Earth-grazing meteoroid of 13 October 1990

Earth-grazing meteoroid of 13 October 1990

All-sky photo with the Earth-grazing meteoroid of 13 October 1990 (the faint near-vertical track just to the right of the pole star) taken at Červená hora, Czechoslovakia. The bright track on the left is the Moon.
Time	3h 27m 16s ± 3s UT
Duration	9.8 s
Date	October 13, 1990 (1990-10-13)
Location	Czechoslovakia, Poland
Coordinates	49°03′00″N 17°39′00″E / 49.050°N 17.650°E / 49.050; 17.650 (begin) 52°40′59″N 17°04′01″E / 52.683°N 17.067°E / 52.683; 17.067 (end)
First reporter	Petr Pravec, Pavel Klásek, Lucie Bulíčková
Recorded by	European Fireball Network
Type	Ordinary chondrite

The Earth-grazing meteoroid of 13 October 1990 was a meteoroid with an estimated mass of 44 kg that entered Earth's atmosphere above Czechoslovakia and Poland and, after a few seconds, returned to space. Observations of such events are quite rare, and this was the second one recorded, after the 1972 Great Daylight Fireball, and the first one from two distant positions, which enabled the calculation of several of its orbital characteristics. The encounter with Earth significantly changed its orbit and, to a smaller extent, some of its physical properties (mass and structure of its upper layer).

Observation

Visual observations were reported by three independent comet observers: Czech astronomers Petr Pravec, Pavel Klásek, and Lucie Bulíčková. According to their report, the event started at 3h 27m 16s ± 3s UT^{[note 1]} and the observed bright meteor (bolide) was moving from the south to the north. It left a track that was visible for 10 seconds.^[1]

Most data about the encounter were acquired using photographic observations by cameras of the European Fireball Network. It was the first event of this type recorded by cameras from two distant locations, at Červená hora and Svratouch (both in what is now the Czech Republic), which enabled the calculation of the meteoroid's orbital characteristics by geometrical methods.^[1] Both were equipped with all-sky fisheye objectives.^[1]

The Červená hora image was especially valuable. It recorded the bolide's trajectory over approximately 110°, starting 51° above the southern horizon, passing the zenith just 1° westward and disappearing only 19° above the northern horizon. The camera was also equipped with a rotating shutter, which divided the image 12.5 times per second to allow the determination of its speed. Over the last 4°, its angular velocity was so low that it got below the resolution of the instrument.^[1] The Svratouch image recorded the trajectory only for about 15°, beginning 30° above the northwest horizon, and the pictured bolide was quite weak. Despite this, the data were sufficient for the calculations.^[1]

Gotfred M. Kristensen also detected a radio reflection of the passing body in Havdrup, Denmark, at 3h 27m 24s ± 6s UT.^{[note 1]} It lasted 78 seconds.^[2]

Encounter data

The meteoroid grazed Earth's atmosphere quite gently (for example in comparison with the 1972 Great Daylight Fireball above the United States and Canada). Becoming visible at a height of 103.7 km south of Zlín, Czechoslovakia, it only approached Earth's surface to 98.67 km northeast of Wrocław, Poland, before disappearing from the sight of the cameras at a height of 100.4 km north of Poznań, Poland. It would probably still have been visible until a height of 110 km above the southern Baltic Sea. Its absolute magnitude (the apparent magnitude it would have at an altitude of 100 km at the observer's zenith) was approximately −6 and did not vary significantly during the encounter. The meteoroid travelled a distance of 409 km in 9.8 seconds during the time it was observed. It was moving with a speed of 41.74 km/s^{[note 2]} that did not change during the flight.^[3] Jiří Borovička and Zdeněk Ceplecha from the Ondřejov Observatory in Czechoslovakia estimated that the deceleration caused by the friction of the atmosphere was only 1.7 m/s² near the bolide's perigee, which meant that its velocity was reduced by only 0.012 km/s.^[1] This corresponds well to computer simulations provided by D. W. Olson, R. L. Doescher and K. M. Watson at the Southwest Texas State University, who concluded that the body was practically not decelerated along the track, with the exception of a very short time near perigee, when the deceleration was 1 m/s².^[4]

The software also calculated the meteoroid's apparent magnitude at the ground directly below the instantaneous position of the bolide. The computation started and ended with the heights of approximately 250 km, long before and after the cameras of the European Fireball Network could observe it. Its apparent magnitude started at a value of +5.7 and grew brighter quite quickly. The program gave an apparent magnitude of −5.7 at the moment when it was seen by one of the cameras and −6.3 at perigee. It did not differ significantly from its absolute magnitude at this point. Then, the bolide got dimmer, with an apparent magnitude of −5.4 at the moment when it was last seen by the cameras and a final calculated value of +6.0 at a height of 257 km. However, these values are not entirely certain, because the program worked with the simplified assumption that the luminous efficacy of the bolide did not change along the track.^[4]

Physical characteristics

The meteoroid was a type I bolide,^[5] i.e. an ordinary chondrite.^[6] When it entered Earth's atmosphere its mass was about 44 kg, which was estimated on the basis of the measured values of its absolute magnitude and velocity. It lost approximately 350 grams during the flight.^[1] Computer simulations showed that it started losing mass approximately at the moment it started to be visible to the cameras of the European Fireball Network, at a height of 100.6 km. It lost mass for 25 seconds, until it reached a height of 215.7 km.^[4] Its surface melted and solidified again after leaving,^[1] so it acquired a typical meteoritic fusion crust.^[5]

Orbit

Orbit of the meteoroid before and after grazing Earth's atmosphere

Because the bolide was recorded by two cameras of the European Fireball Network, it was possible to calculate the trajectory of its flight through the atmosphere, and afterward also the characteristics of both its pre- and post-encounter orbit in the Solar System.^[1] The calculations were published by Czech astromers Pavel Spurný, Zdeněk Ceplecha, and Jiří Borovička from Ondřejov Observatory,^[5][1][3] who specialize in meteor observations. They proved that the encounter changed the meteoroid's orbit significantly. For example, its aphelion (the farthest it travels from the Sun) and orbital period were lowered to almost half of their original values.^[3]

Similar events

Although impacts of meteoroids to Earth's atmosphere are very common, recording a similar flight through the upper layers of the atmosphere is quite rare.^[7] Probably the first one reliably verified happened on 20 July 1860 above the American state of New York.^[8] The Czechoslovak–Polish fireball is sometimes compared to the 1972 Great Daylight Fireball^[5] above Utah, the United States, and Alberta, Canada, which is the first scientifically observed and studied event of this type.^[7] The 1972 bolide was more than a thousand times more massive and it got 40 km closer to Earth's surface.^[5] Observational data from both of them helped to develop a method for computing the grazing trajectories of such bodies, which was later used when calculating the trajectory of another Earth-grazing meteoroid, observed on 29 March 2006 above Japan.^[9]

Notes

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References