Imapct Event
© Live Science
A total of five mass extinctions occurred during the last 500 million history of the planet earth, when more than 75 per cent of existing life forms had gone extinct. Various causes have been ascribed for each of the five mass extinctions.

The most recent, 5th mass extinction took place about 66 million years ago, when a huge meteorite with a radius of about 10 km crashed on the earth and made a crater with 180 km diameter and 20 km deep in the Yucatan village, Chicxulubin Mexico. The impact created a doomsday scenario.

Superheated dust particles and steam filled the sky preventing sunlight to reach earth for decades killing plant life as they could not carry out photosynthesis. Many volcanoes around the earth became active and spewed lava. There were super-tsunamis drowning land animals and plants. The atmosphere of earth became unsuitable to support most of the existing life forms, leading to the extinction of dinosaurs.

Can the 6th mass extinction occur in a similar manner and all of us would disappear before we figure out what hit us? Michael Rampino and Bruce Haggerty (1984) proposed the 'Shiva hypothesis' (named after Lord Shiva, the Hindu God of destruction) based on an earlier paper of Napier and Clube (1979). According to this hypothesis, the earth experiences large impact events with comets every 30 million years when solar system crosses the plane of Milky Way galaxy. This causes gravitational disturbances in the cloud of comets (Oort cloud) surrounding the solar system and sends some of them hurling towards the inner solar system. These may be propelled to collide with the earth by the 'sling shot' gravitational action of Jupiter (NASA's Voyager-1 and 2 used this effect to escape Sun's gravity).

Modelling studies suggest that an object of 1 km size, depending upon its speed and angle of approach is enough to wipe out the humanity according to Rampino, who gave the 'Shiva hypothesis.' This could throw up enough pulverised rock to block the sun for months. The debris falling back on earth would cause wild fires increasing the surface temperature and killing everything living to death. The earth will then go through a process of cooling and a prolonged winter possibly creating new forms of life millions of years later.

US-Congress in 1998 authorised NASA (National Aeronautics and Space Administration) to search for Near Earth Objects (NEO), known as 'Space Guard Survey' to identify 90 per cent of NEO's with diameter of 1 km or more in the next 10 years. The agency achieved this goal by 2010 and submitted its report (Defending the Planet Earth; Near Earth objects, Surveys, Hazards and Mitigation Strategies, "Summary" National Research Council. 2010. Washington DC, The National Academic Press, doi: 10.17226/12842).

Astronomers spotted about 15,000 objects in the earth's neighbourhood, including hundreds of them more than 1 km across. None of them were currently in collision course with the earth.

The second mandate given to NASA was to discover 90 per cent of NEO's with diameter of 140 meters or more by 2020. Since recent studies on impact of NEO's with diameter of 30-50 meters have been found to be highly destructive NASA was asked to detect as many of them as possible without compromising the earlier mandates.

The best-known example of such an impact was of a 30-50 meter, meteorite, which exploded near Tunguska river in the Siberian wilderness of Russia in 1905. It devastated more than 2000 square kilometres of forest, but possibly with no human casualties in this sparsely populated region.

In India, the Lonar Lake located in the Buldhana district of Maharashtra, diameter 1.8 km and depth 137m, was formed, when a meteorite or comet coming at an angle of 35-40 degrees (Arif and coresearchers, 2011) and at a speed of 90,000 km/hr crashed on the site about 52,000 years ago (or earlier at about 570,000 years ago).

The size of the meteorite, which dug up the lake is not known but probably less than 100 meters (if compared with the crater created by Chicxulub meteor).

Mitigation: 'Mitigation' refers to all means of defending the earth and its inhabitants from the possible impact of a NEO. The NEOs need to be characterised with respect to their orbit and physical properties to plan defence from their possible impact on earth. Technically NEO's may be considered as an asteroid or comet, whose orbit approaches earth's orbit to within about one third average distance of earth from the sun. These objects are potentially capable of colliding with earth in the next century.

The survey of NEOs by NASA needs to be completed by 2020-2030, using space missions, ground- based telescopes, also the complementary radar system in Arecibo Observatory in Puerto Rico and the Goldstone Solar System radar in California to accurately determine the orbit, size, shape, surface structure and other properties within their latitude coverage and detection range.

Primarily four types of defences are proposed to be adopted.

1. Civil defence: This option is possible if the warning time is more than 1-2 years.

2. Slow push or slow pull methods: The orbit of the target object is to be changed so that it avoids collision with earth. This option will take decades to be effective and useful only for objects less than 100 meters in diameter.

3. Kinetic impactors: One or more space crafts with massive payloads will be sent to impact directly on the target either in the direction of motion or opposite to it. The effectiveness of this method will depend on the mass of the object and consequences of such impact.

This method is proposed to be tested by a joint mission called AIDA (Asteroid impact and deflection assessment) by European Space Agency in collaboration with NASA on asteroid,Didymos, when it passes earth in October 2022.

Didymos is a binary asteroid consisting of an 800-meter diameter main body and a 170-meter satellite revolving around it. NASA will send a space craft between December 2021 and May 2022 to deliberately crash it on the smaller body trying to dislodge it from its orbit. The accompanying European device HERA will record the effect of such impact. Over ground telescopes and other devices from USA will record the percentage of orbital deviation.

The success of this experiment will validate if orbit of an asteroid moving on a collision course towards earth can be altered by a kinetic impactor.

4. Gravity Tractor: Still in conceptual stage, it consists of robotic probe, which flies with the space rock for months or years gradually nudging it off course by a slight gravitational tug. Gravity could be increased if the mass of robotic probe is higher. To avoid high cost of launching such probes from earth, NASA proposes to hijack one of the asteroidsand use it as a gravity tractor, said Lindley Johnson, program executive of NASA's NEO's observation program (March, 2014).NASA has identified about a dozen of candidates for asteroid capture mission. The space agency wants astronauts to visit redirected asteroids by 2025 as fixed by White House.

5. Nuclear explosion: This would be the last option but is the most powerful technique. This could be used for objects with a diameter of a few kilometres. The idea is to jolt the body and not blow it up, since that would cause more damage. While UN Outer Space Treaty (1967) bars sending nuclear weapons into space, scientists already have a good understanding of the technology. Planetary Defence Coordination office of NASA will decide when US will use such an option to prevent a potential impact of an asteroid with earth (Rosen, 2016).

(Dr Mitra is a former Dean and Professor, OUAT. The article is based on several publications on the subject including the NRC report 2010. Email: