On Sunday 22 April 2012, at about 8.00 am, local time (3.00 pm GMT) witnesses saw a fireball streak across the sky over Nevada and California in broad daylight, from as far east as Elko and Las Vegas to San Francisco in the west. This was accompanied by a sonic boom that rattled houses and knocked some people of their feet.
The fireball passing over Nevada. News Lincoln County.
The fireball occurred at during the Lyrid Meteor Shower, but was not necessarily connected to this; the Lyrids are not generally associated with such large objects. Meteor showers are caused by when the Earth passes through the orbital path of comets, encountering large amounts of small debris from the tail of the comet, larger objects are not generally found in such showers, tending to be individual asteroids.
For an object to have created an audible boom and been visible in daylight it must have been particularly large, and have penetrated deep into the atmosphere. Estimates of the size vary: Bill Cook of NASA's Meteoroid Environments Office at the Marshall Space Flight Center suggested that it might be caused by a 3-4 m object weighing about 70 tonnes, and causing an explosion equivalent to 3.8 kilotons of TNT, Dan Ruby of the Fleischmann Planetarium at the University of Nevada suggests the object was 'a little bigger than a washing machine' and Donald Yeomans of NASA's Near-Earth Object Program Office said 'The meteor was probably about the size of an SUV'.
To have created an audible boom the object probably reached within 10 km of the ground, most visible meteors burn up ten times as high. It is still unlikely any material from the object reached the ground, though the area will probably be visited by a lot of meteorite hunters in the next few months (a meteorite is an object that has fallen from the sky and can physically be touched, a meteor is a shooting star, an object witnessed burning as it enters the atmosphere, and an asteroid is an object orbiting the Sun too small to be called a planet).
Objects the size of this hit the Earth several times a year, but are seldom observed, most falling over the ocean or other uninhabited areas.
See also The Lyrid Meteors, Images of Vesta, Asteroid 2012 DA14 may pass within 21 000 km of the Earth, How the Moon got its strange magnetic field and Comet C/2011 W3 (Lovejoy) survives a close encounter with the sun.
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