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| Earthgrazer: The Great Daylight Fireball of 1972 (Credit & Copyright: Antarctic Search for Meteorites program, Case Western Reserve University, James M. Baker)[1] | |
The Great Daylight 1972 Fireball (or US19720810) was an Earth-grazing meteoroid which passed within 57 kilometres (35 mi) of the surface of the Earth at 20:29 UTC on August 10, 1972. It entered the Earth's atmosphere 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.[2]
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Analysis of its appearance and trajectory showed it was a meteoroid about 3 metres (9.8 ft) in diameter, if a carbonaceous chondrite, to 14 metres (46 ft), if made of cometary ices.[3][4] It was in the Apollo asteroid class in an Earth-crossing orbit that would make a subsequent close approach to Earth in August 1997.[2] In 1994, Czech astronomer Zdenek Ceplecha re-analysed 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).[3]
The meteoroid's 100-second passage through the 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.[4]
The US19720810 meteoroid is described in the preface of the first chapter of Arthur C. Clarke's The Hammer of God.
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. Atmospheric entry of meteoroids is complex and a full calculation requires a full simulation, but a highly simplified calculation can be made using the web-based program[5] by Collins et al.[6] This table shows how sensitive the result is to the entry angle and composition:
| Diameter | Density | Entry angle | Energy lost | Airburst altitude | Airburst energy |
|---|---|---|---|---|---|
| 3 | 3.4 | 1 degree | 1.3 kiloton | 43 km | 0.7 kiloton |
| 3 | 3.4 | 45 degrees | 1.3 kiloton | 39 km | 0.4 kiloton |
| 8 | 0.9 | 1 degree | 6 kiloton | 80 km | 0.4 kiloton |
| 8 | 0.9 | 45 degrees | 6 kiloton | 45 km | 2 kiloton |
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