Bill Napier, David Asher, Mark Bailey and Duncan Steel

A&G (2015) 56 (6): 6.24-6.30

Centaurs as a hazard to civilisation

Figure 4. Four Taurid Complex branches (meteor showers). The separation into branches is not real within the physical stream; it is an artifact of the fact that the Earth's path in space only intersects orbits with particular parameters: for a given size and shape of orbit, the argument of perihelion must take one of four values, corresponding to northern and southern branches, before and after perihelion. Part of orbit above ecliptic drawn thicker, solid; part below ecliptic thinner.


Assessments of the risk posed by near-Earth objects ignore the possibility of a giant comet entering the inner solar system. Bill Napier, David Asher, Mark Bailey and Duncan Steel examine the likelihood and potential consequences of the appearance of such a centaur.

Do we live in dangerous times? The risks to civilization from impacts by asteroids and comets have been appreciated only in the past few decades. 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? 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. 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. 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.

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Last Revised: 2016 January 15th