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Astron. Astrophys. 333, 746-752 (1998)

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1. Introduction

Comet 55P/Tempel-Tuttle, discovered independently on 19 December 1865 and 5 January 1866, is the parent of the Leonid meteor stream. The comet's revolution period is 33.23 years, and its next passage through perihelion will be on 1998 Feb. 28.10. Much more information is known about that stream than about its parent body; the Leonid stream has produced some of the greatest meteor storms in history, most notably that of 1833, with meteor rates up to 100 000 hr-1 (Kresak, 1993), and 1966, with rates up to 150 000 hr-1 (Milon, 1967). In fact, the Leonid showers have been documented back to 902 A.D (Yeomans et al., 1996).

[FIGURE] Fig. 1a-f. Composite images of comet 55P/Tempel-Tuttle a  1995 Feb 04, b  1996 May 12, c  1997 Mar 04, d  1997 Mar 07, e  1997 Apr 17, and f  1997 Jun 17. North is up, East is left; all images have been obtained by co-adding all the frames obtained during that night after being shifted to compensate for the motion of the comet, whose expected position (using the post-recovery orbit) is at the center (0, 0) of the [FORMULA] frame sections shown.

Unlike some comets where the dust evolution is dominated by ejection processes, radiation pressure and planetary perturbations play the major role in the case of 55P/Tempel-Tuttle. This has led to an uneven distribution of material in the orbit, where the material trails behind and outside the orbit of the comet. Leonid showers are most likely when the Earth trails the comet and passes just outside of the comet's descending node (Yeomans, 1981; Yeomans et al., 1996). Moreover, the dust is not spread along the orbit - possibly because of interaction with Uranus (Williams, 1997), therefore causing storms only during a couple of years near the comet's perihelion passage. In non-storm years, the rate of meteors is relatively stable around 10-15 hr-1. Based on an orbital analysis, Wu and Williams (1996) predict that the 1998/1999 Leonid storm will be unimpressive (e.g. average), however, Yeomans et al. (1996) suggest that there is the possibility of a significant display during both years. The 1997 shower was quite unimpressive, with rates peaking at 10-15 meteors per hour (Hainaut, 1997).

Meteor streams provide an independent observation of comet dust from which we have inferred information on the physical nature of the dust, e.g. the chemical properties (spectroscopy) and density (Hajduk and Hajdukova, 1990; McIntosh, 1991). The particles in the stream depend on the irregular mass loss from the parent, which in turn depends on the nucleus properties.

Comet 55P/Tempel-Tuttle is a Halley-family comet, i.e. it has migrated inwards from the Oort cloud via planetary perturbations. Unlike the observable short-period comets originating from the Kuiper belt, the Halley-family comets are probably less evolved than the Jupiter-family comets. Unfortunately virtually nothing is known about this comet: astrometric data are available from the 1699, 1865-66 and 1965 apparitions; from a 2000 year backward orbital integration Yeomans et al. (1996) believe that they may have identified observations of the comet by the Japanese in 1234 and the Chinese in 1035. Although the second comet spectrum ever taken was made of Tempel-Tuttle in 1866, no observations of the comet's physical behavior were made - even at the 1965 apparition, and thus no information is available about the dust and gas production.

The only observations of 55P/Tempel-Tuttle since its discovery in 1865 were during its 1965 apparition. The comet was recovered after an extensive orbital analysis (Schubart, 1965) but only 3 astrometric data points were obtained. In summary, virtually nothing is known about the parent of the Leonid stream. The comet will be making its closest approach to Earth in 132 years (Yeomans et al., 1996), passing within [FORMULA] = 0.36 AU on 1998 January 17. This is the first apparition where modern detectors have been available to watch the development of the activity in the comet. The goal of the present program was to make an early observational recovery of the comet so that both nucleus observations and the development of the activity and measurement of the dust production could be made. In addition to this program aimed at characterizing the nucleus, an extensive "International Leonids Watch" is planned for the 1998/1999 apparition; an increase in shower rates was first observed in 1994 and 1995 (Jenniskens, 1996).

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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