These rare events aren't typically wayward missiles, or satellite debris (as was thought when one such streak recently lit up the skies over Texas), or alien invasions. But they do come from outer space.

Scientists aptly call the objects fireballs because they are the brightest meteors, or "shooting stars," that fall to Earth.

A fireball as bright as the full moon raced across the Spanish skies on July 11, 2008, and was tracked by the Spanish Fireball Network. Researchers used the tracking data to trace the path of the comet backwards through the sky and space; they think the boulder may be a chunk of a comet that broke up nearly 90 years ago. Their conclusions are detailed in the Feb. 11 online issue of the journal Monthly Notices of the Royal Astronomical Society.

Earth and the other planets of the solar system are under constant bombardment from particles that range in size from a sand grain to a boulder and are collectively known as meteoroids. Many meteoroids are the detritus left over from collisions of asteroids and comets and impacts to other planets.

If a meteoroid enters Earth's atmosphere, it starts to burn up, forming a bright streak in the sky, called a meteor. Meteors can come from asteroid or comet fragments. If that meteor is brighter than any of the planets in the sky, it is deemed a fireball (also called a bolide).

A blazing bolide can also create a sonic boom that can be heard up to 30 miles away — these explosive noises were heard over Kentucky on Feb. 13, and over Texas on Feb. 15, causing a number of startled citizens to call local law enforcement.

Initial speculation that these streaks of light and accompanying boom were caused by debris from the Feb. 10 collision of two satellites was later refuted by astronomers, who said it was likely a meteor. Preston Starr, the observatory manager at the University of North Texas, told the Associated Press that the object would have been about the size of a truck and that somewhere between eight and 10 such objects burn up in the atmosphere every year.

Spanish sighting
The bolide that shot across the Spanish skies in July was also seen in Portugal and southern France.

At maximum intensity, it was 150 times brighter than the full moon. It was first picked up by the SPMN above Bejar in the western part of Spain at a height of about 61 miles (98 kilometers) and disappeared from view at about 13 miles (21 kilometers) above the surface of the Earth.

A professional photographer also snapped a picture of the streak from the north of Madrid.

From these images, astronomers Josep Trigo-Rodríguez of the Institute of Space Studies, CSIC-IEEC in Spain, Jose Madiedo of the University of Huelva-CIECEM in Spain and Iwan Williams of the University of London were able to deduce the trajectory and properties of the incoming fireball.

Their work was funded by the Spanish Ministry of Science and Innovation, the National Institute of Aerospatial Technique, and the Junta de Andalucía.

The team thinks the bolide was a dense object, about 3 feet (about 1 meter) across with a mass of about 4,000 pounds (1.8 tonnes). This would be like squeezing an adult elephant down to the size of an armchair.

The rock would have been big enough that chunks of it may have survived the fiery passage through the atmosphere and hit the ground as meteorites. Finding these pieces would be a boon to science if they are, as the team suspects, remnants of a comet breakup.

The bolide traveled an unusual orbit around the sun, as determined by the astronomers, following a path that took it from beyond the orbit of Jupiter to the vicinity of Earth. This orbit is similar to that of a cloud of dust particles known as the Omicron Draconids, which on rare occasions produce a minor meteor shower on Earth.

This collection of meteoroids is thought to originate from the breakup of Comet C/1919 Q2 Metcalf in 1920.

It has been proposed that comets consist of large boulders glued together by a mixture of smaller particles and ice. If the nucleus of the comet disintegrates, the boulders are set loose in space. Finding chunks of the Bejar bolide could help confirm this theory.

"Handling pieces of comet would fulfill the long-held ambitions of scientists — it would effectively give us a look inside some of the most enigmatic objects in the solar system," Trigo-Rodríguez said.