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1972 Great Daylight Fireball
The 1972 Great Daylight Fireball (or US19720810) was an Earth-grazing fireball that passed within (185,000 ft) of Earth's surface at 20:29 UTC on August 10, 1972. It entered Earth's atmosphere at a speed of 15 kilometres per second〔 in daylight over Utah, United States (14:30 local time) and passed northwards leaving the atmosphere over Alberta, Canada. It was seen by many people and recorded on film and by space-borne sensors.〔(Observation of Meteoroid Impacts by Space-Based Sensors ) Astronomical Society of the Pacific, Edward Tagliaferri, 2003, 'It was first detected by satellite at an altitude of about 73 km, tracked as it descended to about 53 km, and then tracked as it climbed back out of the atmosphere', 'object is still in an Earth-crossing orbit around the Sun and passed close to the Earth again in August 1997'〕 An eyewitness to the event, located in Missoula, Montana, saw the object pass directly overhead and heard a double sonic boom. The smoke trail lingered in the atmosphere for a number of minutes.
The atmospheric pass modified the object's mass and orbit around the Sun but it is allegedly still in an Earth-crossing orbit and passed close to Earth again in August 1997.〔 However, IAU's website states that these "suggestions have not been substantiated".
==Description==
Analysis of its appearance and trajectory showed the object was about in diameter, depending on whether it was a comet made of ices, or a stony and therefore denser asteroid.〔〔 Other sources identified it as an Apollo asteroid in an Earth-crossing orbit that would make a subsequent close approach to Earth in August 1997.〔 In 1994, Czech astronomer Zdeněk Ceplecha reanalysed the data and suggested the passage would have reduced the asteroids mass to about a third or half of its original mass (reducing its diameter to .〔(Daylight Fireball of August 10, 1972 ) C. Kronberg, Munich Astro Archive, archived summary by Gary W. Kronk of early analysis and of Zdeněk Ceplecha's paper for Astronomy and Astrophysics in 1994, '3 meters, if a carbonaceous chondrite, or as large as 14 meters, if composed of cometary materials', 'post-encounter ... 2 or 10 meters'〕
The object was tracked by military surveillance systems and sufficient data obtained to determine its orbit both before and after its 100-second passage through Earth's atmosphere. Its velocity was reduced by about and the whole encounter significantly changed its orbital inclination from 15 degrees to 8 degrees.〔(US19720810 (Daylight Earth grazer) ) Global Superbolic Network Archive, 2000, 'Size: 5 to 10 m' 〕
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