Wednesday, October 12, 2016

The 1972 Great Daylight Fireball

The 1972 Great Daylight Fireball (or US19720810) 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 kilometres per second (9.3 mi/s) 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 several minutes.

The atmospheric pass modified the object's mass and orbit around the Sun, but it is probably still in an Earth-crossing orbit and passed close to Earth again in August 1997. IAU's website states that these "suggestions have not been substantiated.”


Analysis of its appearance and trajectory showed the object was about 3–14 m (10–45 ft) 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 asteroid's mass to about a third or half of its original mass (reducing its diameter to 2–10 metres (6.6–32.8 ft).

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 800 metres per second (2,600 ft/s) and the encounter significantly changed its orbital inclination from 15 degrees to 7 degrees.

In Popular Culture

The US19720810 meteoroid is described in the preface of the first chapter of Arthur C. Clarke's The Hammer of God.

The clip featuring the fireball is shown in the 1994 made-for-TV film Without Warning, in which it is described as a 500 metres (0.50 km) asteroid narrowly missing the Earth by thousands of feet.

See Also

  • 2008 TC3
  • Bolide
  • Earth-grazing fireball
  • Earth-grazing meteoroid of 13 October 1990
  • List of asteroid close approaches to Earth
  • Meteor procession

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