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. 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 impacts are not observed. Weather and vegetation rapidly erase the evidence of smaller impact craters and degrade or bury the meteorites so Greenland, Antarctica, Australia and other deserts are exceptionally good places to find them.
Nevertheless, there are many recognized impacts and we list some of them below ranging from events in just the past few years, to large explosions that occurred in the past century, to huge explosions while humans were present on Earth, to global catastrophes eons ago.
2009 - 5-10m (15-35 ft) diameter impactor
Remains of an asteroid explosion over Indonesia.
A large explosion occurred over Indonesia
on 8 October 2009. The explosion was estimated to have released an energy equivalent to 20-50 ktons of TNT and the impacting asteroid was probably 5-10m (15-35 ft) in diameter. It was not
seen ahead of time because the moon was nearly full and 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.
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2008 - 2-3m (7-10 ft) diameter impactor
Trail of asteroid 2008 TC3 over Sudan.
For the first time ever, on 7 October 2008, an asteroid was discovered before
it hit the Earth. This asteroid, called 2008 TC3, was discovered by Richard Kowalski of the Catalina Sky Survey and 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 to within 20sec. Their final prediction was accurate to 1km and 1.5sec. A story in the New Scientist describes the expedition that recovered fragments of the meteorite.
The fact that it occurred almost exactly one year before the Indonesian event described above means that the Earth was passing through the same position in space as when it was struck a year earlier. The timing coincidence may herald the beginning of a new annual meteor shower on 8 October. In the hope of discovering more asteroids that might strike the Earth on the same day some astronomers have tried looking back along the approach direction of 2008 TC3 but they didn't detect anything. ATLAS would do better at keeping an eye on that spot in the sky.
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2007 - 3m (10 ft) diameter impactor
Crater that formed in the southern Peruvian town of Carangas on September 16, 2007.
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
that provided the image to the left (note the tiny people on the crater 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 (see below).
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1947 - 2.5m (8 ft) diameter impactor
Stamp commemorating the 10th anniversary of the Sikhote-Alin meteorite shower.
On 12 Feb 1947 a large meteorite fall
occured near the Sikhote-Alin Mountains near Vladivostok, Russia. It is thought to 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 to the left 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 that are mostly metal-iron are much less common than the stony C-
asteroids their ability to cause damage is greater at any given size because they are stronger and more likely to deliver damaging material to the ground.
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1908 - 50m (160 ft) diameter impactor
The Tunguska asteroid impact in 1908 knocked down and burned trees over an area the size of a large city.
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 left. 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 Cosmic Materials Space Research have used eyewitness accounts to try to reproduce what the Tunguska explosion sounded like.
Tunguska blast area.
The discrepancy between the devastation caused by the Tunguska explosion and the lack of recovered material or a crater 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 that explodes high above the Earth's surface can produce more damage on the surface than a point explosion of equivalent energy on the ground. Asteroids smaller than about ~100m (330 ft) diameter are not expected to make it to the surface intact yet they can still cause great damage.
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50 thousand years ago - 25-50m (80-160 ft) diameter impactor
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). Planetary scientists 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.
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35 million years ago - 3-5 kilometer (1.9-3.1 mile) diameter impactor
Chesapeake Crater profile view.
A much larger impact took place about 35 million years ago that 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.
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65 million years ago - 10 kilometer (6.2 mile) diameter impactor
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, when most animal and plant species on the planet suddenly disappeared, was the Tertiary period.
This K-T fossil boundary (why isn't it 'CT' you ask? It's because the word Cretaceous starts with a 'k' in German) shows a layer of the element iridium which is uncommon on the Earth's surface but much more common in asteroids. The layer also includes 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.
Chicxulub crater gravity map.
Most people believe that the cause of this change was the impact of a large asteroid at Chicxulub
, off of the coast of what is now known as 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 left 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.