On March 23rd, 1989 a roughly 300 meter wide asteroid named '4581 Asclepius' (1989 FC) missed the Earth by only 700,000 kilometers. This might seem like a lot but in practice, this means that the giant rock was passing through the exact position where the Earth was only 6 hours before. If the asteroid had impacted it would have created the largest explosion in recorded history - thousands of times more powerful than the Tsar Bomba (50 megatons), the most powerful nuclear bomb ever exploded by man. Early calculations estimated its passage being as close as 64,000 kilometers from the Earth. Due to large uncertainties in these calculations, the possibility of it striking the Earth was no foolish scenarios. Therefore this event attracted widespread attention in the media and amongst political leaders.
Recent hits recorded:
1908: Tunguska event
It is now commonly believed that on June 30th, 1908 a stony asteroid exploded over Tunguska with the energy of the explosion of 10 megatons of TNT. The explosion occurred at a height of 8.5 kilometers. The object that caused the explosion has been estimated to have had a diameter of 45–70 meters.
1979: Vela Incident
September 22nd, 1979: an event recorded as occurring near the junction of the South Atlantic and the Indian Ocean was possibly a low-yield nuclear test, but was also initially thought to have been caused by the possible impact of an extraterrestrial object. The event, which became known as the 'Vela Incident', was identified by a U.S. Vela defence satellite in Earth orbit. The event alarm triggered multi-year investigations by several organizations which could not conclusively determine if the explosion was of nuclear or non-nuclear origin.
2002: Eastern Mediterranean event
On June 6th, 2002 an object with an estimated diameter of 10 meters collided with Earth. The collision occurred over the Mediterranean Sea, between Greece and Libya, at approximately 34°N 21°E and the object exploded in mid-air. The energy released was estimated (from infrasound measurements) to be equivalent to 26 kilotons of TNT, comparable to a small nuclear weapon.
2008: Sudan event
On October 6th, 2008, scientists calculated that the small Near-Earth asteroid '2008 TC3' just sighted that night should impact the Earth the day after over Sudan, at 0246 UTC, 5:46 local time. The asteroid arrived as predicted. This is the first time that an asteroid impact on Earth has been accurately predicted. However, no reports of the actual impact have so far been published since it occurred in a very sparsely populated area. A systematic search for fragments found a total of 600 fragments, with a mass of 10.5 kilograms. The object is confirmed to have entered Earth's atmosphere above northern Sudan at a velocity of ... 12.8 kilometres per second (+46000 km/h) ... That's were most of the energy of these objects comes from when they impact another celestial body.
On August 10th, 1972 a 3-10 meter small meteor that became known as 'The Great Daylight 1972 Fireball' was witnessed by many people moving north over the Rocky Mountains from the U.S. Southwest to Canada. It was an Earth-grazing meteoroid that passed within 57 kilometres of the Earth's surface. It was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera (see video above).
On March 18th, 2004, a 30 meter asteroid, '2004 FH', is announced to pass the Earth that day at only 42,600 km (about only 1/10 the distance to the moon, and the closest miss ever noticed). Estimation determined that similar sized objects come as close about every two years...nice!!
On March 31st, 2004, two weeks after 2004 FH, meteoroid '2004 FU162' set a new record for the closest recorded approach, passing Earth only 6,500 km away (about only 1/60 of the distance to the Moon). Because it was very small (6 meters), FU162 was detected only hours before its closest approach. If it had collided with Earth, it probably would have harmlessly disintegrated in the atmosphere. That's what they say...
On March 2nd, 2009, near-Earth asteroid 2009 DD45 flew by Earth at about 13:40 UT. The estimated distance from Earth was 72,000 km, approximately twice the height of a geostationary communications satellite. The estimated size of the space rock was about 35 meters wide...a nice chunk of rock already...
On January 13th, 2010 at 12:46 UT, near-Earth asteroid 2010 AL30 passed at about 122,000 km. It was approximately 10–15 m wide. If 2010 AL30 had entered the Earth's atmosphere, it would have created an air burst equivalent to between 50 and 100 kilotons. The Hiroshima 'Little Boy' atom bomb had a yield between 13-18 kilotons.
Although there have been a few false alarms, a number of objects have been known to be threats to the Earth. 2002 NT7 was the first asteroid with a positive rating on the Palermo Technical Impact Hazard Scale, with an approximate value of 1/1000000 and a potential impact date on January 31st, 2019.
Asteroid 1950 DA was lost after its discovery in 1950 since not enough observations were made to allow the plotting of its orbit, and then rediscovered on December 31st, 2000. The chance it will impact Earth on March 16th, 2880 during its close approach has been estimated as 1/300 (astronomically speaking this is super likely). This chance of impact for such a large object is roughly 50% greater than that for all other such objects combined between now and the year 2880. It has a diameter of about a kilometer...80-90% of all living things on Earth would probably not survive a collision with it...
What's more a relatively large number of asteroids like 2004 MN4, 2007 VK184 or 2008 AF4 have had above-normal rankings on the Torino Scale.
Keep watching the skies...