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The Earth-grazing meteoroid EN131090 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; this was the second recorded using scientific astronomical instruments (after the 1972 Great Daylight Fireball) and the first recorded 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). == Observations == 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 03:27:16±3 UT〔The record is in Universal Time (UT), local Central European Time (CET) was 1 hour later.〕 and the observed bright meteor (fireball) was moving from the south to the north. It left a track that was visible for 10 seconds.〔 Most data about the encounter was 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.〔 Both were equipped with all-sky fisheye objectives.〔 The Červená hora image was especially valuable. It recorded the fireball'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 (thus crossing about 60% of the sky). Its camera was also equipped with a rotating shutter that interrupted the exposure 12.5 times per second and divided the captured track of the fireball, allowing the determination of its speed. Over the last 4°, the fireball's angular velocity was lower than the resolution of the instrument.〔 The Svratouch image recorded the trajectory only for about 15°, beginning at 30° above the northwest horizon, and the pictured fireball was quite weak. Despite this, the data were sufficient for the calculations.〔 Gotfred M. Kristensen also detected the fireball in Havdrup, Denmark, using a pen recorder connected to a radio receiver for 78 seconds, at 03:27:24±6 UT.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earth-grazing meteoroid of 13 October 1990」の詳細全文を読む スポンサード リンク
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