1972 Great Daylight Fireball
Atmospheric phenomenon
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The Great Daylight Fireball (also known as the Grand Teton Meteor) was an Earth-grazing fireball that passed within 57 kilometres (35 mi; 187,000 ft) of Earth's surface at 20:29 UTC on August 10, 1972. It entered Earth's atmosphere at a speed of 15 km/s (54,000 km/h; 33,554 mph)[2] 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.[3] An eyewitness to the event, located in Missoula, Montana, saw the bolide pass directly overhead and heard a double sonic boom. The smoke trail lingered in the atmosphere for several minutes.
| Date | August 10, 1972 |
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| Location | United States, Canada |
| External media | |
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| Images | |
 Earthgrazer:The Great Daylight Fireball of 1972 (Credit & Copyright: Antarctic search for meteorites program, Case Western Reserve University, James M. Baker)[1] | |
| Video | |
 Grand Teton Meteor Near Miss! |
The atmospheric pass modified the object's mass and orbit around the Sun. A 1994 study found that it is probably still in an Earth-crossing orbit and predicted that it would pass close to Earth again in August 1997.[3][4] However, the object has not been observed again and so its post-encounter orbit remains unknown.[5]
Description
Analysis of its appearance and trajectory showed the object was about 3 to 14 meters (10 to 46 feet) in diameter, depending on whether it was a comet made of ice or a stony and therefore denser asteroid.[2][6] 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.[3] In 1994, Czech astronomer Zdeněk Ceplecha reanalysed the data and suggested the passage would have reduced the asteroid's mass to about a third or half of its original mass, reducing its diameter to 2 to 10 meters (7 to 33 feet).[6]
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 0.8 km/s (2,880 km/h; 1,790 mph) and the encounter significantly changed its orbital inclination from 15 degrees to 7 degrees.[2] If it had not entered at such a grazing angle, this meteoroid would have lost all its velocity in the upper atmosphere, possibly ending in an airburst, and any remnant would have fallen at terminal velocity.[7]