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The question arises whether the presently observed NEOs, at all sizes, are characteristic of a long-term average, or whether we are living in dangerous times, at a time of significant enhancement in the NEO population.It is clear that no survey of Earth-approaching asteroids lasting just 10–20 years can possibly lead to the identification of over 90% of all Earth-approaching asteroids if there is a significant fraction in larger, intermediate- or long-period (i.e.A new, self-consistent picture has emerged in which there is a gradual pass-me-down of substantial objects (50–100 km across and above) into the inner solar system (Horner 2004a, b, Steel 2014, Napier 2014, 2015).
Neglecting physical destruction and non-gravitational effects, about one in ten centaurs in Chiron-like orbits become Earth-crossers, often repeatedly (Horner 2004a, b, Napier 2015): an example is shown in figure 1.
The rate of arrival of ≥100 km centaurs into short-period, Earth-crossing orbits is of order one per 30 000 yr (Napier 2015).
A more variable but significant threat comes from centaurs, giant comets derived from the trans-Neptunian region that reach the inner solar system generally via short-term, dynamically unstable residence periods in the outer planetary region.
The disintegration of such giant comets would produce intermittent but prolonged periods of bombardment lasting up to 100 000 years.
We argue that the population is variable and that assessments of the extraterrestrial impact risk based solely on near-Earth asteroid counts underestimate its nature and magnitude for timescales of order 10 000 years, i.e.
the interval of interest and concern to our civilization.Jupiter and Saturn-crossing) orbits, because many of these will not come close enough to the Sun during that interval so as to become discoverable.To discover and track all such asteroids would require a survey lasting longer than the maximum orbital periods deemed to be of interest.Centaurs comprise a transitional population deriving from cometary reservoirs beyond Neptune.Some reach Earth-crossing orbits (Hahn & Bailey 1990, Asher & Steel 1993), even though only a minor fraction of the objects that begin such an inward passage actually get here, most being weeded out along the way by the influences of the giant planets.Over the bulk of this interval, the semimajor axis is ∼3 au, corresponding to an orbital period ∼5 years.Tags: Adult Dating, affair dating, sex dating