Earlier this week, researchers at the University of Western Ontario in London spotted a meteor streaking through the sky with the help of their Southern Ontario Meteor Network, a combination of cameras, radar and shock-wave-detecting microphones set up at nine locations in southern Ontario.

They were able to track the meteor from the point it entered the Earth's atmosphere and, based on the altitude at which it stopped giving off light and the speed at which it was travelling when it did, chances are good that it survived the trip and that pieces of it landed somewhere east of Selwyn, Ont.

If some of the pieces are found, it will be cause for celebration for many of those who scour the skies above and the rocks beneath our feet for clues about the origins of our planet and the solar system.

Even though this meteorite, which is what it is called when it strikes the Earth, is likely to be of the most common variety, known as a stony meteorite, its find would still be a rare event. Fewer than one meteorite a year is found in Canada.

What is a meteorite?

Most meteorites that fall to Earth come from the asteroid belt between Mars and Jupiter, which is a veritable "junk pile" of rocks, dust and other space debris that lack the mass and gravitational pull to come together to form planets, explains Kimberly Tait, associate curator of mineralogy at the Royal Ontario Museum in Toronto.

Once this space material starts falling toward Earth, it's called a meteoroid. When it penetrates the Earth's atmosphere, it becomes a meteor, which is the term used to describe the visible "shooting star" portion of the meteoroid's voyage in which it heats up and interacts with the gases in the atmosphere, creating a streak of light.

Most of the shooting stars we see at night burn up in the Earth's atmosphere, but if they survive and fall to the ground, they are called meteorites.

Some meteorites are chunks of Mars or the moon that have broken off when they were struck hard enough by a piece of space debris, but these rarely reach the Earth. Only 65 Martian meteorites have been found to date.

What is it made of?

Meteorites break down into three broad categories. The most abundant are stony meteorites, which are made up mostly of silicate minerals and resemble common rocks found on Earth, except for the fact that they also contain free metals that make them magnetic. More than 90 per cent of meteorites are of this type.

The rarest type of meteorites, about five per cent, are those made mostly of iron (about 90-95 per cent iron content), which are thought to come from the core of planets that no longer exist or large asteroids.

In between are iron-stony meteorites, which are part silicate, part iron.

What does it look like?

A meteorite is usually black and glassy on the outside after it falls to Earth but can start to rust over time.

The black outer layer is called the fusion crust and forms as the rock travels through the Earth's atmosphere and its surface melts.

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A meteorite from the asteroid known as Vesta, which is one of the largest asteroids in our solar system. (Royal Ontario Museum/All rights reserved)

"There's a lot of friction on the material, and it actually goes white hot," said Tait. "That's what you're seeing [from the ground] — that shooting star."

The second test of whether a rock is from space is to see whether it is magnetic. Not every meteorite is magnetic, but 95 per cent are, so seeing how your find reacts with a fridge magnet, for example, is a good way to rule out other types of rock.

But beware, these are not foolproof tests. In southern Ontario, for example, mining slag is often mistaken for meteorites because it has a similar black appearance and is magnetic. What differentiates it from meteorites is the gas bubble-like holes on the surface, which meteorites don't have, Tait said.

When in doubt, space rock enthusiasts can send a photo of their find to organizations like the ROM, which often holds public sessions on how to spot a meteorite in areas where a fall has been reported. ROM officials are considering heading up to Selwyn, Ont., where the latest meteorite has fallen, for just such an event.

How old are meteorites?

Most meteorites are between 4.5 billion and six billion years old. Unlike on Earth, where rocks are constantly cycled through processes such as glaciation and plate tectonics, rocks in space can remain relatively unaltered and retain clues to the origins of our solar system, Tait said.

How fast do they travel?

Speeds of meteors vary but the average speed of those that make it to Earth is 18 kilometres per second at the point of entry into the atmosphere. The meteorite spotted in Ontario this week was travelling a bit slower than that at 14 km/s.

"The slower they hit the atmosphere, the less they burn up, the less forces they experience, so the better the chance they'll survive," said Peter Brown, Canada Research Chair in meteor astronomy and professor of physics and astronomy at the University of Western Ontario.

How do they get here?

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Kimberly Tait, associate curator of mineralogy at the ROM, holds a piece of a meteorite from the Vesta asteroid in the ROM's mineralogy lab. (Royal Ontario Museum/All rights reserved)

To make it to Earth, a space rock has to be hit by something with enough force to knock it out of orbit. It then has to cross the Earth's orbit and be drawn into our atmosphere by the Earth's gravitational pull. If it survives its voyage through the atmosphere without burning up completely, it falls to Earth — usually in fragments.

A good indication of whether a meteor will make it to Earth is the altitude at which it stops being luminous. This is the point at which the meteor has either burned up completely or slowed down enough that the heat generated by the friction between it and the surrounding air is no longer sufficient to melt or sublimate the rock.

Most of the meteors we see at night burn up at an altitude of 90-100 kilometres. But if they make it down below about 40 kilometres, they have a good chance of survival.

"An object that gets that deep into the atmosphere has to be pretty massive to be able to survive that far," Brown said.

The meteor spotted this week went dark at an altitude of 31 kilometres and slowed down from a speed of 18 km/s to about eight km/s, another indication that pieces of that meteor are likely to have made it to Earth.

Where are meteorites found?

Meteorites can be found anywhere on the planet, but since two-thirds of the Earth is water, many of them fall into the ocean or other bodies of water and are never recovered. Open areas unencumbered by trees and other obstructions make meteorites easier to find, which is why the greatest number have been found in places like Antarctica and the desert.

NASA and the National Institute of Polar Research in Japan conduct regular meteorite expeditions to Antarctica, which is also why the largest number of meteorites have been found there.

How many have been found?

More than 35,000 meteorites have been found worldwide. Between 50 and 100 are found each year. In Canada, 59 have been found, according to the Meteoritical Society, an international scientific organization that maintains a database of meteorites found around the world. (The figures refer to the number of unique meteorites discovered, not the actual number of pieces of these meteorites that are found, which is much higher.)

The ROM has 2,500 pieces of 300 distinct meteorites in its collection, which focuses on rare finds — the five per cent of meteorites that are not as common.

"Our collection is focused on the weird stuff," Tait said. "We had Carl Allen from the Johnson Space Centre and NASA here about a month ago, [and] he said it was by far the most comprehensive display of meteorites he's seen in the world."

What is the biggest meteorite found to date?

Meteorites differ in size from a few grams to several dozen tonnes. The largest meteorite found to date is the Hoba meteorite, which fell to Earth roughly 80,000 years ago and was found in 1920 in a farmer's field near Grootfontein, Namibia. It's about three metres wide, three metres long and a metre high and is composed of roughly 84 per cent iron and 16 per cent nickel. It's thought to be between 200 million to 400 million years old.

Notable finds in Canada

2009: Springwater, Sask.: a 53-kg piece of a pallasite meteorite originally discovered in 1931 was found when a team returned to the fall site. Pallasites are a rare type of meteorite — only three have been found in Canada and 84 in the world. They are composed of roughly equal amounts of iron-nickel alloy and a green-brown-coloured silicate mineral called olivine and are thought to come from deep within asteroids. The Springwater pallasite is about 4.5 billion years old.

2009: Grimsby, Ont.: about 13 fragments totalling 215 grams were found in a field in southern Ontario. The meteorite was spotted with the same University of Western Ontario detection system that caught the recent fall. A few days after UWO researchers appealed to the public to keep an eye out for meteorites, a piece was recovered that had hit an SUV in a driveway. "The meteorite wasn't intrinsically special; what was special about Grimsby was that we recorded it in such exquisite detail entering the atmosphere," said Brown. "We're learning a lot from that event."

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A 13-kilogram piece of the Buzzard Coulee meteorite sits by a ruler for measurement. The meteorite landed in a field about 40 kilometres from Lloydminster, Sask., on Nov. 20, 2008. (Geoff Howe/Canadian Press)

2008: Buzzard Coulee, Sask.: the fall to Earth of this meteorite created a fireball so bright that it was spotted in Alberta, Saskatchewan and Manitoba. According to the Meteoritical Society, more than 100 well-substantiated pieces, totalling about 41 kg, and hundreds of others were found strewn across seven kilometres of farmer's field about 40 kilometres southeast of Lloydminster, Sask., near the border with Alberta. The largest piece weighed about 13 kg.

2000: Tagish Lake, B.C.: this meteorite was a rare find because it was recovered frozen, meaning the material inside remained unmelted and pristine, enabling researchers to gather valuable information about it. Several dozen pieces were initially found on the frozen-over Taku Arm of Tagish Lake in January 2000 in northern B.C. near the Yukon border. About 200 other pieces were found a few months later. The pieces in the ROM's collection are still held at –85 C conditions.

How much does a meteorite sell for?

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Supply and demand determines what a meteorite will sell for. The rarer the composition of the meteorite, the higher the price. Prices vary from about five cents per gram up to $1,000 a gram.

To assess a meteorite's value, organizations like the ROM get arm's length appraisers who work in the field and understand the market to evaluate the rock. If a collector wants to donate a rock to the ROM, the museum will issue a tax receipt in the amount at which the rock was assessed to the collector. It also buys meteorites from collectors and research institutions but won't disclose how much it has paid for some of its most precious finds, such as the Springwater pallasite.

Meteorites can be bought anywhere from eBay to specialized sites and auctions.

Who owns a meteorite?

In Canada, the owner of the property on which the meteorite is found owns it even if the meteorite has been found by someone else.

Why do they matter?

Meteorites are clues that contain information about the universe and how our own planet and solar system were formed.

"If you got a piece of granite from your cottage, and you were told to come up with the whole story about what the planet looks like, it would be so hard to do, so every meteorite is really rare," says Tait. "Every little chunk of rock that we get is just another jigsaw piece in this big, big puzzle of what happened and what's going on in our solar system."