There are three classifications of NEO's:
The number of known objects considered to be "potentially hazardous to Earth" stood at 113 in July 1998. These things are confoundedly hard to discover, because they are small, faint, dark, and for a high percentage of their orbits, they are too close to the Sun to be observed anyway. It is estimated that we have discovered less than 10% of the total number that may be out there. Close approaches in the 1990's brought a couple of 6000-tonne rocks (10m diam) within half the distance to the Moon. Some of these objects may be kilometres across, and while that doesn't sound very big, let's see what happens when one of them crashes into a solid body like the Earth.
When an object hits the surface of a planet, a crater is formed and the impactor is fragmented or vaporised. For a collision at 20 km/sec the energy release is equivalent to over forty times the same mass of TNT.
The impact of a 10 km asteroid (or a common-or-garden variety of comet) would be equivalent to detonating 60 million megatonnes of TNT, would cause an earthquake of 12.4 on Richter scale and would result in the formation of a crater 60 km wide, 40 km deep and 40 km high at the rim. 100,000 cubic kilometres of rock would be vaporised and lifted into atmosphere. Gaspra, a main belt asteroid, is 60 km across. It would create a crater the size of NSW if it impacted with Earth
There have been numerous extinction events in the last few hundred million years, which may be attributable to terrestrial impacts. They are spaced about every 13 million years, which correlates with the passage of the Solar System in and out of the plane of the Milky Way. The crater believed to have resulted from the impact which led to the extinction of the dinosaurs lies half submerged on the Yucatan peninsula (Mexico). It is 300 km in diameter and formed 65 million years ago.
"Ah!" you say, "but most of the Earth is covered with water." Well I hope you live on high ground. The resulting tsunami (tidal wave) after an impact of a 10 km asteroid would be 1,300 metres high at 3 km from impact, and 100 metres high after travelling halfway around the planet.
There is a one in one hundred chance that an impactor of 100 metres diameter will arrive in the next 100 years. If it lands in the Pacific Ocean, the resulting tsunami (tidal wave) would wipe out every city on the Pacific rim and travel 25 km inland.
You may have heard of the devastating event which occurred in the USSR in 1908. Luckily, it did not hit at a populated location, but at remote Tunguska. It was caused by an object about 60 metres across travelling at 15 km/second exploding 8 km up in the atmosphere over Siberia. It burned an oval shape into the forest and flattened trees over an irregular area of about 40 x 45 kilometres. A 10-metre diameter meteorite was observed to have entered Earth's atmosphere on 1st February 1994. It was seen by two fishermen and 6 satellites above the Western Pacific. It vaporised in the atmosphere
The main factor which determines whether an impactor will explode in the atmosphere or on impact with the surface of a planet is its composition. Comets are generally composed of about 2/3 water and 1/3 rock and are more likely to break up at least partly in the atmosphere. It is the objects with a higher iron content - asteroids - that are more likely to survive to the surface.
Two well known impact craters still surviving on Earth
* Gosses Bluff, 130 million years old. The original crater would have been about 20 km in diameter, but has since been eroded. The impactor would have been 2-3 km in size. This is one of 19 known impact craters in Australia. * Meteor Crater, or more correctly, Barringer meteorite crater, near Winslow, Arizona. It is 1.2 km in diameter and was caused by the impact of a 30-40 metre nickel-iron meteorite. You may remember this one from the final scenes of the movie "Starman" in which it represented a UFO landing site.
The youngest crater on Earth is less than 3,000 years old and four are less than 5,000 years old. Were it not for our rain-giving atmosphere, liquid oceans and plate tectonics, the Earth would be as battered and beaten as the other terrestrial worlds.
The approach and impact of comet P/Shoemaker-Levy 9 with Jupiter in July 1994 demonstrated that small bodies can break up under the stresses of tidal forces and impact in sequence. Pictures taken of the event have been reassessed to look for other evidence of this type of impact. Callisto has 12 crater chains, Ganymede has 3 and they all lie on or near the edge of the hemisphere facing Jupiter. The craters are closely spaced indicating that the impacts occurred soon after the parent body broke up.
There is also a 50 km-long crater chain on the MOON near the Davy crater in Mare Nubium. Sunrise over this region of the Moon occurs a little after First Quarter and the chain is just visible in backyard telescopes.