How often does the Earth suffer a NEO impact powerful enough to produce catastrophic consequences for the environment? Impact energy determines the degree of devestation produced by an asteroid striking the Earth. According to the  UK Task-Force on NEO hazards, 1,000  megaton TNT (MT) impacts can potentially produce regional physical damage, but the implications of such an impact to our world-wide linked economic system would almost certainly be global.

To estimate the NEO collision frequencies as a function of impact energy we employ our NEO distribution model. Recall that our model provides the distribution of the Near Earth Objects in terms of semi major axis, eccentricity, inclination, physical size and albedo. The only remaining  component needed to compute impact energy is the projectile's bulk density of the objects; we use this value to estimate the projectile's mass.

Fortunately, measurements of the asteroid bulk densities, derived from spacecraft data and asteroid satellite observations, show a sharp correlation between bulk density and asteroid taxonomic class:  asteroids in the C taxonomic class have bulk densities of ~1.3 grams per cubic centimetre, while asteroids in the S taxonomic class have bulk densities of ~2.7 grams per cubic centimetre. Therefore, it is reasonable to assume that the former density is applicable to bodies with albedo smaller than 9%, while the latter value is applicable to higher albedo bodies.

Accordingly, we estimate that the typical interval between collisions with impact energy of 1,000 megatons TNT is ~65,000 years. This interval increases to ~250,000 years for energies of 10,000 megatons and ~1 million year for energies of 100,000 megatons.
 
 

reference impact energy	Average impact interval	average D of impactors	average Hof impactors	Observational completeness of the corresponding population
> 1,000 MT	63,000 y	277 m	20.5	18%
> 10,000 MT	241,000 y	597 m	18.9	37%
> 100,000 MT	925,000 y	1287 m	17.3	49%

Our predicted interval between impacts on Earth  is roughly a factor of 4 larger (or, equivalently, the impact rate is 4 times smaller) than in previous estimates. This difference can be attributed to a more precise evaluation of the NEO population (current estimates are roughly half of previous estimates), a better model of the NEO orbital distribution, and updated information on the bulk densities of real asteroids.
 

The expected number of collisions carrying an impact energy larger  than 1000 megaton TNT, as a function of the impactors' semi major axis (red histogram).  For comparison, the green histogram shows the number of collisions expected from the known NEO population. The completeness of the known population, measured relative to  its total collision probability is only 18%. This result implies that there is an 82% chance that we have not yet discovered the NEO that will make the next 1,000 MT impact on Earth.

The stated Spaceguard goal of the current NEO surveys is to discover 90% of the NEOs with H<18. However, it would be appropriate:

1) to push the value of H to 20.5, which corresponds - on average - to impact energy  ~1,000 megaton TNT;

2) to state the goal in terms of discovering the NEOs carrying 90% of the total collision probability. The comparison between the figure above and that of the raw distribution of NEOs shows that the two goals are not equivalent. For instance, the Atens (a<1 AU), despite the fact that they are only 6% of the total NEO population, carry about 20% of the total collision probability. Thus,  the discovery of Atens - of secondary importance for the original Spaceguard goal - becomes a top priority when collisional hazards are taken into account.
 

NEXT PAGE                                       PREVIOUS PAGE                              FIRST PAGE