by D.I. Steel, R.H. McNaught, G.J. Garradd, D.J. Asher and K.S. Russell
c1998. Reproduced on this web site by permission of Duncan Steel.
Whilst there is some NEO work done from South America and southern Africa, the southern sky is by no means as well covered as that in the north, placing special responsibilities on southern hemisphere observers. (For those who are unfamiliar with the term a Near-Earth-Object or NEO is an asteroid or comet with an orbit approaching that of the Earth.) The comparative sparsity of sky-watchers in this part of the world is nicely illustrated by the fact that most comets found by amateur visual observers in the northern hemisphere are spotted by two or more independent discoverers, and thus get multiple names attached to them, whereas Bill Bradfield, a resident of South Australia, has found 18 comets and he is the sole discoverer of all of them; that is, there are 18 objects called Comet Bradfield (Marsden & Williams, 1996). The recognition of this responsibility on the part of the AANEAS team has meant that, as the northern hemisphere discovery teams improved their strike rates, so we put larger fractions of our effort into tracking (obtaining astrometric measures of) NEOs found elsewhere but passing into the southern sky, rather than concentrating upon searching for new ones ourselves. Nevertheless, we will begin this report by describing our search efforts, before passing on to a discussion of our NEO astrometry program, and then a summary of our other scientific research work pertaining to NEOs.
Only the results from AANEAS are covered herein, so that the outcomes of the research program on radar meteors involving Steel at the University of Adelaide, such as the discovery of interstellar meteors in the terrestrial atmosphere (Taylor et al., 1996a), the identification of heavy organic meteoroids ablating at high altitude (Taylor et al., 1997a), and various advances in radar meteor speed determination techniques (Elford et al., 1995; Taylor et al., 1996b, 1997b; Cervera et al., 1997), receive their only mention in this paragraph.
At Palomar is a smaller Schmidt telescope, with an aperture of 18 inches. This was used for more than two decades by the Shoemaker team, and Eleanor Helin and her team, in the PACS and PCAS search programs (see Carusi et al., 1994a,b, for details and references). The PACS and PCAS mode of operation was to take pairs of short-exposure photographs of the sky separated by about an hour, and then to use stereo microscopes to find which star-like images had moved between the shots, betraying themselves as members of the solar system. Certainly a larger Schmidt telescope such as the UKST could be used for a similar program, and it would be expected to deliver a much higher discovery rate than PACS and PCAS (see Steel, 1995a), but at a much greater expense. In any case the UKST is not available for such work.
Whilst it was written above that we used the UKST for NEO searching, in fact all we did was to exploit the photographs obtained with that telescope for other purposes. For example, in recent years a complete survey of the southern sky has been carried out, using an emulsion/filter combination corresponding to red light ("R plates"). This is a repeat of a survey carried out in the 1970_s in the visual/green region of the spectrum (the "J survey"), with the exposures being sequenced so far as possible such that about 18 years have elapsed between photographs being obtained of the same region of the sky; this more-or-less constant time difference was desired because measurements of proper motions are being made for stars close enough to render an appreciable angular displacement within a couple of decades. This survey in R is called the Second Epoch Sky Survey.
Whilst the J survey plates were slightly better for moving object detection due to the emulsion/hypering combination used, R plates are quite likely to turn up discernable asteroid trails, in part because asteroids tend to be reddish (there is a little joke there, for anyone who knows the history of the UKST). Rather than the 5-10 minute exposures used by PACS and PCAS, regular UKST exposures last for 60-180 minutes, during which time an asteroid may have moved sufficiently so as to produce a trail tens of arcseconds long. McNaught realized that the plates being taken in R should turn up many near-Earth asteroids (NEAs), if they were diligently searched. In the 17 years from 1973-89 a total of five NEAs had been found on UKST plates, and their heliocentric orbits determined, whereas a greater number of comets had been found (such as those bearing the surname of Malcolm Hartley: see Marsden & Williams, 1996) because comets tend to be rather easier to spot on a fresh plate. Indeed it is notable that comets tend to be more vivid on J-survey plates, and this may explain in part why comet discoveries were more profuse on the UKST in the 1970-80's as compared to the first half of the 1990's, when AANEAS was operating. Compared to comets, asteroids are much more difficult to spot, and our experience has shown that NEAs turn up typically on about one R plate in 50 to 100; each plate takes about 40 minutes to search, with experience, using a binocular microscope. The score of five NEAs in 17 years was surpassed by the AANEAS team within one year of starting our search program (see Steel & McNaught, 1991; Steel, 1992a; and Steel et al., 1992a, 1995). In the following five years several dozen NEAs were discovered using this modus operandi (Table 1). In the later years of this period the NEA discovery statistics were aided by the introduction of some UKST exposures using Kodak 4415 film, which is superior in many regards to the glass plates used previously (see Steel et al., 1995).
By the middle of the 1990_s the Second Epoch Sky Survey was approaching completion, and comparatively few R plates (or useful films) were being exposed. A large fraction of the photography being conducted on the UKST made use of emulsion/filter combinations giving sensitivity in the U (ultra-violet), B (blue) or the I (infra-red) bands, and these are very much sub-optimal for NEA searches. In addition, a growing fraction of the UKST observing time was scheduled for operations using the FLAIR multi-object spectrograph (Parker et al., 1995). These changes in operations meant that the NEA discovery rate attainable merely by searching photographic plates and films delivered by the UKST was rather lower than that in 1990-94, so that alternatives were looked for, including a stereo-pair program similar to PACS and PCAS (see Steel et al., 1995). The expenditure required for this, however, did not make it an attractive proposition, and at about the same time it was recognized that a higher NEO discovery rate should be possible using an automated wide-field instrument at Siding Spring equipped with a CCD camera (Steel, 1995a). The plans for stereo photography for NEO detection using the UKST were therefore abandoned.
A list of the Aten, Apollo and Amor asteroids (i.e., NEAs) and comets (of all types) discovered as a result of the AANEAS program is given in Table 1, arranged in chronological order. In Table 2 we list some NEAs and other interesting asteroids recovered in this program, in order of observation. In Table 3 are listed some of the "precoveries" (see section 3 below) resulting from this program.
It might be mentioned here as an aside that as a by-product of scanning the UKST plates looking for anomalous asteroid trails, McNaught also discovered or co-discovered nine comets (see Table 1) and 63 supernovae, all of which were announced in the International Astronomical Union Circulars.
In addition to the above we know that many NEAs were recorded on plates which we inspected, but we were unable to differentiate them from more common asteroids because their angular motions happened to alias main belt objects (e.g., see the plots of RA and Dec motions in Rabinowitz, 1991, and Bowell & Muinonen, 1994). However, with a predicted rough position, angular speed and sky plane vector from a backwards integration, specific NEA detections may be distinguished from amongst a panoply of main belt asteroid tracks.
The identification of such a trail is called a precovery, derived from "pre-recovery" (the act of recovering an object in the past); this is a new word which is another product of AANEAS, to appear in the next edition of the Oxford English Dictionary. The immense value to NEO researchers of the plate archives from the UKST and other wide field instruments for such purposes has been discussed in greater detail by McNaught et al. (1995). The point is that within a week or so of a discovery it is feasible that the observed arc on an NEA (or other) orbit may be extended to some years, or even a decade or two, with a concomitant enhancement in the orbit precision. An example is the distant Centaur asteroid (5145) Pholus: within a few weeks of its discovery (as 1992 AD) precovery positions had been obtained from the large Palomar Schmidt archive, and then the UKST archive back even further, rendering a 15-year arc and thus its rapid numbering and naming; this also allowed orbital evolution studies to be carried out (Asher & Steel, 1992, 1993). A more recent example is that of Comet Hale-Bopp (1995 O1): within ten days of its discovery on 1995 July 23, a precovery image had been identified by McNaught on a UKST plate exposed in 1993 April, rendering a 27 month arc and making it clear that this comet was not _new' in the Oort-Opik cloud sense, having last passed through the inner solar system about three millennia ago (IAU Circ. 6198).
A particularly good example of the utility of the UKST plate library is the case of the Amor-type asteroid 1977 QQ5. Sixteen years after a plate was exposed, McNaught was scanning it in the UKST plate library at the Royal Observatory, Edinburgh, looking for an image of another asteroid which a backwards integration had indicated to be within the field when the exposure was made in 1977. Apart from the target asteroid, he found a strong image of another asteroid which was clearly a fast mover. A propitious set of circumstances meant that that asteroid was also detected on three other plates exposed within a few months of the date in 1977, giving four images in all. These were sufficient to determine a reasonably good orbit and hence the allocation of a preliminary designation as 1977 QQ5, and made the recovery of the asteroid in 1994 December (by Petr Pravec at Ondrejov in the Czech Republic) a formality. This Amor is now numbered, with a secure orbit (Table 1). The discovery of Comet 1978 G2 McNaught-Tritton (Table 1) came about in a similar way.
The reason that we paid such attention to follow-up astrometry was that we were the only team conducting this sort of program in the southern hemisphere. Even given that observers in temperate latitudes in the northern hemisphere, or even as far south as Hawaii, can cover about a third of the sky at southern declinations, still that left about two-thirds of the southern sky for us to monitor; and with the vagaries of the weather, and the seasons reversed between the hemispheres, it was rare for there to be less than a dozen or so NEAs for which astrometry was needed, in locations such that we were the only team capable of filling the gap. Through the 1990_s the Spacewatch team became more and more prolific in their NEA discoveries (see Rabinowitz, 1991; Carusi et al., 1994a; Scotti, 1994; and Gehrels & Jedicke, 1996), and they were joined in early 1996 by the USAF/JPL Near-Earth Asteroid Tracking (NEAT) team under Eleanor Helin as a second group using CCD techniques for NEA searching, and later by the MIT Lincoln Laboratories LINEAR team led by Grant Stokes; the latter teams use the 1.0-m f/2.2 GEODSS (Ground-based Electro-Optic Deep-Space Surveillance) instruments at Haleakala and Socorro respectively. The productivity of these three groups is now very high (and will increase when the Spacewatch 1.8-m search telescope enters operation), and the lack of any southern hemisphere astrometric follow-up (apart from that provided by Garradd using his own small observatory, and a few other amateur observers elsewhere: see Steel & Marsden, 1996) is severely hampering the securement of NEA orbits.
During our operations, typically we would obtain three or four nights per lunation on the 40-inch telescope at SSO, our main instrument for follow-up astrometry. With a 2k x 2k CCD of high quantum efficiency covering about 20 arcminutes we were generally able to reach visual magnitudes near 21 in good conditions. Because of the long read-out time of this CCD, we usually made use of 2 x 2 pixel binning, giving an effective pixel size a little over an arcsecond; since the seeing at SSO is rarely sub-arcsecond, this still suitably sampled the images for astrometric purposes. The relatively wide field of this f/8 telescope meant that we enjoyed some success in recovering NEAs with significant ephemeris uncertainties.
We also made occasional observations using the 3.9-m Anglo-Australian Telescope (AAT). Our work on that large reflector was mostly directed towards trans-Neptunian objects, but we also obtained astrometry on some NEAs in important cases, i.e., especially if their ephemerides indicated that they would not become brighter than magnitude 21-22 within the next decade. For example, we observed (2608) Seneca with the AAT for this reason: although it was a numbered asteroid there was still a danger that it would be lost before it was next bright enough to be observed with a 1-m class telescope. We also used the AAT to obtain positions of the largely-inactive 109P/Swift-Tuttle as it moved through the region of the jovian planets, in order to define its orbit in the absence of non-gravitational forces and aid the prediction of its return date in AD 2126.
At the request of NASA and ESA, we provided astrometric observations for various spacecraft targets, such as the asteroids Gaspra, Ida and Geographos, and 26P/Grigg-Skjellerup, aiding in particular the targeting of the ESA spaceprobe Giotto close to the latter object. An important practical outcome of the AANEAS program related to the astrometric follow-up of ultra-short arc (arc less than a few hours) objects, such as those detected on UKST plates but not immediately recognized. In the 1980's, the standard technique for re-observing such objects was typically to assume it to be a main-belt object at perihelion (Vaisala orbit). This is clearly inappropriate for a possible Earth-crosser, so a new orbit determination technique was developed specifically for NEAs (McNaught, unpublished). This took the discovery trail as being a straight segment of the orbit ellipse and, for assumed distances and orientations, sets of possible orbits could be determined. With suitable weighting, a probability map could be produced to indicate the likely future position for re-observation. In practice, this indicated suitable field centers for re-observation, as the probability map has bounds that make it rather elongated or V-shaped. This technique to derive "the best of all possible orbits" McNaught called Pangloss after Dr Pangloss, in Voltaire's Candide, who believed in the equally implausible "best of all possible worlds".
An obvious region of interest is the calculation of the expected collision rate with the Earth for the presently-known population of asteroids (Steel, 1995b) and comets (Marsden & Steel, 1994; these authors also discuss the warning times which might be obtained for long-period comets, which would only be discovered on the apparition in which an impact is to occur). Steel (1993a) investigated the impact rate for the theoretical distribution of near-parabolic comets arriving from the Oort-Opik cloud. The general theme of the effects of cometary impacts upon the biosphere was extensively reviewed by Steel (1997b).
There has been much interest of late in the idea of panspermia through rocks being transferred between the Earth and Mars. Ejections from the planets through large impacts depend critically upon the impact speeds, and the probability distributions of those speeds have been investigated, indicating that comets are much more likely to eject debris into heliocentric orbits than are asteroids (Steel, 1997c, 1998). Similarly the arrival speeds at a planet affect the likelihood of intact survival of chemical species, this having implications for the origin of the primordial organic molecules on Earth (Steel, 1992b). (See also "Swapping rocks")
Much of our theoretical work has been concerned with the dynamical evolution and origin of NEOs. In particular we have been interested in the _Taurid Complex,' made up of 2P/Encke, possibly one or two other comets, several NEAs, and several Earth-intersecting meteoroid streams. In a series of papers we have investigated the membership of this putative complex, which we identify as being the remnants of a giant comet trapped in a cis-jovian orbit within the past 20,000 years and undergoing over that time a hierarchical disintegration (Steel et al., 1991a,b, 1992b, 1994; Steel, 1992c, 1996a; Asher et al., 1993a,b, 1994a; Clube & Asher, 1995). The hypothesis that the Tunguska bolide of 1908 might have originated as a fragment of 2P/Encke has been investigated from various perspectives (Steel & Snow, 1992; Steel & Ferguson, 1993; Steel, 1995c; Asher & Steel, 1997). The dispersal of the Taurid Complex asteroids under non-gravitational forces at such times as they may have been active comets has been investigated, and the same mechanism coupled with scanning across jovian mean-motion commensurabilities has been shown to be a viable avenue through which 2P/Encke could have reached its cis-jovian orbit on a reasonable timescale (Steel & Asher, 1996a,b). The possible link between various meteor showers and the Taurid asteroids has been investigated (Asher & Steel, 1995), and the general question of the association of NEOs with meteoroid streams reviewed (Steel, 1991a,b, 1994a, 1995d, 1996b). Another smaller grouping associated with the large NEA (2212) Hephaistos, but perhaps originating in an early fragmentation of the Encke/Taurid pregenitor, has also been identified (Steel & Asher, 1994).
In related work, the past and future epochs in which 2P/Encke could produce meteor storms have been delineated (Steel & Asher, 1996c), and similar investigations carried out for the recently-discovered fragmented Earth-crossing comet P/Machholz 2 (Asher & Steel, 1996a,b).
The first asteroid to be found on an eccentric intermediateor long-period (i.e., comet-like) orbit was discovered in the AANEAS program: (5335) Damocles (formerly 1991 DA). We made physical observations of this object with the AAT to search for cometary activity (Steel et al., 1992c) and have also conducted extensive investigations of its possible dynamical evolution (Steel & Asher, 1992; Asher et al., 1994b). Our work on the dynamical evolution of the outer solar system (Centaur) asteroid (5145) Pholus was mentioned in section 3 (Asher & Steel, 1992, 1993).
Another asteroid of interest, in this case having a small rather than large orbit, is our discovery (5786) Talos (formerly 1991 RC). This NEA has an orbit very similar to (1566) Icarus, prompting the suggestion that they might be genetically related (Steel et al., 1992d). Another pair of NEAs with similar orbits whose dynamical evolution we have investigated is (2101) Adonis and 1995 CS (Steel, 1997d); there are several groupings of NEAs which have been suggested as having genetic significance, apart from the Taurid and Hephaistos groups mentioned above (Steel, 1994b).
Our observations have sometimes rendered significant but unpredictable by-products. Whilst obtaining astrometry on 1995 HM using the SSO 40-inch we noted a large drop in brightness within a few minutes, and therefore continued to obtain measurements of its apparent magnitude over several nights. These measurements resulted in the determination of a spin period of about 97 minutes for this small NEA, the shortest known for any asteroid, and also a high-amplitude light-curve indicative of a highly-elongated structure (Steel et al., 1997). This is the first asteroid to be identified as rotating under tension, implying it to be a monolith rather than a rubble pile held together by self-gravity.
Returning to NEO search programs, as aforementioned the possible search effectivenesses of different telescopes at SSO/AAO have been assessed (Steel, 1995a); the general question of what areas of the sky require scanning, and in what way, has been discussed, including the clues rendered by the measured meteoroid orbit distributions (Steel, 1993b, 1996b, 1997e); and the relative probabilities of NEA trails remaining within different (small) numbers of CCD pixels have been derived, this being of especial importance for space-based searches (Steel, 1997f).
The advent of CCD search techniques and hence the ability to detect NEAs as small as 5-10 metres in size passing through cis-lunar space has opened up a new domain, which will require a re-definition of the terms _meteoroid' and _asteroid' (Beech & Steel, 1995), and allow the contents of near-Earth space to be investigated with unprecedented sensitivity, perhaps throwing up some surprises (Steel, 1995e).
Another line of enquiry has been the flux of natural meteoroids and interplanetary dust to satellites in orbit, this work rendering techniques for interpreting data from exposure experiments, and assessing the hazard to vehicles such as the Space Station (Steel, 1991c,d, 1992d,e). Such research led to an invitation from NASA to provide advice on the necessary shielding for the Cassini spaceprobe.
The impact hazard to humankind has been of public interest in recent years due at least in part to its association with the demise of the dinosaurs. The history of suggestions concerning this link has been investigated (Steel, 1994c, 1995f), as have some aspects of hypotheses of the origin of lunar craters (Steel, 1992f). Many articles and reviews on this hazard and related matters have been published (e.g., Steel, 1992g,h, 1993c, 1995g,h), and numerous public lectures given including the Chalklin Lecture of the Royal Society of New Zealand, and the Robinson Lecture of Armagh Observatory, Northern Ireland (Steel, 1997a). AANEAS personnel have appeared in dozens of TV programs, and literally hundreds of newspaper articles and radio interviews. A popular-level book on the subject of the impact hazard has been published (Steel, 1995i), and this has recently gone into paperback; it has already been used as the basis for six major TV documentaries and as the factual basis for a Hollywood movie script.
Regarding the international organization of science, two AANEAS members (Steel and Russell) served on NASA_s NEO Detection Committee (Morrison, 1992) whilst Steel also served on the NEO Interception and Deflection Committee (Canavan et al., 1993). Steel served as Secretary of IAU Commission 22 (Meteors and Interplanetary Dust) from 1988-94, as Secretary of IAU Commission 15 (Physical Studies of Comets, Minor Planets and Meteorites) from 1991-1994, as a member of the IAU Working Groups on the Prevention of Interplanetary Pollution (1988-1997) and on Near-Earth Objects (1991-1997; Carusi et al., 1994b), as a Council Member of the International Meteor Organization (1989-1993), and as a Trustee of the International Institute for the Problem of the Asteroid Hazard, in St Petersburg, Russia (1991-96).
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Steel, D.I. & Marsden, B.G. (1996), Earth, Moon & Planets, 74, 85-92.
Steel, D.I. & McNaught, R.H. (1991), Aust. J. Astron., 4, 42-48.
Steel, D. & Snow, P. (1992), pp.569-572 in Asteroids, Comets, Meteors 1991 (eds. A. Harris & E. Bowell), Lunar and Planetary Institute, Houston, Texas.
Steel, D.I., Asher, D.J. & Clube, S.V.M. (1991a), Mon. Not. Roy. Astron. Soc., 251, 632-648.
Steel, D.I., Asher, D.J. & Clube, S.V.M. (1991b), pp.327-330 in IAU Colloq. 126: Origin and Evolution of Interplanetary Dust (eds. A.C. Levasseur-Regourd and H. Hasegawa), Kluwer Publishing, Dordrecht, Holland.
Steel, D.I., McNaught, R.H. & Russell, K.S., (1992a), pp.219-221 in Proc. 30th LiŠge Int. Astrophys.Colloq.: Observations and Physical Properties of Small Solar System Bodies (eds. A. Brahic, J.C. Gerard & J. Surdej), Universit‚ de LiŠge, Belgium.
Steel, D.I., Asher, D.J. & Clube, S.V.M. (1992b), pp.189-200 in Periodic Comets (eds. J.A. Fern ndez & H. Rickman), Universidad de la Rep£blica, Montevideo, Uruguay.
Steel, D., McNaught, R.H. & Asher, D. (1992c), pp.573-576 in Asteroids, Comets, Meteors 1991 (eds. A. Harris & E. Bowell), Lunar and Planetary Institute, Houston, Texas.
Steel, D.I., McNaught, R.H. & Asher, D.J. (1992d), Minor Planet Bull., 19, 9-11.
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Steel, D.I., McNaught, R.H., Garradd, G.J., Russell, K.S. & Asher, D.J. (1995), pp.159-169 in IAU Colloq. 148: The Future Utilization of Schmidt Telescopes (eds. J. Chapman, R. Cannon, S. Harrison and B. Hidayat), Astron. Soc. Pacific Conf. Series, 84.
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Taylor, A.D., Steel, D.I. & Elford, W.G. (1997a), Adv. Space Res., in press.
Taylor, A.D., Elford, W.G. & Steel, D.I. (1997b), COSPAR Conf. Proc., 10, Asteroids, Comets, Meteors 1996 (ed. A.-C. Levasseur-Regourd), in press.
(5604) 1992 FE 1994 TF2 1994 XL1
(4953) 1990 MU 1990 SM (5645) 1990 SP (5189) 1990 UQ
(7822) 1991 CS 1991 DG 1991 RB (5786) 1991 RC
1991 VH (8176) 1991 WA (6455) 1992 HE 1993 KH
(7888) 1993 UC (7350) 1993 VA 1993 VB (8201) 1994 AH2
(7482) 1994 PC1 1996 FG3 1996 JG 1996 XX14
(7977) 1977 QQ5 1990 SA 1991 FB 1992 NA
(7474) 1992 TC 1992 UB (6053) 1993 BW3 1993 BX3
(8037) 1993 HO1 1993 UB 1994 LW (7839) 1994 ND
1994 PN 1996 FO3 1996 PC1
Other interesting asteroids discovered include several dozen Mars-crossing objects, one of which is the large, intermediate-period asteroid (5335) Damocles = 1991 DA.
Comets: New Style Old Style Name
C/1978 G2 1978 XXVII McNaught-Tritton
C/1990 M1 1991 III = 1990g McNaught-Hughes
C/1991 C3 1990 XIX = 1991g McNaught-Russell
C/1991 Q1 1992 XI = 1991v McNaught-Russell
C/1991 R1 1990 XXII = 1991w McNaught-Russell
130P/1991 S1 1991 IX = 1991y McNaught-Hughes
C/1993 Y1 1994 XI = 1993v McNaught-Russell
P/1994 N2 1994 XXXI = 1994n McNaught-Hartley
P/1994 X1 1994 XXIV = 1994u McNaught-Russell
For equivalent old style/new style designations, see Marsden and Williams (1996).
1991 RC 1984 KB 1991 WA 1992 JB
1993 PC 1994 AH2 1989 QF 1994 PM
1993 XN2 1993 PB 1992 LC 1991 DG
1991 VH 1989 JA 1995 LG
1992 CH1 1991 FA 1992 SL 1990 VB
1993 HO1 1989 OB 1991 OA 1991 JX
1992 NA 1987 WC 1993 DQ1 1989 RS1
1987 PA 1993 HA
Other unusual asteroids:
1992 BB 1991 NR2 1988 JB1 1992 AB
1988 GB 1985 UJ 1992 KD 1993 OZ2
1991 YA 1977 OX 1992 FL1 1992 DC
1992 TA 1988 PH4 1987 QX
1993 HO1 1986 DA 1993 MO 1992 AA
1989 RS1 1996 HW1
Other unusual asteroids:
1991 NR2 (5145) Pholus 1994 AE2 1989 NA
1991 TC 1994 SE 1994 JF1
C/1995 O1 Hale-Bopp P/1996 N2 Elst-Pizarro