Without doubt, the Leonid stream is the most famous of all the known meteoroid streams. The reason for this is not hard to find, the display of meteors that it produces at times far surpassess anything that any other shower can produce. The showers of 1799, 1833 and 1966 all have numerous engravings or photographs recording the splendidness of the displays. The recorded history of the appearances of spectacular Leonid displays dates back for two millenia. Though the associated parent comet, 55/P Tempel-Tuttle, was only discovered in 1861.

The parent comet of the Leonids, 55P/Tempel-Tuttle, was recovered in 1997. An improved orbit for the interval 1366 to 1997 together with predictions before 1366 were calculated by Nakano (1997). These orbital elements are used to determine the longitude of the descending node of the cometary orbit so that this can be compared with the solar longitude of meteors found in historic records. A number of catalogues listing the appearances of meteors showers have already been published. Most notable of these are Imoto and Hasegawa (1958), Zhuang (1977), Mason (1995), Rada and Stephenson (1992), Kidger (1993) and Hasegawa (1996). Nearly fifty records of the Leonids from AD855 to the end of the 19th century are found in these catalogues.

The recognition of the astronomical origin of meteors is intimately connected to the Leonid meteors, and their tendency to “storm” with a period of 33 years. It was the Leonid storm of 1799, observed by Alexander von Humboldt among others, that first established the geographical extent of the phenomenon, with observations reported over 90 degrees of longitude and 60 degrees of latitude. During the 1833 storm, Olmsted (1834) at Yale and Twining (1834) at West Point established the radiant point in Leo, and the cosmic origin was clinched when H. A. Newton (1863), also at Yale, determined that the cycle repeated in intervals of sidereal years, not tropical years. Inspired by the 1866 storm, Adams (1866), Schiaparelli (1867), and Le Verrier (1867) determined a 33.25 year period and other orbital elements for the meteor stream; two years after the discovery of Comet Tempel-Tuttle in 1865, that comet was recognized as the parent body of the Leonid stream after von Oppolzer (1867) calculated a period of 33.17 years for the comet. Astronomers confidently predicted another storm in 1899, but none appeared, though there were good showers in 1901-1903. The years 1930-32 also brought fairly good showers, but 1966 brought the strongest storm witnessed in modern times, with an estimated 100,000 meteors per hour. Steady progress was made in meteor studies in the 20th century, especially with new photographic and radar techniques. The current question is what will happen in the 1997-1999 period; the only certainty is that the event will be studied with an even wider array of techniques.

The annual November Leonid meteor showers and the occasional Leonid meteor storms (e.g. 1833, 1966) are far better known than is the Leonid parent body, comet 55P/Tempel-Tuttle. Since A.D. 902, enhanced Leonid meteor showers have been recorded around the time of the parent comet’s returns to perihelion (Yeomans 1981). To date, the parent comet itself, however, has not been seen for more than a few days at any apparition except in late 1865 and early 1866. The impending perihelion return of comet Tempel-Tuttle again raises the possibility of strong meteor displays in 1998-99 as well as a chance to observe the elusive parent comet.

The recorded history of the appearances of spectacular Leonid displays have been discussed for example in Yeomans (1981) and also in other papers at this meeting. The similarity in the orbits and that of comet 55P/Tempel-Tuttle is a strong indication that they are related. Further proof of this comes from the fact that the most spectacular displays associated with this stream are seen at time intervals that are multiples of the orbital period of the comet. The explanation for the occurrence of storms is thus that a well populated group of meteoroids are located close to the parent comet. Such a grouping of meteoroids would be formed if the comet were still active, not an unreasonable assumption. However, this raises an other question, namely why have the older of these meteoroids ejected perhaps several thousand years ago, not spread about the orbit. Standard models (eg Williams 1992) suggest that in 20 or so orbits meteoroids will have spread all the way from being close to the comet to being at the opposite point of the orbit. Hence, meteors in similar numbers should be seen every November coming from the Leonid stream. This is not the case, the average zenithal hourly rate of meteors in non-storm years being less than 10 as opposed to several thousand during a storm.

We know the Leonid shower primarily from its record of visual appearances over the last millenium. The period of the stream and its status as a cosmic phenomenon are all intimately linked to historical visual observations of the stream. The advent of modern observational techniques in meteor astronomy such as radar and photography has not decreased the value of visual recordings of the stream; on the contrary, visual observations form our primary means of long-term monitoring of the shower.

Radar techniques can be a powerful adjunct for studying meteor storms in general because of their sensitivity, capacity for continuous observations and their use of current techniques facilitating the handling of data. While some radars are limited to only a range-time presentation (and so adequate for a time-profile description of a storm event), some facilities have the additional capacity to determine directional information or orbital elements which are valuable in delineating the dynamics and spatial structure of stream meteoroids. The following outlines the application of radar techniques to meteor storm recording and summarizes current and planned facilities.

Video techniques, which provide high sensitivity, portability, moderate spatial resolution and excellent temporal resolution promise to be one of the most valuable methods for study of the forthcoming Leonid storm(s). While an unintensified video camera will detect very bright meteors (typically about 0 magnitude), some sort of image intensifier is needed to attain high meteor rates. Most current systems use a second or third generation microchannel plate (MCP) image intensifier lens or fibre-optically coupled to a charge coupled device (CCD) video detector.

We are observing meteor showers using the MU radar located at Shigaraki, Japan. This is a Mesosphere, Stratosphere and Troposphere radar, belonging to the Kyoto University, with a frequency and a peak power of 46.5M.ffz and 1MW, respectively. This system is characterized by the versatility of the antenna array. By using four antennas out of 475 Yagi arrays together with four receiver channels, we can observe more than several hundred meteor echoes per hour. The beam applied at the meteor observation is a static “doughnut beam”, which is the rotational symmetrical pattern to the vertical line. This beam is effective enough to detect meteor echoes of which the zenith angle is smaller than about 50°, which characterizes the MU radar as an all sky monitoring radar of meteor echoes. For the low velocity meteors, most of the echoes satisfy a condition of the perpendicular reflection, and we can derive the position of the radiant point statistically, and succeeded its application for the Geminids (Watanabe et al. 1992a). For the high velocity meteors, most of the echoes are in the overdense condition. However, we can discriminate the echoes of a specific meteor shower out of the data by checking the echo duration, the height, the strength or the combination of them. We have been monitoring the Perseids from 1990, and succeeded to reveal the unusual activity in 1991 (Watanabe et al. 1992b). For the Leonids, we are continuing the observation from 1991. The drastic increase of the activity has been detected around 23h UT on November 17, 1994. The number of the observed long duration echoes, which came from possibly bright Leonids meteors, were 80 per hour. This should be the beginning of the Leonids activity toward the 1998-1999. We are planning to continue this observation until the end of the activity in the 2000’s.

Since September 1996 there has been in operation a new forward-scatter system for meteor observation transmitting signal simultaneously along two mutually almost rectangular baselines with the transmitter at Budrio (ϕB = 44.6° λB = 11.5° E) near Bologna, Italy and the receivers at Modra (ϕL = 48.3° N; λL = 17.3° E), Slovakia and Lecce (ϕL = 40.3° N; λL = 18.2° E) in Southern Italy. The equipment utilizes a continuous wave transmitting frequency at 42.7 MHz, with a mean power of 0.25 kW.

Observations in Central Asia in 1965-1966 by both photographic and radar methods allowed a determination of the radiants and orbits of Leonids (Babadzhanov and Getman 1970). Photographs showed that meteoroids undergo quasi-continuous fragmentation (QCF) in the Earth’s atmosphere. Taking account of QCF, the density of the Leonid meteoroids were found to lie between 1 and Agcm-3 the average being 2gcm-3 (Babadzhanov 1994), in agreement with the density range of between 0.2 and 6gcm-3 given by Maas et al (1990) for dust grains from comet P/1 Halley, with values below 0.6 being rare. Further, the icy grains have a density of about 1gcm-3 while silicate grains have a mean density 2.5 times higher.

The possibility of a storm of Leonid meteors in November 1998 and 1999 offers exciting opportunities for using unusual meteor observing techniques, because of high flux and because the Leonids are unique probes for the study of the parent comet 55P/Tempel-Tuttle and the Earth’s upper atmosphere.

The past few years have seen the emergence of new observational approaches to the study of pulsating stars. Massive photometric surveys are discovering new variables by the tens of thousands. For example, the number of Cepheids and RR Lyrae has increased manifold over the past two years – many of these new distant pulsating stars have now better light curves than their well-known nearby counterparts. The new observational breakthrough has opened many theoretical issues. Some of these can be tackled with the set of new opacities (from Lawrence Livermore and from the Opacity Project), others seem to require new theoretical insights.

The last two decades brought a rich harvest of information about the outer planets, their satellites, and their ring systems that dramatically improved our knowledge about these bodies and evolutionary processes in the Solar System. This progress was due both to the successful Pioneer and Voyager missions, and to progressively improved techniques of ground-based observations. These data give insight into numerous new phenomena and mechanisms operating on the outer planets and also give important clues about the intimate relationships between all objects of the Solar System family. Results from the Galileo probe and orbiter have taken us further along that track.

Results from the Galileo probe and orbiter are compared with Voyager and ground-based results. The probe made measurements to the 20-bar level, well below the nominal cloud base at 5 bars and well below the level where sunlight is absorbed. The winds increased with depth within the clouds and then remained constant below cloud base. This rules out most “thin weather layer” models of Jovian meteorology. Water, H2S, and NH3 were depleted relative to “solar” abundances at the nominal cloud base but increased with depth below 10 bars. Apparently the probe entered a dry downdraft that penetrated to 20 bars. Consistent with this result, the orbiter found water varying by factors of 100 from place to place, with the lowest values at the sites that resemble the one where the probe went in. The orbiter also revealed that these sites are regions of horizontal convergence of the cloud top winds, which is consistent with the downdraft hypothesis.

The Galileo Near Infrared Mapping Spectrometer (NIMS) obtains spectral images in the wavelength range 0.7 to 5.2 μm with a spectral resolving power of approximately 200. This spectral range allows NIMS to sense cloud-reflected solar radiation, thermal emission produced in the deep atmosphere, and auroral emission from the thermosphere of Jupiter. Using 5 μm thermal emission spectroscopy, the amount of water vapor in the deep atmosphere, at approximately the (i to 8 bar level, is found to vary by a factor > 100. Deep atmosphere ammonia was also found to vary, with a spatial behavior different from that of water vapor. No evidence is found for a massive water cloud. Using reflected solar radiation in conjunction with thermal emission, two cloud layers are found, the upper at 0.5 bars and a lower one at 1-1.3 bars. The inferred absorption properties of these clouds are consistent with ammonia crystals (the upper cloud) and ammonium hydrosulfide particles (the lower cloud).

UVS results from Jupiter show the Io torus colder and brighter than the Voyager observations. Aurora near Jupiter’s poles emit 1-4 MR from altitudes of 300 – 400 km. Ganymede and Callisto have extended hydrogen exospheres. The near-UV reflectance of the Galilean satellites shows SO2, magnetospheric alteration, and O3. Remote sensing of Jupiter’s stratospheric haze determines that it thickens and darkens towards higher latitudes.

Insights into both the detailed composition of Jupiter’s atmosphere and unexpected local meteorological phenomena were revealed by in-situ measurements from the Galileo Probe Neutral Mass Spectrometer taken on December 7, 1995. Measurements of the neutral atmospheric composition from a pressure of 0.5 bar to approximately 21 bar revealed the mixing ratios of the major species helium and hydrogen as well as numerous minor constituents including methane, water, ammonia, ethane, ethylene, propane, hydrogen sulfide, neon, argon, krypton, and xenon. This instrument measured the isotope ratios 3He/4He, D/H, and 13C/12C as well as the isotopes of neon, argon, krypton, and xenon. A summary is given of progress that has been made in refining preliminary estimates of the abundances of condensable volatiles and noble gases as a result of an ongoing laboratory study using a nearly identical engineering unit. The depletion of simple condensable species to depths well below their expected condensation levels is explained by a local downdraft in the region of the probe entry. The mass spectrometer data suggests that different species may recover at different depths and this may be due to lateral mixing of Jovian air.

Galileo radio-occultation measurements show that all four of the Galilean satellites possess ionospheres. Peak ionospheric densities for the icy satellites are several thousand electrons per cubic centimeter, and the distributions are not spherically symmetric. Io’s ionosphere is much denser and remarkably similar to that measured by Voyager.

One important aspect of satellite-magnetosphere interactions concerns surface modification effects resulting from ion sputtering and/or electrostatic transport. For example, the Galileo measurements showed that the net mass loss rate at Europa could be as much as 40 meters of material per 100 million years. Magnetospheric ion-sputtering effects therefore have the interesting potential of providing an independent method to date the age of Europa’s surface. Ion sputtering can also lead to the ejection of large amounts of H atoms and H2 molecules into the Jovian magnetosphere as well as the formation of extended oxygen exospheres in the vicinity of the Galilean icy satellites. The newly discovered ionosphere of Europa could be of sputtering origin as well.