http://www.astro.washington.edu/deutsch/misc/asteroid/
    has:

Five photos in 30 minutes.
Asteroid 1997 XF11 taken on March 11 '98 over an interval of 30 minutes at Apache Point, New Mexico. One million times too faint to be seen with the naked eye ...

It is calculated that an asteroid the size of 1997 XF11 colliding with the Earth at more than 17,000 miles an hour would explode with an energy of about 320,000 megatons of dynamite. That equals almost 2 million Hiroshima-sized atomic bombs.

Such an asteroid hitting the ocean would create a tidal wave hundreds of feet high, causing extreme flooding along thousands of miles of coastline. If it struck land, it would blast a crater 20 miles across and clog the sky with dust and vapor that the sun would be darkened for weeks or months.

Images courtesy of
The University of Washington and the Astrophysical Research Consortium.