Current Status of the European Fireball Network

Introduction

Photographic meteor observation has a long tradition among both professional and amateur astronomers. One reason for that is the ability to determine most precise positions of meteor trails from photographs. Main limitation is the low sensitivity resulting in a very small number of meteor records compared with other observing techniques. Especially all-sky cameras as used in fireball networks can only detect the brightest meteors (i.e. fireballs) caused by very large meteoroids colliding with the Earth.
Those cameras, however, can monitor the whole sky at once. If a fireball is recorded from two or more stations, the true path of the meteoroid through the atmosphere as well as its heliocentric orbits before the encounter with Earth can be calculated. For meteorite candidates (fireballs that may have deposited material on the ground) the impact area can be determined, where a possibly surviving meteorite should be found. Fireball camera networks provide fundamental data for the computation of the mass flux near Earth and the probability of collisions with larger bodies. They close the observational gap between millimeter and centimeter large meteoroids observed as shooting stars every day by amateur astronomers, and larger bodies of some ten to hundred meters diameter found routinely by the Space Watch Telescope.

History

1959, after the fall of the Pribram meteorite in Czechoslovakia, the first network of fireball cameras ever was initiated in Europe [1]. Some years later it was expanded by installation of new stations in Southern Germany. Together they were called the European Fireball Network (EN).
In the sixties and seventies two more networks were installed in the United States (Prairie Network, 1965) [2] and Canada (MORP, 1971) [3]. However, both networks had to shut down after a few years of operation (1974 and 1985, respectively), whereas the EN is still monitoring the sky each night. Today, it covers an area of approximately 10^6 square kilometers with 34 camera stations in Germany, the Czech and Slovak Republics, Belgium, Austria and Switzerland (Figure 1). It is completed by associated camera stations of the Arbeitskreis Meteore (AKM) in Germany and the Dutch Meteor Society (DMS) in the Netherlands.

Equipment and Operation Scheme

Since their foundation, the (currently) 22 stations that belong to the German part of the network have been equipped with all-sky mirror cameras (Figure 2). The heart of such a system is a 36 cm parabolic mirror, above which the camera is mounted. The camera contains a shutter rotating with a frequency of 12.5 Hz. This allows the computation of the fireball's apparent angular velocity as well as the geocentric entry velocity and the deceleration of the meteoroid in the atmosphere. The exposure time is controlled by a digital clock.

Figure 3 shows a typical fireball photograph, obtained at Wendelstein Observatory (EN #88) on November 9, 1995.

Contrary to this, the (currently) 12 Czech stations have been upgraded several times. Today, they are equipped with Zeiss Distagon fish-eye lenses. Together with a larger film format this results in more precise fireball trajectories and a better limiting magnitude. Three guided fish-eye cameras are operated additionally. They allow the determination of fireball appearance times from pairs of guided/unguided exposures, whereas the timing of fireballs for most of the German part of the network relies on casual visual observations.
At each German EN station one exposure per night is obtained regardless of the weather, whereas the Czech stations are only operated when clear sky is expected. Due to daylight, clouds and moonlight, the effective monitor time is limited to about 3 hours per day on average.

Data Processing

Most German EN camera stations are maintained by amateur astronomers and weather stations. After their exposure, the films are developed and sent to the network coordinator. He inspects the photographs and selects successful fireball recordings.
The measurement of the negatives and calculation of orbital elements is a complex and time consuming task. The recordings of the Czech EN stations have been measured manually at Ondrejov Observatory for years. For the analysis of a single fireball which is recorded from several stations, a single persons needs approximately one week! Due to the enormous amount of work only a part of all recorded fireballs can be analyzed in detail. For the German stations there is currently no detailed fireball analysis at all. Especially spectacular events are measured at Ondrejov Observatory, too. However, most of the photographs are waiting in the fireball archive for their processing.
To improve the situation, it was decided to automate the measurement using computers. In the frame of his graduate work, the author developed a comfortable measurement software package at the German Aerospace Research Establishment (DLR) [4]. In the future, fireball negatives will be scanned using a Polaroid slide scanner with a resolution of 2,700 dpi. The images are then measured digitally on a UNIX workstation (Figure 4). The program creates a data file that can later be processed with the existing Czech Firbal software package to compute atmospheric trajectories and heliocentric orbits.

Results

Today, the EN obtains approximately 100 photographs of 50 different fireballs each year. In 1996 it has been especially successful recording 71 fireballs (156 photographs). One of the most spectacular events happened in the evening hours of August 18, 1996. 15 fireball cameras and several visual observers reported a -10 mag kappa-Cygnid that entered the atmosphere near Göttingen (Germany) and evaporated completely.
Since 1959 a number of meteorite candidates have been observed and successive ground searches were carried out (table 1).

  date          time           location               impact point     terminal mass
                [UT]          (computed)               (computed)        (computed)
                                                   longitude   latitude      [kg]
07.04.1959  19:30:21 ± 1s   Pribram (+)            14° 11' E*  49° 40' N*    50
15.10.1968  19:53:30 ± 30s  Cechtice (+)           15° 03' E   49° 37' N      0.15
10.04.1969  21:44:30 ± 90s  Otterskirchen (+)      13° 20' E   48° 39' N      5
24.11.1970  01:47:00 ± 1m   Mt. Riffler            10° 21' E   47° 08' N      0.9
30.08.1974  01:25:00 ± 5m   Leutkirch (+)          09° 54' E   47° 51' N      9.6
02.05.1976  19:12:00 ± 20s  Kamyk (+)              14° 19' E   49° 39' N      0.07
01.06.1977  21:46:00 ± 2m   Freising               11° 39' E   48° 28' N      0.7
12.06.1977  23:03:00 ± 2m   the Alps               06° 29' E   46° 06' N     30
27.05.1979  20:38:50 ± 50s  Zvolen (+)             19° 08' E   48° 34' N      1.2
09.10.1983  18:55:21 ± 43s  Zdar (+)               15° 55' E   49° 36' N      1.5
04.12.1983  17:09:48 ± 5s   Neuberg I              15° 32' E   47° 43' N      4
03.08.1984  21:05:53 ± 12s  Valec (+)              16° 43' E   49° 09' N     16
13.08.1985  23:32:00 ± 5s   Valmez (-)             17° 56' E   49° 25' N      2.1
04.10.1987  02:57:00 ± 1m   Janov (-)              17° 28' E   50° 15' N     75
24.12.1987  02:25:23 ± 56s  Freiberg               13° 27' E   50° 52' N     10
14.05.1988  23:15:50 ± 5s   Brdy                   14° 06' E   49° 47' N      1
07.05.1991  23:03:53 ± 3s   Benesov (+)            14° 37' E   49° 47' N      3
22.09.1991  16:48:00 ± 30s  Dobris (#,-)           14° 15' E   49° 43' N    100
09.05.1992  04:06:00 ± 30s  Neuberg III            15° 36' E   47° 39' N     10
22.02.1993  22:12:45 ± 2s   Meuse                  04° 48' E   49° 25' N      2.7
07.08.1993  21:08:15 ± 15s  Polna                  15° 55' E   49° 32' N      0.2
25.10.1995  02:25:53 ± 1s   Tizsa                  20° 47' E   47° 48' N      2.6
23.11.1995  01:29:00 ± 1m   Jindrichuv Hradec (-)  15° 02' E   49° 08' N      2.0
                                                                          
*	coordinates of the largest recovered fragment "Luhy" (5.6 kg)
+	systematic ground search in the predicted area
-	only non-systemantic attempt to recover the meteorite: people in the area 
        were informed by radio, local newspapers and postings
#	daylight fireball: all data rely on approximately 200 visual observations

Despite the large number of meteorite candidates only one fireball could be recorded, from which the meteorite was recovered afterwards. The other ground searches failed for various reasons, and reported meteorite falls in the last 38 years (table 2) occured either in daytime or under unfavourable weather conditions.

  date      time     location         longitude  latitude   mass   type
            [UT]                                             [kg]          
07.04.1959  19:30  Pribram*           14° 02' E  49° 40' N  5.600  stone
26.04.1962  11:45  Kiel               10° 09' E  54° 24' N  0.738  stone
12.06.1963  12:58  Usti nad Orlici    16° 23' E  49° 59' N  1.260  stone
16.09.1969  07:15  Police nad Metuji  16° 01' E  50° 31' N  0.840  stone
14.11.1985  18:17  Salzwedel **       11° 12' E  52° 48' N  0.043  stone
01.03.1988  12:30  Trebbin            13° 10' E  53° 13' N  1.250  stone
04.07.1990  18:33  Glanerbrug         06° 57' E  52° 13' N  0.855  stone

*	first photographed meteorite fall in the history of meteor science
** 	photographed by one German EN station

The two other camera networks had been successful only once in that respect, too. The American Prairie-Network recorded the fall of the Lost City meteorite [2], and the Canadian MORP captured the Innisfree meteorite fall [3]. In addition, the fireball caused by the Peekskill meteorite was recorded by chance with several video systems in 1992 [5].

The EN detection rate of meteorite candidates [6] is in good agreement with predictions based on previous studies of MORP fireball data [7]. According to them, about 15% of the meteorite encounters taking place in the EN area are photographically recorded (Figure 5). This coincides with the estimate, that the EN enjoys clear sky conditions of only 3 hours per day, on average.
Furthermore, the records of meteorite falls and recoveries suggest, that 1% or less of all meteoritical material of the considered mass range deposited on the ground is actually recovered. Therefore it can be estimated, that the joint probability of recording a fireball photographically and recovering the meteorite is only about 0.0015. Given the MORP flux rate, a meteorite of 100 g or 1 kg mass would be recorded and recovered in the European Fireball Network area only every 20 or 100 years, respectively. This conservative estimate, however, does not account for the possibility, that a fireball photograph makes the recovery of meteorite actually feasible.

Conclusions

The European Fireball Network has been recording several hundred fireballs since operation was initiated in 1963. Even though only one meteorite fall could actually be photographed so far, it provided a wealth of information on the population of meteoroids in near-Earth space.
We hope that with the digital analysis of fireball photographs more data of the orbits and properties of meteoritic material will be gained from the network.

Acknowledgements

The German part of the European Fireball Network is funded by the DLR Institute of Planetary Exploration. The Czech camera stations are maintained by Ondrejov Observatory and funded by the Academy of Sciences of the Czech Republic.

References