NEOShield: the physical properties of the most frequent impactors

NEOShield is a consortium of 13 research institutes, universities and industrial partners from 6 countries. The aim of the project is to explore mitigation options in the event that a NEO is found to be on a potential collision course with Earth, and to pave the way for demonstration missions to test proposed mitigation techniques. Finding an accessible and appropriate target NEO for a demonstration mission is an important aspect of mission design. We are carrying out a statistical investigation of the properties of the known NEO population, using the latest published data, with the aim of estimating the most likely mitigation-relevant physical properties of the first NEO to trigger a space-borne mitigation action. Our investigation focuses on the physical properties of the most frequent serious impactors. We define a serious impactor to be one with the potential to lead to major loss of life and damage to infrastructure. At the low end of the size range our definition includes atmospheric events such as the 1908 Tunguska explosion, caused by a body with a diameter, D, of around 50 m. We consider the upper limit of our size-range of interest to be around D = 200 m, because for objects above this size the impact frequency drops below 1 per 10 000 years. However as there are large uncertainties related to the calculation of diameter, we include NEOs with H > 20 mag (D < 300 m). Another reason for considering objects larger than 200 m is the fact that present guidance, navigation, and control (GNC) technology may dictate a minimum size larger than 200 m for the target of a feasible demonstration mission to ensure a high chance of success (although research to improve GNC performance is part of the NEOShield project). Very little is known about NEOs with D < 300 m, as only 2% of the more than 5400 discovered NEOs in this size range have had any physical properties measured besides H magnitudes. Our investigation includes recently published data from the NEOWISE and Warm Spitzer ExploreNEOs surveys, without which this fraction would be much smaller.