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
undiscovered.
In fact, one was discovered in 1998 as the long blue streak in the
above archival image taken by the Hubble Space Telescope.
In 2002 June,
the small 100-meter asteroid
2002 MN was discovered only after it
whizzed by the Earth,
passing
well within the orbit of the Moon.
2002 MN passed closer than any asteroid since
1994 XM1, but not as close as
2004 MN4 will pass in 2029.
A collision with a large asteroid
would not affect Earth's orbit so much as
raise dust
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.
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