The 1972 Great Daylight Fireball (or US19720810) was an Earth-grazing meteoroid that 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 at 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 (9.8 ft) in diameter, if a carbonaceous chondrite, to 14 metres (46 ft), if made of cometary ices. It was in the Apollo asteroid class in an Earth-crossing orbit that would make a subsequent close approach to Earth in August 1997. 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 (6 ft 7 in to 32 ft 10 in).
The meteoroid's 100-second passage through the atmosphere reduced its velocity by about 800 metre per second (2,600 ft/s) and the whole encounter significantly changed its orbital inclination from 15 degrees to 8 degrees.
The US19720810 meteoroid is described in the preface of the first chapter of Arthur C. Clarke's The Hammer of God.
Possible effects of a collision 
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 air burst, and any remnant would have struck Earth 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 by Collins et al. This table shows how sensitive the result is to the entry angle and composition:
See also 
- ^ Astronomy Picture of the Day. 2009 March 2.
- ^ a b c US19720810 (Daylight Earth grazer) Global Superbolic Network Archive, 2000, 'Size: 5 to 10 m'
- ^ a b 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'
- ^ a b Daylight Fireball of August 10, 1972 C. Kronberg, Munich Astro Archive, archived summary by Gary W. Kronk of early analysis and of Zdenek 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'
- ^ Robert Marcus, H. Jay Melosh, and Gareth Collins. Computing Effects of an Impact on Earth
- ^ Collins, Gareth S. et al. Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth Meteoritics & Planetary Science 40, Nr 6, 817–840 (2005) "The curvature of the Earth is also ignored."
- 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)'
- Abe, Shinsuke; et al. 2006. (PDF). Earth-grazing fireball on March 29, 2006 (full details: orbit, charts, spectra, composition) Retrieved 2008-07-07
External links