Protection of the Earth from undesirable impacting bodies is not just a science fiction project for some improbable future. The cost might be comparable to, even smaller than, the world's current military expenditures. We could choose to do it now. We could choose to protect ourselves from asteroids and comets rather than from each other.
Impacts of near-Earth objects (NEOs) onto our planet are natural events where the effects of each single impact mainly depend on NEO size, structure, relative velocity, and impact location. To determine if a newly discovered object might impact on Earth one day, the object's orbit has to be numerically computed into the future. The accuracy of this orbit prediction basically depends on the optical and eventually radar measurements available for that object, and of course on the completeness and precision of the numerical perturbation models. Care has to be taken when handling few observations, long prediction periods, and unendorsed NEO properties, which might lead to large uncertainties in collision probability prediction (e.g., Giorgini et al. 2002).
NEOs larger than 150 m in diameter and approaching Earth's orbit closer than 7.5 million km (0.05 AU) are called Potentially Hazardous Objects (PHOs). Due to their susceptibility to small orbit disturbances on short timescales they are candidates for future collisions with Earth. Typical near-Earth asteroid (NEA) impact velocities onto the Earth range from 11.18 to about 25 km s–1, whereas comets typically impact at higher velocities up to 73.65 km s–1 if on a retrograde orbit (e.g., Gritzner 1996).