The hunt for rocks from space
In a hunt that makes the proverbial needle in a haystack look like easy quarry, scientists have begun the search for remains of a suspected meteor which lit up the skies over the south-western US this week. How do they know where to start looking, and why do they bother?
The meteor, which appeared as a dazzling streak of flame, was probably a chunk of space rock about the size of a football, scientists say.
They believe tiny pieces of the meteor - meteorites - could have survived the fall to Earth, and they have begun gathering data to aid the search.
The fireball flew eastward over southern California, was observed in Nevada and Arizona, and was last seen disintegrating in the sky over Phoenix, the Arizona state capital, according to media reports, eyewitnesses and astronomers.
Many of those who saw the phenomenon telephoned the authorities after capturing it on mobile phone cameras. The footage spread across Twitter and the news media on Thursday.
"It was closer than a shooting star, and you could see it breaking up into pieces," said Sgt Mark Clark of the Scottsdale, Arizona police department, who witnessed it.
If found, those meteorites could yield further clues about the origins of our solar system and the chemistry and physical make-up of other celestial bodies.'Building blocks'
"Most meteorites are older than any of the rocks that are found on the earth," said Prof Peter Brown, professor of physics and astronomy at the University of Western Ontario.
"They are essentially the primordial building blocks of the solar system."
The fireball was most likely caused by a piece of space rock travelling about up to 20 miles (32km) per second, about 30 miles high when it burned up, scientists said.
"Fireballs happen somewhere on the Earth every day," said Paul Chodas, a research scientist with Nasa's Jet Propulsion Laboratory.
"This one was over a populated area, on a convenient time, in the early evening, and so it was widely seen and reported."
It appeared at about 1945 local time on Wednesday (0245 GMT, Thursday).
Scientific investigators are expected to consult a variety of sources as they try to get to the bottom of the fireball and attempt to narrow the meteorite search area.
These include eyewitnesses and a range of records and instruments, including satellites, astronomical cameras, radar, amateur video, CCTV and even dashboard cameras on police cars.
"Our colleagues will try to gather data and videos that they can use to triangulate the path and then they will be able to calculate where the meteorites are likely to be found," said Mr Chodas.
"With just word of mouth descriptions, it's not enough."Crowdsourced search
Meteorites that survive the fiery fall through the Earth's atmosphere continue travelling tens of kilometres before they finally hit the ground.
To find them, searchers also take into account the direction and speed of winds high in the earth's atmosphere.
Once a search area has been modelled, often scientists will in effect crowdsource the meteorite hunt, asking local residents about damaged roofs and cars and asking them to join the search.
"The trouble is there are lots of rocks all over the world, and the vast majority of unusual rocks that people think might be a meteorite do not pan out," said Alan MacRobert, senior editor at Sky and Telescope Magazine.
"If it attracts a magnet at all, if it has a thin, darkened, molten crust just a millimetre or two thick, that's a good sign."
In 2000, researchers in Canada's Yukon territory recovered 1kg of meteorites from a 25 sq mile (64 sq km) area, after a local pilot discovered the first fragments while driving across a frozen lake.
Jim Brook collected the samples without touching them, grabbing them in plastic bags and storing them in his freezer until he could provide them to the meteor scientists.
Eight years later, a team of more than 40 searchers, including students and staff from the University of Khartoum, found 47 meteorites in the Nubian desert of northern Sudan.
Astronomers had tracked the falling body through space from a telescope in Arizona, then predicted the broad area of its impact.Where to look
Eyewitnesses in the town of Wadi Halfa and at a train station between there and Khartoum reported witnessing the fireball, and US government satellites also sensed it.
"We just lined 45 people up and did a foot search," said Dr Peter Jenniskens, a meteor astronomer with the Seti Institute in California, who helped lead the hunt.
"We had everybody about 10-20 metres apart and started walking the desert."
It took about two hours, "but that was because we knew where to look".
Dr Brown of the University of Western Ontario, who is familiar with the Sudan meteorites, said the space rocks were found within 100m of their predicted target.
With the Arizona fireball - should it have dropped meteorites - researchers will be aided by the fact it would have fallen over a more densely populated area relative to northern Canada and the Sudanese desert, scientists say.
"We've populated our country so densely there's a chance something may have fallen into a building or a car," Dr Jenniskens said.
"It would be a great help."