#SOHO has found 4000 comets!

With #AsteroidDay coming up, we thought we'd dig into the potentially risky side of these cosmic snowballs

Have there been comet impacts with Earth?
Do we know of craters formed by comets?
Are comets more/less dangerous than asteroids?

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No discovered comet has ever impacted Earth - but that doesn't mean cometary material hasn't fallen in the past.

The Tunguska event of 1908 saw a large explosion over the Eastern Siberian Taiga, which flattened millions of trees over roughly 2000 square kilometres

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The Tunguska impact released a lot of energy but left no material on the ground. One of the leading theories for the absence of solid mass was that it was caused by a comet, mostly made of ice, which did not survive long on the ground

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Comet Swan, Gerald Rhemann

However, we now know that even a rocky object can fragment completely during the 'airburst'. The Chelyabinsk event left one small 500kg fragment - had that hit the swamps of Siberia it could have sunk in and disappeared

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Chelyabinsk meteor, Alex Alishevskikh

Some meteorites of very primitive composition, such as the Tagish Lake meteorite that fell in Canada in 2000, are thought to closely resemble cometary matter, and are therefore likely fragments of comets.

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Tagish Lake meteorite, Mike Zolensky

Since we don't have any actual comet sample in a lab, it's hard to know for sure. Of course, most meteors are generated by tiny fragments of comets, so in a certain sense they do impact us every year.

Perseid meteor shower, associated with the comet Swift–Tuttle

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Most of the threat from an asteroid or comet impact comes from the atmospheric "explosion" - the airbust - which occurs before any material touches the ground and creates a shock wave that is responsible for most of the damage.

Chelyabinsk impact, Aleksandr Ivanov
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Comets are less dense than asteroids, therefore for a given diameter they are also less massive and carry less energy. However, most comets are in higher-eccentricty and inclination orbits, resulting in much higher relative velocities with respect to Earth

Orbit of Comet 67P

Because of this, comets will have much more kinetic energy than asteroids, more than offsetting their smaller mass due to a lower density, (thanks to the square power of the velocity)

Therefore, an average comet of a given diameter will likely cause more damage than an average asteroid of the same size

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Halley's Comet, 1910

Currently, we know of ~ 100 near-Earth comets (going up by one or two entries per year). Fortunately, the chance of an impact from an 'NEC' is significantly lower than for near-Earth asteroids, of which we currently know there are 22 933!

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1998 OR2, @NAICobservatory