2005 YU55, an asteroid 400 metres wide will pass by the Earth on Wednesday, coming closer to this planet than the Moon.
It will be the first time since 1976 that an object of such a size has been so close - within about 323,000km of the Earth's surface.
Astronomer Duncan Steel says that is quite close in astronomical terms.
Dr Steel says asteroids of a similar size strike the Earth about once every 50,000 - 100,000 years.
He says it will not collide with Earth, but if an asteroid of similar size did hit, it would leave a crater about 6km wide and cause devastation.
Dr Steel says another large asteroid is due to head this way in 2036 which has perhaps a 1 in 40,000 chance of striking the Earth.
2005 YU55 will race past Earth at about 40km per second. Dr Steel says observers will need a powerful telescope to see it.
The asteroid was detected in 2005.
