GIANT comets a 'serious threat' to life on Earth ?

The discovery of hundreds of giant comets in the outer planetary system  over the last two decades means that these objects pose a much greater hazard to life on Earth than asteroids.

The giant comets, termed centaurs, move on unstable orbits crossing the paths of the massive outer planets Jupiter, Saturn, Uranus and Neptune.

The planetary gravitational fields can occasionally deflect these objects towards Earth. Centaurs are typically 50 to 100 km across, or larger, and a single such body contains more mass than the entire population of Earth-crossing asteroids found to date.

Calculations of the rate at which Centaurs enter the solar system indicate that one will be deflected onto a path crossing Earth's orbit about once every 40000 to 100000 years.

Whilst in near-Earth space they are expected to disintegrate into dust and larger fragments, flooding the inner solar system with cometary debris and making impacts on our planet inevitable. Known severe upsets of the terrestrial environment and interruptions i the progress of ancient civilization, together with our knowledge of interplanetary matter in near-Earth space, indicate the arrival of a centaur around 30000 years ago. This giant comet would have strewn the inner planetary system with debris ranging in size from dust all the way upto lumps several kilometers across.

 Specific episodes of environmental upheaval around 10800 BC and 2300 BC identified by geologists and paleontologists are also consistent with this new understanding of cometary populations. Some of the greatest mass extinctions in the distant past, for example the death of the dinosaurs 65 million years ago, may similarly be associated with this giant comet hypothesis.