on spacecraft onboard data storage and downlink capability beyond the level currently in use. Airglow emission from the venusian nightside should not impede meteor searches significantly in this case. A census of three-dimensional minimum orbit-to-orbit distances, (rather than the heliocentric nodal distances used in Beech, 1998) has been carried out on sets of known comet, asteroid, and meteor stream orbits. It revealed that (a) a similar number of Halley-type comets but a substantially lesser population of Jupiter family comets approaches Venus than the Earth, (b) Venus is likely intercepted by several minor streams detectable at the Earth, possibly the December Geminids related to the Asteroid (3200) Phaethon, and also by the orbits of several asteroids that are suspected of being extinct or dormant cometary nuclei. On the basis of the above, it is reasonable to expect prominent meteor activity at Venus related to four HTCs listed in Table 2, at least one JFC, namely 45P/Honda–Mrkos– Pajdusakova, owing to the particularly close approach of its current orbit to that of Venus, and the Asteroid (3200) Phaethon. In all these cases, key expected characteristics of benefit to future observational surveys such as sky radiant position, atmospheric impact speed and critical solar longitude have been tabulated in this paper. In addition, any showers related to minor streams or asteroids (excluding Phaethon) would likely be characterised by low Zenithal Hourly Rates, detectable only by customised instrumentation. Further work on the theoretical front such as detailed dynamical modelling of meteor stream formation associated with the most promising candidates would be useful in refining these predictions to the level currently achievable at the Earth (Kondrateva and Reznikov, 1985; McNaught and Asher, 1999) in order to optimise observation strategies. The same purpose would be served by modelling the ablation and, consequently, the light emission from these meteors in more detail than has been carried out here. From the observational point of view, a search intended to verify the existence of these putative venusian streams using the results presented here will allow a first-order char-

acterisation of the venusian meteor “year” as a prelude to multi-year dedicated surveys.

Acknowledgments Astronomical research at Armagh Observatory is funded by the DCAL. The author thanks Chris Trayner, David Asher, and Jonathan McAuliffe for their comments on an early draft version of the manuscript; Peter Jenniskens and an anonymous referee for suggestions which improved the paper in its final version.

References Adolfsson, L.G., Gustafson, B.A.S., Murray, C.D., 1996. The martian atmosphere as a meteoroid detector. Icarus 119, 144–152. Arlt, R., Bellot-Rubio, L., Brown, P., Gyssens, M., 1999. First global analysis of the 1999 Leonid storm. J. Inter. Meteor Org. 27, 286–295. Babadzhanov, P.B., 2003. Meteor showers associated with the near-Earth Asteroid (2101) Adonis. Astron. Astrophys. 397, 319–323. Baggaley, W.J., 1973. Observations of meteors associated with Comet Grigg–Skjellerup. Observatory 93, 23–26. Beech, M., 1998. Venus-intercepting meteoroid streams. Mon. Not. R. Astron. Soc. 294, 259–263. Beech, M., Brown, P., 1995. On the visibility of bright venusian fireballs from Earth. Earth Moon Planets 68, 171–179. Betlem, H., ter Kuile, C.R., de Lignie, M., van’t Leven, J., Jobse, K., Miskotte, K., Jenniskens, P., 1998. Precision meteor orbits obtained by the Dutch Meteor Society—Photographic Meteor Survey (1981–1993). Astron. Astrophys. Suppl. Ser. 128, 179–183. Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.-M., Levison, H., Michel, P., Metcalfe, T.S., 2002. Debiased orbital and absolute magnitude distributions of near-Earth objects. Icarus 156, 399–433. Bronshten, V.A., 1983. Physics of Meteoric Phenomena. Reidel, Dordrecht. Carbary, J.F., Darlington, E.H., Heffernan, K.J., Harris, T.J., Meng, C.I., Mayr, M.J., McEvaddy, P.J., Peacock, K., 1994. Ultraviolet and visible imaging and spectrographic imaging (UVISI) experiment. In: Cindrich, I., Del Grande, N.K., Gowrinathan, S., Johnson, P.B., Shanley, J.F. (Eds.), Aerial Surveillance Sensing Including Obscured and Underground Object Detection. In: Proc. SPIE, vol. 2217, pp. 204–212. Carusi, A., Kresak, L., Perozzi, E., Valsecchi, G.B., 1985. Long-Term Evolution of Short-Period Comets. Adam Hilger, Bristol. Chamberlin, A.B., McFadden, L.-A., Schulz, R., Schleicher, D.G., Bus, S.J., 1996. 4015 Wilson–Harrington, 2201 Oljato, and 3200 Phaethon: search for CN emission. Icarus 119, 173–181. Chanover, N.J., Anderson, K.S.J., Slanger, T.G., Cosby, P.C., Huestis, D.L., 2001. New observations of the Venus visible nightglow. Bull. Am. Astron. Soc. 33, 1039. Abstract. Christou, A.A., Beurle, K., 1999. Meteoroid streams at Mars: possibilities and implications. Planet. Space Sci. 47, 1475–1485. Coradini, A., Capaccioni, F., Capria, M.T., De Sanctis, M.C., Espianasse, S., Orosei, R., Salomone, M., Federico, C., 1997. Transition elements between comets and asteroids. Icarus 129, 317–336. Denning, W.F., 1915. Meteoric shower of Giacobinid’s comet. J. British Astron. Assoc. 26, 348. Denning, W.F., 1923. Radiant points of shooting stars observed at Bristol chiefly from 1912 to 1922 inclusive. Mon. Not. R. Astron. Soc. 84, 43– 56. Drummond, J.D., 1981. A test of comet and meteor shower associations. Icarus 45, 545–553. Drummond, J.D., 1982. Theoretical meteor radiants of Apollo, Amor, and Aten asteroids. Icarus 49, 143–153.

Meteors in the venusian atmosphere

Esposito, L.W., Knollenberg, R.G., Marov, M.Ya., Toon, O.B., Turco, R.P., 1983. The clouds and hazes of Venus. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. Univ. of Arizona Press, Tucson, pp. 484–564. Gartrell, G., Elford, W.G., 1975. Southern hemisphere meteor stream determinations. Austral. J. Phys. 28, 591–620. Gavajdová, M., 1994. On the population of very bright meteors in meteor streams. Contr. Astron. Obs. Skalnaté Peso 24, 101–110. Giorgini, J.D., Yeomans, D.K., Chamberlin, A.B., Chodas, P.W., Jacobson, R.A., Keesey, M.S., Lieske, J.H., Ostro, S.J., Standish, E.M., Wimberly, R.N., 1996. JPL’s on-line Solar System data service. Bull. Am. Astron. Soc. 28, 1158. Abstract. Hasegawa, I., 1990. Predictions of the meteor radiant point associated with a comet. Publ. Astron. Soc. Japan 42, 175–186. Hasegawa, I., 2001. Parent objects of α Capricornid meteor stream. In: Warmbein, B. (Ed.), Proceedings of the Meteoroids 2001 Conference, 6–10 August 2001, Kiruna. In: ESA SP, vol. 495, pp. 55–62. Hawkins, G.S., Southworth, R.B., 1958. The statistics of meteors in the Earth’s upper atmosphere. Smithson. Contrib. Astrophys. 2, 349–364. Holman, D., Jenniskens, P., 2001. Leonid storm flux from efficient visual scanning of 1999 Leonid storm video tapes. J. Inter. Meteor Org. 29, 77–84. Huestis, D.L., Slanger, T.G., 1993. New perspectives on the Venus nightglow. J. Geophys. Res. 98, 10839–10847. Hughes, D.W., 1975. Cosmic dust influx to the Earth. Space Res. 15, 531– 539. Hughes, D.W., Williams, I.P., 2000. The velocity distributions of periodic comets and stream meteoroids. Mon. Not. R. Astron. Soc. 315, 629– 634. Jenniskens, P., 1994. Meteor stream activity. I. The annual streams. Astron. Astrophys. 287, 990–1013. Jenniskens, P., 1995. Meteor stream activity. II. Meteor outbursts. Astron. Astrophys. 295, 206–235. Jenniskens, P., Nugent, D., Tedesco, E., Murthy, J., 2000. 1997 Leonid shower from space. Earth Moon Planets 82/83, 305–312. Jenniskens, P., Lyytinen, E., de Lignie, M.C., Johannink, C., Jobse, K., Schievink, R., Langbroek, M., Koop, M., Gural, P., Wilson, M.A., Yrjölä, I., Suzuki, K., Ogawa, H., de Groote, P., 2002. Dust trails of 8P/Tuttle and the unusual outbursts of the Ursid shower. Icarus 159, 197–209. Jopek, T.J., Valsecchi, G.B., Froeschlé, C., 2002. Asteroid meteoroid streams. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. Univ. of Arizona Press, Tucson, pp. 645–652. Kirkwood, D., 1861. Cometary astronomy. Danville Quart. Rev. 1, 614– 638. Kondrateva, E.D., Reznikov, E.A., 1985. Comet Tempel–Tuttle and the Leonid meteor swarm. Solar Syst. Res. 19, 96–101. Krasnopolsky, V.A., 1983. Venus spectroscopy in the 3000–8000 Å region by Veneras 9 and 10. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. Univ. of Arizona Press, Tucson, pp. 459–483. Kresaková, M., 1980. The summer ecliptical meteor showers and Comet Lexell. Bull. Astron. Inst. Czech. 31, 193–206. Larsen, K.M., 1994. The uncelebrated and unlucky Ursids. Astron. Soc. Pacific Mercury 23, 18–20. Larson, S.L., 2001. Determination of meteor showers on other planets using comet ephemerides. Astron. J. 121, 1722–1729. Lindblad, B.A., 1971. A stream search among 865 precise photographic meteor orbits. Smithson. Contrib. Astrophys. 12, 1–13.

33

McIntosh, B.A., 1991. Debris from comets—the evolution of meteor streams. In: Newburn, R.L., Neugebauer, M., Rahe, J.H. (Eds.), Comets in the Post-Halley Era, vol. 1, pp. 557–591. McNaught, R.H., Asher, D.J., 1999. Leonid dust trails and meteor storms. J. Inter. Meteor Org. 27, 85–102. Nakamura, K., 1930. On the observation of faint meteors, as experienced in the case of those from the orbit of Comet Schwassmann–Wachmann, 1930d. Mon. Not. R. Astron. Soc. 91, 204–209. Olsson-Steel, D., 1988. Identification of meteoroid streams from Apollo asteroids in the Adelaide radar orbit surveys. Icarus 75, 64–96. Öpik, E.J., 1955. Meteors and the upper atmosphere. Ir. Astron. J. 3, 165– 181. Öpik, E.J., 1958. Physics of Meteor Flight in the Atmosphere. Interscience, New York. Porubcan, V., Stohl, J., 1987. The meteor complex of P/Encke. In: Proc. Tenth European Regional Astronomy Meeting of the IAU, Czechoslovakia, pp. 167–171. Rendtel, J., Arlt, R., McBeath, A. (Eds.), 1995. Handbook for Visual Meteor Observers. In: IMO Monograph, vol. 2. IMO, Potsdam. Rickman, H., Fernandez, J.A., Gustafson, B.A.S., 1990. Formation of stable dust mantles on short-period comet nuclei. Astron. Astrophys. 237, 524–535. Seiff, A., 1983. Appendix A. Models of Venus’ atmospheric structure. In: Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I. (Eds.), Venus. Univ. of Arizona Press, Tucson, pp. 1045–1048. Slanger, T.G., Cosby, P.C., Huestis, D.L., Bida, T.A., 2001. Discovery of the atomic oxygen green line in the Venus night airglow. Science 291, 463–465. Southworth, R.B., Hawkins, G.S., 1963. Statistics of meteor streams. Smithson. Contrib. Astrophys. 7, 261–285. Trayner, C., Bailey, N.J., Haynes, B.R., 2000. Image compression of video meteor images. In: Arlt, R. (Ed.), Proc. of the International Meteor Conference 1999. IMO, Potsdam, pp. 38–46. Treiman, A.H., Treiman, J.S., 2000. Cometary dust streams on Mars: preliminary predictions from meteor streams at Earth and from periodic comets. J. Geophys. Res. 105, 24571–24581. Verniani, F., 1961. On meteor ablation in the atmosphere. Nuovo Cimento 19, 415–442. Weissman, P.R., A’Hearn, M.A., McFadden, L.A., 1989. Evolution of comets into asteroids. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 880–920. Weissman, P.R., Bottke, W.F., Levison, H.F., 2002. Evolution of comets into asteroids. In: Bottke Jr., W.F., Cellino, A., Paolicchi, P., Binzel, R.P. (Eds.), Asteroids III. Univ. of Arizona Press, Tucson, pp. 669–686. Whipple, F.L., 1938. Photographic meteor studies. Proc. Am. Philos. Soc. 79, 499–548. Whipple, F.L., 1940. Photographic meteor studies. III. The Taurid shower. Proc. Am. Philos. Soc. 83, 711–745. Whipple, F.L., 1950. A comet model. II. Physical relations for comets and meteors. Astrophys. J. 111, 464–474. Whipple, F.L., 1967. On maintaining the meteoritic complex. In: Weinberg, J.L. (Ed.), The Zodiacal Light and the Interplanetary Medium. In: NASA SP, vol. 150. US Govt. Prntg. Office, Washington, DC, pp. 409– 427. Whipple, F.L., 1983. 1983 TB and the Geminid meteors. IAU Circ. 3881. Williams, I.P., Wu, Z., 1993. The Geminid meteor stream and Asteroid 3200 Phaethon. Mon. Not. R. Astron. Soc. 262, 231–248. Wolfram, S., 1999. The Mathematica Book, fourth ed. Wolfram Media, Cambridge Univ. Press, Cambridge.