Large Asteroid That Zipped Passed Earth Has Its Own Moons

3-mile-wide asteroid, Florence, passed by Earth, coming at a distance of 4.4 million miles (7 million kilometres) at its nearest, NASA says.

No bigger asteroid has come as close to the Earth since the tracking of asteroids began.

"Florence is the largest asteroid to pass by our planet this close since the NASA program to detect and track near-Earth asteroids began," Paul Chodas, manager of NASA's Center for Near-Earth Object Studies, said in a statement.

The asteroid is so big that it has two tiny moons of its own.

The images obtained by NASA when Florence was at its closest on 31 August and 1 September, found the moons 100-300 metres wide, having orbits of around 8 hours for the inner moon and between 22 and 27 hours for the outer moon.

According to scientists, asteroids larger than half-mile to a mile (1 to 2 kilometers) could have worldwide effects if it hit the planet.

Asteroids usually don't penetrate Earth's atmosphere as they burn up in the atmosphere. But in 2013, an 18 meters wide asteroid exploded over Chelyabinsk, Russia, injuring hundreds of people and shattering glass in buildings.

The asteroid that currently poses the biggest threat to Earth, according to NASA, is Asteroid 1950 DA. The asteroid is about 1 mile wide and can impact the Earth in the year 2880. The chances for the impact are 1 in 8,000.