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Centaurs As a Hazard to Civilization

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Centaurs As a Hazard to Civilization CENTAURS Centaurs as a hazard to civilization Assessments of the risk posed small bodies from the main asteroid belt. asteroids predates the discovery of the vast The timescale for NEO supply from such a trans-Neptunian and centaur populations, by near-Earth objects ignore source is tens of millions of years, leading and the finding that centaurs, in chaotic the possibility of a giant comet to an expectation that the population of orbits, leak from the outer solar system into entering the inner solar system. NEOs does not fluctuate substantially over the inner planetary region at a significant Bill Napier, David Asher, such intervals. Thus crater-counting on rate, largely via the Jupiter family of comets. planetary surfaces, and the understanding A new, self-consistent picture has emerged Mark Bailey and Duncan Steel of what such counts imply, is based on an in which there is a gradual pass-me-down examine the likelihood and assumption that such craters are produced of substantial objects (50–100 km across and Downloaded from potential consequences of the at a constant rate over geological/astro- above) into the inner solar system (Horner nomical time; but this core assumption is et al. 2004ab, Steel 2014, Napier 2014, 2015). appearance of such a centaur. contraindicated by the terrestrial cratering Napier (2015) discussed how the occa- record (the only craters for sional arrival of a massive o we live in dangerous times? The which we have individual “The idea that impacts centaur will dominate the http://astrogeo.oxfordjournals.org/ risks to civilization from impacts datings), which shows that can affect our planet’s supply of Earth-crossing Dby asteroids and comets have been these impacts on our own climate and life is now debris. The size distribution appreciated only in the past few decades. planet have been largely established” of centaurs is shallow (Bauer Programmes such as NASA’s Spaceguard episodic in nature. That is, et al. 2013), i.e. more of the observations seek to map near-Earth it seems that the population of NEOs has mass is in the larger objects. These being objects (NEOs) as a way to quantify the varied substantially across time. The ques- comet-like bodies, we may assume they are risk to Earth. But does the current count tion arises whether the presently observed subject to wholesale fragmentation. This of NEOs reflect the population over time? NEOs, at all sizes, are characteristic of a leads to an expectation that when a centaur We argue that the population is variable long-term average, or whether we are living arrives in an orbit with perihelion within and that assessments of the extraterrestrial in dangerous times, at a time of significant Jupiter’s heliocentric distance there will be by guest on November 22, 2015 impact risk based solely on near-Earth enhancement in the NEO population. a temporarily enhanced mass of material asteroid counts underestimate its nature in the inner solar system, spread across and magnitude for timescales of order Perpetual vigilance all sizes from dust at <100 microns to tens 10 000 years, i.e. the interval of interest It is clear that no survey of Earth-approach- of kilometres across. The transition rates and concern to our civilization. A more ing asteroids lasting just 10–20 years can into the near-Earth environment indicate variable but significant threat comes from possibly lead to the identification of over that fluctuations in the mass of near-Earth centaurs, giant comets derived from the 90% of all Earth-approaching asteroids interplanetary material may amount to trans-Neptunian region that reach the if there is a significant fraction in larger, two orders of magnitude over a timescale inner solar system generally via short-term, intermediate- or long-period (i.e. Jupiter 30–100 000 yr. dynamically unstable residence periods in and Saturn-crossing) orbits, because many While the terrestrial record of impacts is the outer planetary region. The disintegra- of these will not come close enough to the incomplete and does not support the notion tion of such giant comets would produce Sun during that interval so as to become that all mass extinctions arise from asteroid intermittent but prolonged periods of bom- discoverable. To discover and track all such impacts, the idea that impacts can affect our bardment lasting up to 100 000 years. Mass asteroids would require a survey lasting planet’s climate and life is now established extinction/geological boundary events on longer than the maximum orbital periods (see box, “Impacts and extraterrestrial bod- Earth show such a pattern, as do levels of deemed to be of interest. This links to the ies: the story so far”). These new astronomi- dust and meteoroids in the upper atmos- problem of long-period or near-parabolic cal and terrestrial lines of evidence merit a phere. Over the past 10 000 years, Earth has comets: perpetual vigilance is necessary reappraisal of the celestial hazard faced by been experiencing the intermittent arrival to protect civilization (Marsden & Steel contemporary civilization. Here we concen- of dust, meteoroids and comet fragments 1994). The claim that the NASA Spaceguard trate on the implications over timescales of from the disintegration of comet 2P/Encke, programme has (so far) led to the discovery order 10 000 yr. trapped within the orbit of Jupiter. and orbit determination of 93 or 95% of The assessments by Chapman & NEOs larger than 1 km in size is justifiable, The centaur flux Morrison (1994) and others of the major so long as one recognizes that the term (5145) Pholus, the second-discovered impact hazard posed to modern civiliza- “NEOs” as used there implies only aster- centaur after (2060) Chiron, and tion are based on the assumption, usually oids in short-period (i.e. cis-jovian) orbits, (15760) 1992 QB1, the second trans- unspoken, that the contemporary NEO and not those asteroids in larger orbits – Neptunian object (TNO) after Pluto, were population is in a steady state, in terms of and no comets at all. both discovered as recently as 1992. Discov- the populations at different sizes, being The basic model of NEO supply by ery rates have increased such that known maintained primarily through leakage of gravitational perturbations upon main-belt centaurs now number in the hundreds and 6.24 A&G • December 2015 • Vol. 56 • www.astrongeo.com CENTAURS Impacts and extraterrestrial bodies: the story so far The idea that the geological and biological 90 km diameter Popigai crater, are associated 1 Correlations between evolution of the Earth proceeds in isolation with regional, not global, effects. The evidence from its surroundings, developed by Darwin, of iridium excess, shocked quartz and the volcanism, extinctions Hutton, Lyell and others, was the default like at extinction boundaries has either been and bombardment position in the Earth sciences until the 1970s. ambiguous or absent (MacLeod 2013). Further, This perspective began to change when some of the mass extinctions, such as that VEB E = BV = BV = E terrestrial craters such as Odessa (Texas), in the Late Devonian, 365 Myr ago, appear to 2 1 Barringer (Arizona) and Henbury (Australia) have occurred in steps (McGhee 1996), while ● were associated with iron meteorites; when others coincide in time with massive volcanic/ 15–17 spacecraft missions in the 1960s indicated that flood basalt activity (see table 1). 29–31 planetary surfaces are generally pockmarked The fact that the main asteroid belt 34 35 ● with craters as predicted by Öpik (1951); when between Mars and Jupiter fails by an order of 49 returned Apollo samples were shown to magnitude to supply the required transient 55–61 display evidence of shock metamorphism, population of large near-Earth objects (NEOs), 65–67 66 65 ● ● ● demonstrating the well-known lunar craters to or to account for the number of large impact 87–90 be of impact (rather than volcanic) origin; and craters (Menichella et al. 1996, Minton & 94 when wide-field telescopic surveys unveiled Malhotra 2010) is another complication. 109 –119 a significant population of small (~1 km) This perspective raises the question of Downloaded from 122–122 122 ● near-Earth asteroids (Helin & Shoemaker 1979). whether, and to what extent, cosmic impacts 132–134 Impact cratering is now considered a common in the present epoch (the Holocene) might 146 144 and continuing geologic process. have deleterious effects limited to restricted ● 166 Napier & Clube (1979) indicated that impact areas, as with the 1908 Tunguska impact or the 180–183 183 ● rates were high enough to be associated with 2013 Chelyabinsk event, or globally. Chapman http://astrogeo.oxfordjournals.org/ the mass extinctions seen in the geological & Morrison (1994) considered that the arrival 199–202 200 200 ● ● ● record and they proposed possible extinction of a ~2 km asteroid would result in the death 250–253 251 255 (●)(●) ● agencies: collapse of food chains as a result of a large fraction of the human population, 257–260 260 ● of impact-generated fine dust in the upper and that there is a probability of about 1% 322 atmosphere blocking out sunlight; ozone-layer of such an event in the next 10 000 yr. The 360 destruction; straightforward blast; and others. combination of small probability and large 364–367 The concept of food-chain collapse caused by consequences (for substantial NEOs) has led 370–370 dust was not new: Hoyle & Wickramasinghe to systematic searches for large near-Earth 375 377 ● (1978) had suggested that such dusting might asteroids (>1 km) in cis-jovian orbits and their 385 be caused by an encounter with the coma of discovery appears to be above 93% complete by guest on November 22, 2015 445 445 ( ) a comet. Earlier suggestions of links between (cf.
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