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Historical Impacts

Asteroid impacts that are large enough to notice are rare. Until the 18th century European academics simply did not believe that stones could fall from the sky and skepticism persisted even into the 19th century. We now understand that impacts occur all the time in different sizes and places. Most impactors are very small, gravel or baseball sized rocks that burn up as meteors leaving only their burnt dust to float down to the surface and increase the mass of the Earth by tens of thousands of tons every year. Since most of the Earth's surface is ocean or far enough from human habitation most ground impacts are not observed. Weather and vegetation rapidly cover up smaller impact craters so that Greenland, Antartica, Australia and other deserts are exceptionally good places to find meteorites.

Nevertheless, there are many recognized impacts that have occurred over the years and we list some of them below ranging from global catastrophes eons ago, to huge explosions that have occurred during human presence on the Earth, to large explosions that have occurred in the past century, to impacts that have occurred in the past few years.

Chicxulub

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Fossil evidence shows an amazing, abrupt change in life on Earth that occurred about 65 million years ago. Prior to that moment was the Cretaceous period when the dinosaurs were common, as well as many other species that no longer exist today. After that moment was the Tertiary period, when thousands of animal and plant species suddenly disappeared.

This K-T fossil boundary also shows a layer of the element iridium which is uncommon on the Earth's surface but much more common in asteroids, as well as burned material that suggests vast firestorms. The amount of burnt materioal in this layer is consistent with everything on the Earth's surface being burned! There are also tsunami (tidal wave) deposits around the Caribbean dating from that time.

Most people believe that the cause of this change was the impact of a large asteroid at Chicxulub, off of the coast of Yucatan, Mexico. There is the unmistakeable site of an ancient asteroid impact, a 180km (100 miles) circular crater filled with minerals that show the effects of high temperature and pressure, shock, and shattering. The gravity map on the right shows concentric rings due to the impact and the white dots label places where sinkholes are found, suggesting creation by subsidence of the crater wall.

Chesapeake Crater

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A slightly smaller event about 35 million years ago created the Chesapeake Bay and inundated the East Coast as far inland as the Blue Ridge Mountains. The impact crater is 40km (25 miles) across, and the damage done to the geological layers affects the aquifers in the area to this day.

The Popigai Crater in Siberia was probably created by a 5-8km (5 miles) diameter impactor and may have happened at the same time as Chesapeake Bay. The shock and heating from the impact transformed graphite into diamonds throughout the impact area. The Wikipedia article also has a nice set of pointers to other topics on asteroid impact cratering on Earth.

Meteor Crater

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The Meteor Crater in Arizona was created by the impact of an asteroid about 50,000 years ago. This iron asteroid is thought to have been about 25-50m (80-160 ft) in size while the crater is about 1.2km (0.8 mile) in diameter and 200m (650 ft) deep! The explosion was equivalent in energy to a few Mton of TNT, similar to the explosion in 1908 over Tunguska (see below). We think that such impacts occur every thousand years, and there may have been dozens of similar ones since the creation of the Meteor Crater. What makes the Barringer Meteor Crater exceptional is its preservation due to its desert location. Since the impact of this object 50,000 years ago it is likely that some other impact took place that was ten times bigger - but no evidence remains. The Barringer Meteor Crater today is a fascinating tourist attraction and well worth a visit.

Tunguska

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On 30 June 1908 a huge explosion occurred in Siberia over Tunguska. Nineteen years later in 1927 Leonid Kulik mounted an expedition to the site and found dramatic devastation. Trees were scorched and denuded of branches over a "ground zero" region 8km (5 miles) in diameter, and trees were blown down pointing away from this epicenter over a vast region of 70x55km (45x35 miles) as illustrated on the right. It is thought that this was a stony asteroid 50m (160 ft) in size and the explosion released about 10 Mton of energy. People were knocked off their feet at distances of 60km (35 miles) from the impact.

Scientists at the Cosmic Materials Space Research have used eyewitness accounts to try to reproduce what the Tunguska explosion sounded like.

The discrepancy between the apparent magnitude of the Tunguska explosion, the lack of recovered material, and the fact that ground zero was apparently not incinerated is probably because the asteroid was completely consumed in the atmosphere, but still created a huge shock wave that punched down to the ground. Detailed simulations by Boslough and Crawford support this conclusion; their work is summarized in an article by the Planetary Society. The result is that an asteroid impact can produce a greater blast on the surface than a point explosion of equivalent energy, and an asteroid smaller than the threshold size of ~100m (330 ft) required for a fireball to reach the ground can still cause great damage.

Sikhote-Alin

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On 12 Feb 1947 large meteorite fall occured near the Sikhote-Alin Mountains near Vladivostok, Russia. It is thought to have have been an iron meteorite that weighed about 100 tons (i.e. about 2.5m or 8 ft in diameter) that scattered many fragments and craters over a wide region. The illustration shows a stamp to commemorate the 10th anniversary of the Sikhote-Alin meteorite shower which reproduces a painting by eye witness P. J. Medvedev. Although this asteroid was far smaller than the usual impactor that punches through the atmosphere and therefore carried much less energy, its iron composition made it unusually effective for delivering its explosive power to the ground. Thus, although M-type asteroids are much rarer than C- or S-type at a given size, their ability to cause damage is greater.

Peru

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On 15 Sep 2007 an asteroid struck the Earth in Carancas, Peru, and left a 13m (40 ft) wide, 4.5m (15 ft) deep crater. The explosion shattered windows a kilometer away (about half a mile). Details are found in the Wikipedia article as well as a National Geographic article where an image of the crater can be seen (note the tiny people on the rim). An article in Astronomy Now describes the analysis carried out by Peter Schultz who says that the asteroid was a chondrite about 3m (10 ft) in size that probably burned up in the atmosphere and did not survive all the way to the ground. However, the hypersonic approach velocity creates a self-sustaining shock, like a shaped explosive charge, that kept the material and energy directed towards the eventual impact site. This may be a small version of the Tunguska explosion ninety years earlier.

Sudan

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On 7 October 2008, for the first time ever, an asteroid was discovered before it hit the Earth. This asteroid, called 2008 TC3, was discovered by Richard Kowalski of the Catalina Sky Survey, the full story is at the JPL NEO website. The asteroid is estimated to have been about 2-3m (7-10 ft) in diameter and the impact released about 1-2 kton of energy over a barren area in Sudan.

It's an amazing story because of the hard work and quick wit of Kowalski and the Catalina Sky Survey, the way the Minor Planet Center coordinated hundreds of observations by amateurs and professionals over the next 20 hours, and the astounding accuracy with which the folks at JPL were able to predict the eventual impact. Within a few hours they knew where it would land within 100km and when it would land within 20sec, and their eventual prediction was accurate to 1km and 1.5sec. A story in the New Scientist describes the expedition that recovered fragments of the meteorite.

Indonesia

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Almost exactly a year after the impact in Sudan, on 8 October 2009, a large explosion occurred over Indonesia. This explosion was estimated at 20-50kton, much larger than the one over Sudan, and the impacting asteroid is thought to have been 5-10m (15-35 ft) in size. This asteroid was not seen ahead of time, both because the moon was nearly full but also because there is no survey that scans all the sky, all the time. No fragments were recovered and there were no reports of blast damage, possibly because the explosion was over the ocean. The fact that it occurred one year later means that the Earth was passing through the same place in space as when it was struck in Sudan, and it may herald the beginning of a new, yearly meteor shower on 8 October. Astronomers looked back along the approach direction of 2008 TC3 on 7-8 October 2010, but didn't detect anything large that passed by. ATLAS would do better at keeping an eye on that spot in the sky.