The year 2000 is not a particularly good one for predicted returns of periodic comets, however a newly discovered comet may reach naked eye visibility in July. The periodic comet predicted to be brightest is 73P/Schwassmann-Wachmann 3, which may reach 7^m at the end of the year. This comet underwent several outbursts at its last return, reaching naked eye brightness and the expected magnitude at this return is uncertain. A couple of long period comets discovered in previous years are still faintly visible and there are some poor returns of short period comets. Recent theories on the structure of comets suggest that any comet could fragment at any time, so it is worth keeping an eye on some of the fainter periodic comets, which are often ignored. Ephemerides for new and currently observable comets are published in the Circulars, Comet Section Newsletters and on the Section, CBAT and Seiichi Yoshida's web pages. Complete ephemerides and magnitude parameters for all comets predicted to be brighter than about 18^m are given in the International Comet Quarterly Handbook¹; details of subscription to the ICQ are available from the comet section Director. The section booklet on comet observing² is available from the BAA office or the Director; a new edition is likely to be printed in 2000.

First some comets, which were at perihelion in previous years. Comet 1995 O1 (Hale-Bopp), the great comet of 1997, is still fading slowly and could be 13^m at the beginning of the year, though this is likely to be the last year with visual observations. It is only observable from Southern Hemisphere locations as it loops round Mensa.

141P/Machholz 2 (1999 P1) was recovered by Rob McNaught in early August, not far from its predicted track. Its likely brightness is still uncertain, but it could be fading from 8^m at the end of 1999.

29P/Schwassmann-Wachmann 1 is an annual comet which has frequent outbursts and seems to be more often active than not at the moment, though it rarely gets brighter than 12^m. In the first half of 1999 it was in outburst on several occasions. The randomly spaced outbursts may be due to a thermal heat wave propagating into the nucleus and triggering sublimation of CO inside the comet. It begins the year in Scorpius and reaches opposition in the same constellation in June. It passes through Ophiuchus and into Sagittarius and is in solar conjunction in December. This comet is an ideal target for those equipped with CCDs and it should be observed at every opportunity. Unfortunately opportunities for UK observers may be limited, as its altitude does not exceed 11° from this country.

This year sees comet 2P/Encke's 58th observed return to perihelion since its discovery by Mechain in 1786. The orbit is quite stable, and with a period of 3.3 years apparitions repeat on a 10 year cycle. This year the comet is not particular well seen, but there are short observing windows from the Northern Hemisphere prior to perihelion, which is in September, and in the Southern Hemisphere after the comet reaches perihelion. There is some evidence for a secular fading and any observations will help confirm this. Another suggestion is that Encke has two active regions, an old one with declining activity, which operates prior to perihelion and a recently activated one present after perihelion. The comet is the progenitor of the Taurid meteor complex and may be associated with several Apollo asteroids.

9P/Tempel 1 was first observed in 1867, but was lost between 1879 and 1967 following an encounter with Jupiter in 1881 which increased the perihelion distance from 1.8 to 2.1 AU. Further encounters in 1941 and 1953 put q back to 1.5 AU and calculations by Brian Marsden allowed Elizabeth Roemer to recover it in 1967. Alternate returns are favourable, but perturbations will once again increase the perihelion distance in the middle of the next century. This return is an unfavourable one, but Southern Hemisphere observers will be able to follow it as it fades after perihelion. It is an important comet to observe as it is a potential spacecraft target, so all observations will be welcome.

A few comets not due to return until 2001 may become visible towards the end of the year. Horace Tuttle was the first discoverer of 41P/Tuttle-Giacobini-Kresak in 1858, when he found a faint comet in Leo Minor. Nearly 50 years later, Professor M Giacobini discovered a 13^m object whilst comet hunting, which was observed for a fortnight. A C D Crommelin linked the apparitions in 1928 and made predictions for future returns, but the comet wasn't recovered and it was given up as lost. In 1951, Lubor Kresak discovered a 10^m comet in 25x100 binoculars whilst participating in the Skalnate Pleso Observatory's program of routine searches for comets. After further observations the comet was identified with the lost comet and a better orbit computed. At the 1973 return, which was similar to the 1907 return, it underwent a major outburst and reached 4^m, before fading and then undergoing a second outburst. Alternate returns are unfavourable and this is one of them, but the comet has been observed at a few of them and it should be possible to observe it from equatorial regions in December. If it undergoes a further outburst, more widespread observation may be possible.

47P/Ashbrook-Jackson was discovered in 1948 following an approach to Jupiter in 1945, which reduced the perihelion distance from 3.8 to 2.3 AU. Although intrinsically relatively bright, the large perihelion distance keeps it faint. Alternate returns are favourable, but this is not one of them, although the comet will be reasonably well placed for Southern Hemisphere observers at 13^m.

Professor A Schwassmann and A A Wachmann of Hamburg Observatory discovered their third periodic comet, on minor planet patrol plates taken on 1930 May 2. Initially of magnitude 9.5 it brightened to nearly 6^m, thanks to a very close approach to Earth (0.062 AU) on June 1. The initial orbit was a little uncertain and the comet wasn’t found at this or succeeding apparitions until 1979. The comet passed within 0.9 AU of Jupiter in 1953, and 0.25 AU in 1965. In August 1979, Michael Candy reported the discovery of a comet on a plate taken by J Johnston and M Buhagiar while searching for minor planets; this had the motion expected for 73P/Schwassmann-Wachmann 3, but with perihelion 34 days later than in a prediction by Brian Marsden. Missed again at the next return, it has been seen at the last three returns. The 1930 approach to Earth is ninth on the list of well determined cometary approaches to our planet. In May 2006 it will make another close approach (0.082 AU), when it could again reach 7^m or brighter. This small miss distance makes it a convenient spacecraft target, and the Contour mission is scheduled to intercept it, as well as comets 2P/Encke and 6P/d’Arrest and possibly a new discovery. Following its outburst in 1995, 73P/Schwassmann-Wachmann 3 is expected to show fresh cometary surfaces, whilst 2P/Encke is an old comet and 6P/d’Arrest an average one. With the orbit approaching so closely to the Earth, an associated meteor shower might be expected, and the comet has been linked to the Tau Herculid shower, though the radiant now lies in the Bootes - Serpens region. Strong activity was reported in 1930 by a lone Japanese observer, but little has been seen since then. It is likely that any future activity would be in the form of a short-lived outburst, confined to years when the comet is at perihelion. The comet will be brightening towards the end of the year on its way to perihelion in late January 2001. If it maintains the level of activity seen at the last return it might be glimpsed in the morning sky around the beginning of December, although the solar elongation is not good.

Two newly discovered comets should become visible during 2000. 1999 S4 (LINEAR) offers the prospect of a naked eye comet in July. The comet should become visible to large aperture telescopes in January. It brightens slowly, but also closes with the Sun and reaches conjunction in March. We should pick it up again as a binocular object in the northern sky in June, becoming naked eye in July, when it passes through Perseus, Camelopardalus, Ursa Major and Leo. It doesn't stray far from the Sun and then heads south and back towards conjunction. UK observers will loose it by August, but Southern Hemisphere observers may follow it into September and will pick it up again as a telescopic object in November.

The other new comet, 1999 T1 (McNaught-Hartley), may reach binocular brightness. Initially at far southern declinations it will come within range of large apertures in July and binoculars in October. It moves far enough north for observation by UK observers in December when the 9^m comet moves through Hydra, Virgo and Libra.

A number of fainter comets may be of interest to CCD observers or those with large aperture telescopes. These include: 1997 BA6 (Spacewatch) (slowly fading from 13^m in January), 1999 H3 (LINEAR) (fading from 13^m in January), 1999 L3 (LINEAR) (fading from 13^m in January), 1999 K5 (LINEAR) (14^m between May and September), 1999 T2 (LINEAR) (13^m from August) and 95P/Chiron (16^m at opposition in late May in Libra). Ephemerides for these can be found on the CBAT WWW pages. CCD V magnitudes of Chiron would be of particular interest as observations show that its absolute magnitude varies erratically.

Several other comets return to perihelion during 2000, however they are unlikely to become bright enough to observe or are poorly placed. 71P/Clark, 76P/West-Kohoutek-Ikemura, 64P/Swift-Gehrels, 108P/Ciffreo, 112P/Urata-Niijima, 137P/Shoemaker-Levy 2 and P/Lovas 2 (1986 W1) have unfavourable returns. 14P/Wolf, 17P/Holmes, 33P/Daniel, 70P/Kojima, 87P/Bus, 114P/Wiseman-Skiff, D/Kowal-Mrkos (1984 H1), D/Shoemaker 2 (1984 W1), P/Shoemaker-Levy 5 (1991 T1), 1999 J2 (Skiff), 1999 K8 (LINEAR) and 1999 N4 (LINEAR) are intrinsically faint or distant comets. 5D/Brorsen has not been seen for over a century and is unlikely to be recovered, however if it still exists and resumes activity it could be a binocular object in the dawn sky between late August and early October, however it could be virtually anywhere along its orbital track.

Looking ahead, 2001 sees favourable returns of comets 19P/Borelly, which may reach 9^m and 24P/Schaumasse, which may reach 10^m.

Comet T q P N H1 K1

9P/Tempel 1 Jan 02.6 1.50 5.51 9 5.2 23.4

C/LINEAR (1999 L3) Jan 04.8 1.99 10.0 10.0

114P/Wiseman-Skiff Jan 11.7 1.57 6.66 2 11.5 15.0

137P/Shoemaker-Levy 2 Feb 05.8 1.87 9.37 1 14.5 10.0

112P/Urata-Niijima Mar 04.4 1.46 6.65 2 14.0 15.0

P/Lovas 2 (1986 W1) Mar 11.7 1.45 6.75 1 10.0 10.0

C/Skiff (1999 J2) Apr 05.9 7.11 6.4 5.0

108P/Ciffreo Apr 18.4 1.71 7.24 2 9.2 15.0

64P/Swift-Gehrels Apr 21.9 1.34 9.18 4 9.0 20.0

C/LINEAR (1999 K8) Apr 24.4 4.20 1.9 15.0

C/LINEAR (1999 N4) May 25.5 5.50 6.0 10.0

17P/Holmes May 11.8 2.17 7.07 8 10.0 15.0

76P/West-Kohoutek-Ikemura Jun 01.3 1.60 6.45 4 11.0 15.0

D/Shoemaker 2 (1984 W1) Jun 15.5 1.32 7.84 1 13.0 10.0

33P/Daniel Jun 23.5 2.16 8.07 8 10.5 20.0

C/LINEAR (1999 K5) Jul 04.6 3.25 6.0 10.0

C/LINEAR (1999 S4) Jul 24.4 0.75 7.0 10.0

P/Shoemaker-Levy 5 (1991 T1) Aug 18.6 1.99 8.68 1 13.0 10.0

2P/Encke Sep 09.7 0.34 3.30 57 10.0 8.8

70P/Kojima Sep 14.8 2.00 7.05 4 11.0 15.0

5D/Brorsen Oct 05± 0.54 5.5± 5 9.5 10.0

D/Kowal-Mrkos (1984 H1) Oct 26.8 2.68 9.31 1 12.0 15.0

14P/Wolf Nov 21.1 2.41 8.21 14 5.3 30.0

C/LINEAR (1999 T2) Nov 24.7 3.02 6.0 10.0

71P/Clark Dec 01.9 1.56 5.51 5 8.6 15.0

C/McNaught-Hartley (1999 T1) Dec 09.6 1.15 5.0 10.0

87P/Bus Dec 29.8 2.18 6.51 3 10.0 15.0

The date of perihelion (T), perihelion distance (q), period (P), the number of previously observed returns (N) and the magnitude parameters H1 and K1 are given for each comet.

Note: m1 = H1 + 5.0 * log(d) + K1 * log(r)

References and sources

1. Nakano, S. and Green D. W. E., Eds, International Comet Quarterly 2000 Comet Handbook, (1999).

2. Shanklin, J. D., Observing Guide to Comets, (1996).

3. Marsden, B. G. Catalogue of Cometary Orbits, 13th edition, IAU CBAT, (1999).

4. Kronk, G. W., Cometographia, Cambridge University Press, (1999).

Jonathan Shanklin