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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.The size distribution of centaurs is shallow (Bauer 2013), i.e. These being comet-like bodies, we may assume they are subject to wholesale fragmentation.This leads to an expectation that when a centaur arrives in an orbit with perihelion within Jupiter's heliocentric distance there will be a temporarily enhanced mass of material in the inner solar system, spread across all sizes from dust at , the second trans-Neptunian object (TNO) after Pluto, were both discovered as recently as 1992.Mass extinction/geological boundary events on Earth show such a pattern, as do levels of dust and meteoroids in the upper atmosphere.Over the past 10 000 years, Earth has been experiencing the intermittent arrival of dust, meteoroids and comet fragments from the disintegration of comet 2P/Encke, trapped within the orbit of Jupiter.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.
The rate of arrival of ≥100 km centaurs into short-period, Earth-crossing orbits is of order one per 30 000 yr (Napier 2015).Programmes such as NASA's Spaceguard observations seek to map near-Earth objects (NEOs) as a way to quantify the risk to Earth.But does the current count of NEOs reflect the population over time?Thus crater-counting on planetary surfaces, and the understanding of what such counts imply, is based on an assumption that such craters are produced at a constant rate over geological/astronomical time; but this core assumption is contraindicated by the terrestrial cratering record (the only craters for which we have individual datings), which shows that these impacts on our own planet have been largely episodic in nature.That is, it seems that the population of NEOs has varied substantially across time.
This links to the problem of long-period or near-parabolic comets: perpetual vigilance is necessary to protect civilization (Marsden & Steel 1994).