The simulations that will be described in subsequent chapters require models for the asteroid and comet populations, however. Estimates for the total number of asteroids having diameters larger than values of particular interest are shown in Fig. 3-1 by the solid curve. Specific population estimates at sizes of interest are indicated in the figure, where our uncertainties are bounded by the dashed lines. For example, we estimate there are 2,100 ECAs larger than 1 km in diameter, with an uncertainty of a factor of two.
Active comets can also cross the Earth's orbit with the potential for collision.
Comparing these numbers with those for the ECA population in Fig. This contribution is negligible compared to the estimated uncertainty in the ECA population. As stated previously, an object that displays no apparent atmosphere or tail is classified as an asteroid even if its orbital properties are similar to that of a short-period comet. Dormant or extinct short-period comet nuclei are therefore likely members of the ECA population, and such objects are implicitly included in the ECA estimates given above. Although about 700 long-period comets are known to have passed through the inner solar system during recorded history, their total population is difficult to characterize.