The 1972 Great Daylight Fireball (or US19720810) was an Earth-grazingmeteoroid that passed within 57 kilometres (35 mi) 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. 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.
Analysis of its appearance and trajectory showed it was a meteoroid about 3 metres (10 ft) in diameter, if a carbonaceous chondrite, to 14 metres (46 ft), if made of cometary ices. It was 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 meteoroid's mass to about a third or half of its original mass (reducing its diameter to 2 to 10 metres (6 ft 7 in to 32 ft 10 in).
The meteoroid's 100-second passage through Earth's atmosphere reduced its velocity by about 800 metres per second (2,600 ft/s) and the whole encounter significantly changed its orbital inclination from 15 degrees to 8 degrees.
^ abDaylight 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'
Abe, S. et al. (abstract) Earth-grazing fireball on March 29, 2006 European Planetary Science Congress 2006. Berlin, Germany, 18–22 September 2006, p. 486. code:2006epsc.conf..486A, 'the first and second Earth-grazing fireballs observed on August 10, 1972(Jacchia, 1974; Ceplecha, 1979) and on October 13, 1990(Borovicka and Ceplecha, 1992)'