Rocks from space hit Earth every day.
The larger the rock, though, the less often Earth is struck.
Many kilograms of space dust pitter to Earth daily.
Larger bits appear initially as a bright meteor.
Baseball-sized rocks and ice-balls
streak through our atmosphere daily,
most evaporating quickly to nothing.
Significant threats do exist for rocks near
100 meters in diameter, which
strike the Earth
roughly every 1000 years.
An object this size could cause significant
tsunamis were it to strike an ocean,
potentially devastating even distant shores.
A collision with a massive asteroid,
over 1 km across, is more rare,
occurring typically millions of years apart, but
could have truly global consequences.
Many asteroids remain
In fact, one was discovered in 1998 as the long blue streak in the
above archival image taken by the Hubble Space Telescope.
Further, just last year the 50-meter wide asteroid
2012 DA14 was discovered that will
pass inside the orbit of Earth's
geosynchronous satellites this coming Friday.
A collision with a large asteroid
would not affect Earth's orbit so much as
that would affect Earth's climate.
One likely result is a global extinction of many species of life,
possibly dwarfing the ongoing
extinction occurring now.
R. Evans & K. Stapelfeldt