Section B-1: Some major impacts of asteroids and comets;
Section B-2: Documented impacts in the last decade;
Section B-3: Close approaches.

B-2: Documented impacts on Earth in last decade

Note that between August 1972 and March 2000, sensors in these satellites detected 518 impact events, all in the kilotonne of TNT class or above (objects above a few metres in diameter); this averages 30 events per year. Most of these events were primarily bursts in the upper atmosphere and were not detected at ground level.

B-3: Close approaches
This Section analyses Near Earth Objects which have been observed over recent years to have come closer than two or four lunar distances of the Earth.The first two lists below cover close approaches in the past; the remaining two are predictions for the future.

Although objects of all sizes are included, those under a few tens of metres in diameter would burn up in the upper atmosphere. The conclusion is that no known object presents a serious hazard over at least the coming 50 years. However, only about half the 1 kilometre objects have yet been discovered and a much smaller proportion of small ones.The Section is based on information kindly supplied by Brian Marsden and Gareth Williams of the Minor Planet Center, Boston (and some data from Andrea Milani�s �riskpage� website, see Bibliography).

1: Objects which were discovered as they approached the Earth to within 2 lunar distances. These are given chronologically by the date of closest approach.The lunar, or Earth to Moon, distance is about 400,000 kilometres.

2: Objects 200 metres and larger which were discovered after they had passed within two lunar distances some time in the last century. This conclusion was reached by calculating the past orbit of each newly discovered object.

3: Objects 200 metres and larger that have been seen more than once and are predicted to pass within four lunar distances over the next hundred years. In addition there are several predicted possible approaches by objects seen only once, although none of them should come within two lunar distances.

4: Objects for which the first orbital calculations indicated that they might impact Earth during the next 50 years. Further work has dismissed this possibility or made it improbable. There are nine objects of widely varying size. For three of them (marked * below) the possibility of impact was dismissed during the month or so after discovery as more observations were made and their orbits more accurately determined. For two of them (marked **, above and below) intervening close approaches (see list 3 above) did not allow current observations to eliminate the possibility of a later impact; fortunately, an impact was eliminated following the recognition of observations on old photographs (which existed because these objects are large and therefore bright). In the four remaining cases at least two would probably be too small to do damage. For the other two the available observations are yet to be performed to allow an impact to be dismissed. However, even before that, the chances of the object 1998 OX4 hitting the Earth is thought to be only about 1 in 2,000,000, and the much smaller object 1995 CS, 1 in 200,000.The objects are arranged in order of earliest possible impact date.