Long-period comets They are part of the Oort cloud, a vast diffuse envelope of encircling comets. Period may thousands of years of more.
Comet Hale Bopp – visible for 18 months in 1996 - 1997
All comets are mainly made of ice and rocky debris.
IMPACTS Energy release depends on speed and size. Asteroids may impact at >30,000mph. Long-period comets >150,000mph. M-16 rifle ~2250 mph.
Given equal size, the mass of an asteroid is greater than that of a comet.
Atmosphere may heat the asteroid to 3,000 o C. Impact instantly released enormous heat and energy, melting and vaporizing not only rock but also much of the asteroid itself. Crater may end up filled with crushed and shattered rock.
Tunguska originally thought to be ~60m in diameter. If so, similar impacts every ~2,000 years. But, with new estimate of 40m or less, once every ~300 years. Total potential hazard from impacts has increased.
NEAR-EARTH OBJECTS (NEOs) PROJECT Monitors and identifies near earth-objects for potential impact of objects >1km (.62mi.) diameter. Average time interval between impacts of this size >100,000 years.
However, objects 40m(or less) – 100m in diameter can cause devastating local effects (like Tunguska). Smaller potentially damaging events occur perhaps every couple of centuries.
Would we have warning before an impact? Maybe, maybe not.
November 2011 Asteroid http://neo.jpl.nasa.gov/news/news171.html Near Earth Objects Tracking http://neo.jpl.nasa.gov/risk/#legend