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‘Centaurs’ as Potential Impactors

An asteroid shaped something like a sweet potato swept by the Earth on December 24, though at a sufficient distance (28 times further away than the Moon) to pose no hazard to our planet. 2003 SD220 was making the first of five predicted encounters within the next twelve years, and the measurements made by the planetary radar system at Arecibo on the asteroid’s shape, rotation and other properties will now be useful in refining its orbit. The Jet Propulsion Laboratory’s Goldstone Solar System Radar and the National Radio Astronomy Observatory’s Green Bank Telescope and Very Long Baseline Array also contributed data on the asteroid.

Highly elongated and more than 2 kilometers long, 2003 SD220 is of interest as a possible target for a future space mission, not only because asteroids should give us clues about conditions in the early Solar System, but also because near-Earth asteroids need to be well understood in case we one day find ourselves dealing with a potential impactor.

Christmas NEA

Image: Delay-Doppler image of asteroid 2003 SD220 as observed from the Arecibo Observatory using the NASA-funded planetary radar system on December 3-5 and December 14–15. Credit: Arecibo Observatory.

But as we assess impact probabilities over long periods of time, are asteroids the prime concern? New work out of Armagh Observatory and the University of Buckingham (UK) indicates that we should keep an eye on the outer Solar System, where hundreds of giant comets have been discovered within the last two decades. Known as centaurs, these objects ply unstable orbits that cross the paths of the gas giants. Such encounters can change their trajectories, meaning we need to assess their orbits with care.

The typical centaur is in the 50 to 100 kilometer range — some are larger — with more mass, as this Royal Astronomical Society news release tells us, than the entire population of Earth-crossing asteroids found to date. A centaur deflected onto a trajectory that crosses Earth’s orbit is expected to appear every 40,000 to 100,000 years, according to this work. So while the odds of a direct impact by an intact centaur are slight, these objects are likely to disintegrate as they approach the inner system, with cometary debris posing a potential hazard.

Is cometary dust and debris rather than a single large impact a likely scenario for past extinction events? The paper is currently behind a firewall, but lead author Bill Napier (University of Buckingham) had speculated on the possibility in a paper that appeared earlier this year:

The main asteroid belt seems inadequate to supply the larger impactors, and large cometary populations have now been revealed extending to beyond the planetary system, with the potential to be thrown into the near-Earth environment. The top-heavy mass distribution of these comets, their likely disintegration history when thrown into short-period orbits, and the potential of their debris to cause deleterious effects, make them likely candidates for the mass extinctions observed in the terrestrial record and the associated geological disturbances. The mechanisms involved are a matter for detailed stratigraphic studies; it is suggested here that prolonged atmospheric perturbations arising from fireball storms and dusting are the most energy efficient means of collapsing food chains, yielding both marine and land extinctions.


Image: The outer solar system as we now recognise it. At the centre of the map is the Sun, and close to it the tiny orbits of the terrestrial planets (Mercury, Venus, Earth and Mars). Moving outwards and shown in bright blue are the near-circular paths of the giant planets: Jupiter, Saturn, Uranus and Neptune. The orbit of Pluto is shown in white. Staying perpetually beyond Neptune are the trans-Neptunian objects (TNOs), in yellow: seventeen TNO orbits are shown here, with the total discovered population at present being over 1,500. Shown in red are the orbits of 22 Centaurs (out of about 400 known objects), and these are essentially giant comets (most are 50-100 km in size, but some are several hundred km in diameter). Because the Centaurs cross the paths of the major planets, their orbits are unstable: some will eventually be ejected from the solar system, but others will be thrown onto trajectories bringing them inwards. Credit: Duncan Steel.

The image above gives us a sense of how our view of the Solar System has changed over the past century, as we’ve moved from nine planets and an asteroid belt moving through supposedly empty space to a wild jumble of objects including not just asteroids and comets but a vast population of trans-Neptunian objects. You can see how the centaurs cross planetary orbits in the image, and it’s possible that some are captured along the way. Saturn’s moon Phoebe may well have been a centaur that was drawn into Saturn’s gravity well long ago.

Napier puts the new work into perspective:

“In the last three decades we have invested a lot of effort in tracking and analysing the risk of a collision between the Earth and an asteroid. Our work suggests we need to look beyond our immediate neighbourhood too, and look out beyond the orbit of Jupiter to find centaurs. If we are right, then these distant comets could be a serious hazard, and it’s time to understand them better.”

The paper is Napier et al., “Centaurs as a Hazard to Civilization,” Astronomy & Geophysics 56 (6): 6.24-6.30 (abstract). The earlier Napier paper referenced above is “Giant Comets and Mass Extinctions of Life,” Monthly Notices of the Royal Astronomical Society 448 (2015), pp. 27-36 (preprint).


Comments on this entry are closed.

  • Dani Eder December 28, 2015, 14:33

    A couple of comments:

    (1) The distinction between asteroids and comets is an accident of history. Objects that formed beyond the “frost line”, where water and other light compounds could condense, retained them as water and other ices. When they come too close to the Sun, these ices evaporate, producing “hairy” tails from which the word comet is derived. However, if an asteroid made of rock gets close enough to the Sun, it too will evaporate. We just could not see that happen before modern instruments. Earth just happens to be in a good position for the ancients to see water type comets, because we live where water is liquid, and therefore also evaporates from icy bodies.

    (2) Apparent brightness of objects farther from the Sun decreases as the fourth power of distance. First, sunlight dims as the inverse square of distance, then the apparent area of the object, which can return photons to us, also decreases as the square of distance. So there is a strong bias to finding large objects when they are far away.

    The nearest Centaur class objects, those whose perihelions are around 5 AU, are as small as 2-3 km in diameter. When we get to the Scattered Disk, whose orbit come no closer than Neptune, they have to be at least 19-25 km to see.

  • Alex Tolley December 28, 2015, 15:07

    Interesting that breakup of these comets might cause multiple impacts and increase the likelihood of impact and total damage to the biosphere. Just as well that the geologic record shows so few major impactors, although the Tunguska impact which may have been an asteroid or comet would be a disaster today if it hit a populated area.

    Formation of methane in comet impacts: implications for Earth, Mars, and Titan suggests that CH4 and CO2 could form with a comet impact which would have potentially large GHG implications. One might expect short pulses of anomalously high temperatures with associated biodiversity loss in the geologic record if this is indeed the case.

  • Harry R Ray December 28, 2015, 15:31

    ASSUME THE FOLLOWING: ONE: The once every 40 to 100 years scenario is IDENTICAL for EVERY middle aged star. TWO: EVERY(instead of JUST “most likely”)occorrence results in fragmentation. Could someone with the math skills(i.e. Jason Wright et al) now be able to determine the odds AGAINST the Kepler Space Telescope detecting EVEN ONE OF THESE EVENTS in its four year mission time? CAVAET: With SPECIFIC regards to KIC8462852, Stein Sigurdson stated that it would require an object 200 kilometers in diameter to cause what happened at KIC8462852. Centaurs like Chiron and Chereklo fit the bill, but certainly NOT the MAJORITY of the known ones. In astronomy, USUALLY the LARGEST of ANY NEW CLASS of objects are discovered FIRST. It is INCREDIBLY SERINDIPITOUS that this paper came out IMMEDIATELY AFTER KIC8462852 piqued our collective curiosity!

  • RobFlores December 28, 2015, 15:34

    At 50Km an Ice ball, even if gets heated up, wont lose so much mass.
    on the way in. And I don’t think we have an arsenal and delivery system powerful enough to do much to it. Good thing they are being monitored
    that should give a few years warning before one of these centaurs puts
    the 100 mark on the side of the Earth.
    How much of a tell tale do these leave. Over a 100 Mya time period
    several hits of many sizes might cause climate upsets, pushing species that would only be moderately at risk of extinction into oblivion. Is there substantial Irridium (the classic telltale that asteroids leave after an Earth Impact) on these icy bodies.

  • Andrew Palfreyman December 28, 2015, 18:39

    A 50 Km iceball undergoing continuous 1 TW laser reflection orthogonal to its path would undergo only about 60 Km deflection over a time period of 100 years, if I’ve calculated correctly. We would need rather more than that if it was auguring straight in.

  • Jim Franklin December 28, 2015, 18:55

    Yet more scaremongering. The odds of one of the larger Centaurs being deflected into the inner solar system in the next few million years is very slight. The Earth has not been impacted by anything larger than 15km in the last 500 million years and claims that these objects will break up if they are redirected our way are spurious and ill-informed.

    We are assuming they are “comets”, but at the physical size of many Centaurs found to date it is safe to say they contain vast quantities of silicates and metals as well as possible volatiles. The amount of break up for these object may be minimal to zero or it could be catastrophic – the fact is we know too little about them to make any rational judgments.

    Finding them in important, it assist in our understanding go planetary system formation and evolution, but we should not loose sight of the fact that the Apollo, Aten and Amor families pose the biggest threat and we should be spending more resources of the study of these populations and how they evolve and may pose a threat to humanity.

  • Daniel Högberg December 30, 2015, 5:28

    I cant believe that this is not more seriously debated in the world.. One hit and it could all be over and everything we have accomplished in art, science and society would have been in vain. We live in a false security that one of these potential impactors will be detected in good time, but does that really matter in the end? As we spend less and less on the exploration of space, how will we ever be able to have the extreme space-tech required to deflect one of these “world-killers” should the day come?

  • Mike Fidler December 30, 2015, 10:08

    Over 21 years ago Comet Shoemaker–Levy 9 broke into pieces and collided with Jupiter. It was a Centaur – how quickly we forget.

  • Michael December 30, 2015, 12:41

    @Daniel Högberg December 30, 2015 at 5:28

    ‘I cant believe that this is not more seriously debated in the world.. One hit and it could all be over and everything we have accomplished in art, science and society would have been in vain…’

    Daniel that may be so but on a positive note there would be no more Justin Bieber!

  • Hop David December 30, 2015, 15:27

    I had heard of Centaurs. But I believe Centauri Dreams is where I first heard their orbits can easily be perturbed by crossing the path of a gas giant. Since then I’ve read the same observation in J.S. Lewis’ Asteroid Mining 101.

    A large Centaur thrown earthward is a real possibility. It’s pretty scary.

    Lewis had also talked about the fragility of volatile rich meteors, that they would be more likely to blow up in the upper atmosphere. But I hadn’t thought of the possibility of large icey meteors wreaking extinction level events without leaving craters. That is also a real possibility.

  • Mark Zambelli December 31, 2015, 8:38

    Centaurs will get their turn for close scrutiny but it’ll just take a bit of time. We’re a fledgling race with only sixty years of experience beyond the atmosphere yet we began at such a pace we’ve already learned such a wealth of things that our system has been transformed. Rosetta is studying a comet from way out there… with those findings in hand it’ll be easier to make sense of other objects in the outer suburbs. With possible mission plans for the next sixty years just think of what further treasures we’ll uncover; there’ll be more data from the bodies we have already visited and are returning to for intense, specific study, ontop off all the new bodies we are visiting for the first time and I would imagine by then we’ll have data on a small number of Centaurs. These missions will add to the other huge datasets we’ll also be aquiring through the next sixty years of ground/space-based telescopes across multiple wavelengths. Highlighting, or even recognizing, potential threats at this early stage can only do good in the longterm and will open them up for study with whatever means we have at our disposal.

    As an aside, I’ve always found the mythology of the Centaurs fascinating. I was born at the beginning of December so the pagan nonsense of astrology has allocated me the Centaur as its sign (no I don’t believe in horoscopes but then I’m a typical Sagittarian! ;) ). I love the artwork depicting Centaurs, either archers or teachers, and I find a poetic resonance between the myth and Paul’s main article… namely the Centaur, as a learned healer was the bridge between earth and the heavens. How fitting.

  • Mike Fidler January 1, 2016, 3:58

    Why not look for new Centaurs with the Cassini spacecraft at Saturn – It should have plenty of time to take long exposures while it’s further away from Saturn and the data should be able to be highly compressed!

  • Michael January 1, 2016, 14:04

    @Mike Fidler January 1, 2016 at 3:58

    ‘Why not look for new Centaurs with the Cassini spacecraft at Saturn – It should have plenty of time to take long exposures while it’s further away from Saturn and the data should be able to be highly compressed!’

    Not sure the optic system is designed in a way that would make it worth while.

    Anyway they plan to potentially ‘exterminate’ the only living organisms on-board their ‘starship’ and only home!

    Magnum actum iniustum

    RANT over!

  • Mattia January 3, 2016, 13:20

    We are working on this, but the paper was blocked a bit, before this one…..now I have to clarify, why there are many similarities….

  • Mattia January 3, 2016, 14:07

    Dynamical studies of Centaurs and their sources: interactions with the Main Belt
    Galiazzo, Mattia; Carruba, Valerio; Wiegert, Paul
    AA(The university of Western Ontario), AB(UNESP), AC(The university of Western Ontario)
    IAU General Assembly, Meeting #29, id.#2252424
    Publication Date:
    Abstract Copyright:
    (c) 2015: American Astronomical Society
    Bibliographic Code:
    Centaurs are objects whose orbits are found between those of the giant planets. Their main source is presumed to be the Trans-Neptunian objects (TNOs), and they are among the sources of Near-Earth Objects. Their dynamical evolution is heavily influenced by close encounters with giant planets and some of them were most likely scattered into their current orbits. After experiencing close encounters with the giant planets, Centaurs may have their eccentricity increased to values large enough to reach the main belt (a<3.8 au) and even the region of the Near-Earth asteroids, with a perihelion, (q<1.3 au). Some may become short-period comets and a fraction falls into the Sun or become sun-grazers. In this work we propose to investigate the interactions of dynamically evolving Centaurs and other members of the outer Solar System (i.e. Plutinos and Trans-Neptunian objects which cross the orbit of Neptune) with main belt asteroids to determine if chaotic scattering caused by close encounters and impacts may have played (and/or still play) a role in the dynamical evolution of the main belt. Preliminary results show already that TNOs (larger than 100 km size) are capable of reaching the inner solar system and may cross the main belt several times during an interval of time at least of ~10 kyrs. Centaurs such as 1995 S1 can generate significant deflections to the orbits of main belt asteroids after close encounters. In particular, we plan to investigate if close encounters with large (diameters larger than 100 km) outer Solar System bodies could have been responsible for the scattering of V-type asteroids from the Vesta family beyond the 3J:1 mean motion resonance, into the central and outer main belt.

  • Mike Fidler January 15, 2016, 9:56

    Centaurs as a hazard to civilization.
    Bill Napier, David Asher, Mark Bailey and Duncan Steel
    Very well written article!