Cosmic catastrophe - 2

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Asteroid impacts

A collision with an asteroid could be even more devastating than a comet collision.
Comets are icy snowballs with low tensile strength - they fragment easily, and would dissipate some energy through ablation within the atmosphere (but airbursts can also be very destructive).
Asteroids are made of rock and/or iron - much higher strength material, so a larger fraction survives to impact.

The orbit of asteroid 1997XF11 for 2028

Most asteroids are confined within the `asteroid belt' - between the orbits of Mars and Jupiter. A relatively small number, however, follow orbits which take them through the inner Solar System. At present, ~160 are known: there may be as many as 2000 with diameters exceeding 1 km. An impact with such a body would have global implications.

Meteor Crater, Arizona: 1200 m x 70 m : 20 Megatons, D~10 metres?

The Caloris impact crater on Mercury

1. Large impact crater plus fragmentation due to seismic shocks
2. localised winds exceed 500 km/hr
3. Injection of dust into stratosphere for months
   Sunlight fails to reach ground, crops fail, lower temperature
4. large-scale fires, atmospheric smoke
   increased greenhouse effect, higher temperatures after dust dissipates
5. increased vulcanism, probably for centuries
6. acid rains, ozone layer destruction - many years, probably centuries, to recover

Death of the dinosaurs

This is almost certainly the fate meted out to the dinosaurs - the Cretaceous/Tertiary impact (iridium layer) suggests a 10-km scale impact. See this site and the New Scientist dino page. We may even have found a piece of the killer asteroid.
expected frequency - once per 100 million years

The dinosaur killer is associated with the Chicxulub crater, a 170-km diameter crater lying off the Yucatan peninsula. Nuemrous other terrestial impact craters are known: see this link.

The Chicxlub impact crater

Nearer to home, Clearwater Lakes, in Quebec, form a pair of impact craters, 32 km and 22 km in diameter respectively, aged ~290 million years

More terrestial impact craters pictured here.

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