### The rate of small impacts on Earth

#### Abstract

Asteroids tens to hundreds of meters in diameter constitute the most immediate impact hazard to human populations, yet the rate at which they arrive at Earth's surface is poorly known. Astronomic observations are still incomplete in this size range; impactors are subjected to disruption in Earth's atmosphere, and unlike the Moon, small craters on Earth are rapidly eroded. In this paper, we first model the atmospheric behavior of iron and stony bodies over the mass range 1-10^12 kg (size range 6 cm-1 km) taking into account deceleration, ablation, and fragmentation. Previous models in meteoritics deal with rather small masses (<10^5-10^6 kg) with the aim of interpreting registered fireballs in atmosphere, or with substantially larger objects without taking into account asteroid disruption to model cratering processes. A few earlier attempts to model terrestrial crater strewn fields did not take into account possible cascade fragmentation. We have performed large numbers of simulations in a wide mass range, using both the earlier "pancake" models and also the separated fragments model to develop a statistical picture of atmosphere-bolide interaction for both iron and stony impactors with initial diameters up to ~1 km. Second, using a compilation of data for the flux at the upper atmosphere, we have derived a cumulative size-frequency distribution (SFD) for upper atmosphere impactors. This curve is a close fit to virtually all of the upper atmosphere data over 16 orders of magnitude. Third, we have applied our model results to scale the upper atmosphere curve to a flux at the Earth's surface, elucidating the impact rate of objects <1 km diameter on Earth.We find that iron meteorites >5 xv10^4 kg (2.5 m) arrive at the Earth's surface approximately once every 50 years. Iron bodies a few meters in diameter (105-106 kg), which form craters 100 m in diameter, will strike the Earth's land area every 500 years. Larger bodies will form craters 0.5 km in diameter every 20,000 years, and craters 1 km in diameter will be formed on the Earth's land area every 50,000 years. Tunguska events (low-level atmospheric disruption of stony bolides >108 kg) may occur every 500 years. Bodies capable of producing hazardous tsunami (~200 m diameter projectiles) should strike the Earth's surface every ~100,000 years. This data also allows us to assess the completeness of the terrestrial crater record for a given area over a given time interval.

#### Keywords

TOF-SIMS;Presolar grains;Volatile trace element depletion;Nebular condensation