In early March 2009 an asteroid whipped over the Pacific Ocean near Tahiti travelling 72,000 kilometres from the Earth, according to NASA's Near Earth Object program. The asteroid could be seen from Australia, Japan and China.

The space rock was first observed by Rob McNaught at Siding Spring Observatory in Australia, when it was "a dim speck about 1.5 million kilometres away."

Asteroids pass near the Earth's atmosphere frequently — the next one, Asteroid 4179 Toutatis — is expected on Dec. 12, 2012.

What is an asteroid?

Asteroids, also known as "minor planets," are metallic, rocky bodies orbiting the sun that are too small to be classified as planets.

This false-colour picture of asteroid 951 Gaspra is a mosaic of two images taken by the Galileo spacecraft from a range of 5,300 kilometres, some 10 minutes before its closest approach to the spacecraft on Oct. 29, 1991. (NASA/NEO)Unlike comets, which are thought to have originated in the outer solar system beyond Jupiter, asteroids are closer to the sun. Most asteroids are gathered in belts between Mars and Jupiter, about 300 to 600 million kilometres away from the sun, or roughly two to four times the distance from the Earth to the sun.

In addition to the relative orbits, asteroids are different in composition to comets, which also contain water-ice.

It's generally assumed that asteroids are the remnants of material from the early days of the solar system that were never able to collide and merge to form another planet because of the disruptive influence of Jupiter's gravity.

It is estimated the total mass of all asteroids would form a body approximately 1,500 kilometres in diameter, or half the size of the moon.

The largest asteroid astronomers know of is Ceres, which is about 1,000 kilometres in diameter. Asteroids can be much smaller, though the smallest pebble-like objects in space, like the debris from comets, are often referred to as meteoroids.

Why is it some asteroids end up close to Earth?

Though tens of thousands of asteroids congregate in the asteroid belts, orbiting the sun in an elliptical path, a few have strayed off course. Astronomers believe these asteroids are fragments whose course was altered either by collisions or Jupiter's gravitational influence. Those asteroids whose orbits bring them within 195 million kilometres of our planet are called "near-Earth asteroids."

NASA's Near-Earth Object (NEO) program office tracks the paths of both near-Earth asteroids and comets. As of Jan. 20, 2008, the NEO office said it has discovered 5,086 near-Earth asteroids.

Have asteroids or comets ever collided with Earth?

An overhead look at the approximately 206-million-year-old Manicouagan Crater in northern Quebec. Unlike other impact craters found elsewhere on Earth, the atmosphere and erosion have not damaged the crater significantly. (NASA/NEO)While meteoroids usually burn up when they enter Earth's atmosphere, larger asteroids can make quite an impact.

Scientists have found evidence of asteroid collisions with Earth, the most notable being the impact site off the coast of the Yucatan Peninsula in Mexico. The site, buried by ocean sediments today, is thought to be a record of a collision that occurred 65 million years ago. Scientists have theorized that the collision wiped out the dinosaurs by causing an upheaval in the planet's climate.

In a paper published in the journal Science in 2007, researchers at the Southwest Research Institute's office in Boulder, Colo., traced the origins of the Yucatan asteroid to an earlier collision of two asteroids about 190 million years ago in the asteroid belt that may have sent a third fragment hurtling toward Earth.

A more recent collision occurred when a small object — thought to be a comet with a diameter of less than 100 metres — landed in the Tunguska region of Siberia in 1908. The resulting shockwave knocked down trees for hundreds of square kilometres and burned an area about 80 kilometres across.

Is the Earth in any danger?

The NEO office lists 910 known asteroids that can be classified as potentially hazardous to Earth. But potentially hazardous doesn't mean an asteroid will hit Earth, only that there is a possibility. The NEO office uses a scale — called the Torino scale — to measure that potential danger to Earth from both the probability of an impact and the size of the asteroid.

The scale starts at zero, given to events of "no likely consequences." Phrases such as "regional devastation" start creeping in at about four on the scale, while it ends, at 10, with what the NEO office describes as a certain collision "capable of causing global climatic catastrophe that may threaten the future of civilization as we know it." Such collisions happen once every 100,000 years on average, or less, the NEO office says.

Among recently observed asteroids, all but one scored a Torino scale rating of zero. The exception, asteroid 2007 VK184, scored a one on the scale, indicating an event that "merits careful monitoring." 2007 VK184 has an estimated diameter of 130 metres and has four potential chances to strike the Earth between 2048 and 2057, giving it a total chance of impact of one in 2,940, slightly better odds than scoring a straight flush in a game of seven-card stud.

But those odds could change. The asteroid 99942 Apophis once rated a four on the Torino scale and was given a one in 42 chance of striking Earth in 2036 before later study dropped the likelihood of a collision to one in 45,000.