ASTEROIDS AND THE THREAT OF THEM HITTING EARTH

I am fond of Astronomy. Once a friend sent me a video which made me feel disturbed because it showed an asteroid hitting the earth whose impact had the capacity to bring in a Catastrophe on the earth and exterminate all life. I then thought that I should investigate a little more about the probability of earth taking such a hit.

 Asteroids with a diameter of over 50 meters strike the earth every 1000 years. One such strike took place in 1908 at Tunguska in Siberia. This is the largest impact event over land in Earth's recent history. Luckily the impact occurred in a forest and trees in the surrounding area with a diameter of 8 KM got affected. The general agreement among scientists is that this meteor is a few tens of meters across.

Asteroids with an average diameter of 1KM strike the earth every 500,000 years.

Bigger collisions with asteroids of 5 KM in diameter occur once every 10 million years. The smallest of these impacts would generate 20,000 times as much energy as the greatest bomb ever produced by us and leave a crater 95 km across. One such strike is supposed to have wiped out the Dinosaur population from the earth 65 million years ago.

 The largest meteor impact crater on the earth is the Barringer crater in the Arizona desert formed 50,000 years ago. This is 1200 meters across and 170 meters deep. This is believed to have been caused by a meteor weighing 150,000 MT at the time of striking the earth and an impact speed of 13 KM/hr.

 There are only 3 asteroids which are 500 KM and over across that can cause the tremendous devastation and wipe out all life from earth as shown in the video. They are 

 1) Ceres-Dia 952 KM,

2) 2Pallas- 544 KM,

3) 4Vesta-529 KM.

 Luckily for us none of them have any chance of colliding with the earth. Of the asteroids Vesta is the only one that attains brightness that is visible to the naked eye. The largest Comet known to us is the Bernardinelle-Bernstein which is 120 Km across.

 The TORINO SCALE is a scale for measuring the possibility of an asteroid or a comet hitting the earth. This is valuated between 0 and 10.

On this scale a collision is certain if the object reaches 8 on the scale. None of the objects evaluated by astronomers till date managed to cross 1 till the Asteroid 99942Apophis did so in 2004. In fact at one time Apophis was upgraded to 4 which means a 1% chance of a collision capable of regional devastation. With later observations however, it has been downgraded to 0.

 Apophis is 370 meters across with an estimated mass of 6 crore tons and it travels with a speed of over 30 Km/sec. Such an Asteroid hitting the earth can be devastating.

 Subsequently another asteroid 2004 VD 17 reached a level of 2, but subsequent observations made the scientist downgrade it to 0. All the asteroids discovered earlier have been downgraded to 0. Incidentally, a 0 level indicates almost nil chances of impact.

 Presently there are only 3 asteroids with a risk level of 1. Level 1 means a collision is highly unlikely.

 The asteroid 2007 VK184 discovered in 2007 has a probability of 1 in 3030 to hit the earth. It has a diameter of 130 meters and travels at a speed of 16 KM/Hr.

 The asteroids 2011 AG5 and 2011 BM45 discovered in Feb 2011 are rated 1.

 Considering the above data it is highly unlikely that in the near future the earth would be hit a by an asteroid of substantial size to cause lasting damage.

A NEO (Near Earth Objects) Observations program had been launched by NASA and the EU in 1998 and is tracking the NEO’s. An NEO is by definition is all those natural bodies that approach up to a distance of 1.3 AU (Astronomical Unit the distance from Sun to Earth 151 million KM).

If an NEO crosses the earth’s orbit and if it is larger than 140 meters, then it is considered a PHO (Potentially Hazardous Object). Most of the PHO’s known are asteroids but a few are Comets. 

There are over 30500 known NEO Asteroids, and over 100 NEO Comets. NASA is continuously tracking these bodies through its NEO Program. Of these 2300 are PHO’s.

The TUNGUSKA explosion mentioned above occurred near the Tunguska river in Russia northwest of lake Baikal on June 30 1908. Luckily, it was a sparsely populated area and there were only 3 reported deaths. However, it flattened 80 million trees over an area of 2150 sq mtrs.

This blast is attributed to a meteor air burst in the atmosphere of a stony meteoroid about 50-60 meters across. It approached earth at a speed of 27 Km/Sec. No crater was found as the object was supposed to have deteriorated at an altitude of 5 to 10 Km instead of directly hitting the earth. This is the largest impact event in the Earth’s recorded history.

An explosion of this size would have been capable of destroying a large metropolitan area.

In the more recent times on 15^th February 2013 a 20 meter asteroid entered the earth at a velocity of 19 Km/Sec near CHELYABINSK in Russia. It weighed 12000-13000 tons. The meteor exploded in the air at a height of 30 Km. The bulk of the shock of explosion was absorbed by the atmosphere creating a large shockwave which is equal to a 400-500 kiloton TNT blast, some 30 times the power of 30 Hiroshima bombs.               

This object approached the earth undetected before its atmospheric entry because its source direction was close to the sun. About 1500 people were injured as the shockwave hit 6 cities and damaged 7200 buildings.

Among these the Chelyabinsk explosion was the most recent and mankind is very lucky that both in this case as well as the Tunguska event the meteors exploded in the atmosphere. Otherwise a 12000 ton body hitting any area with a velocity of 19 Km/ Sec can cause a devastating impact.

These 2 events in recorded history clearly prove that such events are likely to happen again if not with larger asteroids, certainly so with smaller ones. We have to be particularly worried about Chelyabinsk like events because the Chelyabinsk meteor approached the earth undetected as it came from the direction of the Sun. Despite the NASA’s observations on NEO it could not be located  beforehand.