How often does a one-kilometer object hit Earth?

Answer 1

A meteorite with a 1km diameter will undoubtedly cause global damage. It is believed that a meterite a bit larger was responsible for the extinction of dinosaurs. Rocks this big only appears to hit Earth once every 100 million years.

Answer 2

It would catastrophic. Depending on the composition of the object, and where it hit, the effects would be "very bad" at a minimum, and "globe-altering" at worst. The impact would probably devastate any continent that it hit, and would likely cause earthquakes around the world. If it were to crash into the Indian Ocean, the tsunami would probably scour western Australia, eastern Africa, and most of India, and would inundate most of the middle east all the way to the Mediterranean Sea.

The "best" case might be an impact in Antarctica, which would not cause hundreds of millions of immediate fatalities, but probably would cause long term climatic changes including the worst components of "nuclear winter" and global warming; the Antarctic ice cap would probably melt substantially, and the debris thrown into the air would probably block most sunlight for a decade.

Answer 3

Fortunately, very rarely! Because those "intermediate" sized asteroid impacts do not cause global damage, we can't be entirely certain when in the distant past they happened. But not more often than every 3,000 years or so.

The last major impact may have struck in the southern Indian Ocean about 3500 years ago. Parts of western Australia appear to bear the marks of ancient inundations from the sea, and there is an interesting structure called the Burckle Formation on the floor of the Indian ocean. A tsunami generated by a large impact could easily have caused the hills we see today, and similar marks in India and Africa. Did I mention that this would also be a fairly good match for the legends of Noah and the Flood, or the Sumerian "Gilgamesh" epics? Many middle eastern cultures share legends of a major flood.

Before that, perhaps the impactor in northern Canada that correlates with the Younger Dryas mini-ice age, and the extinctions of most large life forms in North America about 14,000 years ago, including the Clovis People. Although at this point, we're well into the realm of speculation and conjecture, and have strayed from real facts.

However, there is a definite periodicity to major extinction events, about every 25 million years or so.

Answer 4

every million years

Answer 5

not very