STONED EARTH

What are the chances of an asteroid crashing into the Earth and wiping out civilisation as we know it? Aswin Sekhar measures the prospect

  • Published 23.10.17
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What would happen if a body from outer space hit Earth is a question that has exercised scientists and the general public alike. Of course, movies and TV shows exaggerate the effects - the Earth isn't likely to get sliced into pieces because of an asteroid impact. But deep in our past, an asteroid hit did wipe out the dinosaurs.

The biggest such occurrence in recent history is, of course, the Tunguska impact event. On the morning of June 30, 1908, thousands of square kilometres of forest in icy Siberia were wiped out within hours. Scientists now say this was caused by a small body from space which entered the Earth's atmosphere and caused a blast that was approximately 1,000 times stronger than the atomic bomb dropped on Hiroshima. The impact had the potential to level a metropolis, though the celestial body involved was only between 60 to 200 metres long.(Longer if it was icy and light, shorter if it was dense rock.)

So we can only imagine what happened when a 15 kilometre long object from space - with its high impact velocity and hence high-impact energy - crashed in Chicxulub in Mexico. It not only formed a huge crater but also spelled doom for the dinosaurs. This has happened about 200 times in the long history of our planet - we have the craters to prove it.

"Visible on all planetary bodies, the scars of impact craters vividly illustrate that exchange among solid bodies is an active agent of the solar system and planetary surface evolution," says Stephanie Werner, a leading cratering scienceexpert and president of the Division on Planetary Science at the European Geosciences Union.

In the recent past - on the morning of February 15, 2013, to be exact - a super bolide or a large meteor impacted the city of Chelyabinsk in Siberia. Only 15 metres long - much smaller than the Tunguska one - it disintegrated many kilometres above the ground. The resulting shock waves severely damaged several buildings and caused thousands of serious human injuries - mainly retinal damage due to brightness, eardrum punctures due to piercing sound and cuts or bruises caused by flying glass. This incident was widely reported and led to public awareness about the risks of celestial impacts.

"We need to devote significant resources to understand the impact of comets and asteroids on the Earth," says Mark Bailey, a renowned comet expert and Director Emeritus of the Armagh Observatory in Northern Ireland. "Advances in solar system astronomy are playing a major role in facilitating a mature assessment and discussion of Earth's current near-space environment and the possible hazards that it presents on both short (decadal) and long (millennial) timescales," he adds.

Latest research shows that Tunguska-like impacts happen every 100 years or so while Chelyabinsk-like impacts occur once every few decades. There is no tearing need to panic; the chances of such an impact occurring over an uninhabited area - a large area on Earth is still sparsely populated (such as Siberia) - is much more.

We, however, must keep our eyes peeled so that we get advance warning of any such impending impact. How do we do that?

The Minor Planet Center at Harvard Smithsonian Center for Astrophysics in the US is where observations of all known asteroid positions is collated. "Our job is to receive and process data reported from thousands of observatories. The primary goal is to identify near-Earth objects (NEOs), particularly those that may be passing close to the Earth in the near future. Those objects typically require additional observations in order to refine our knowledge of their orbits and trajectories," says Matt Holman, an exoplanetary dynamicist and the director of International Astronomical Union Minor Planet Center. It is their job to keep track of all asteroids near the Earth and their trajectories. And that isn't easy.

"Each time an asteroid passes near a planet, its orbit is modified, and the sequence of future encounters with the same, or another, planet becomes difficult to predict; thus, a potential collision with the Earth occurring, say, decades after the current epoch may be 'hidden''behind the curtain of uncertainty generated by other intervening Earth encounters," says Dr Giovanni Valsecchi, a well known celestial mechanics expert at the Istituto Nazionale di Astrofisica in Rome.

Should we be worried about future impacts? "Astronomers are surveying the sky and monitoring the trajectories of known asteroids. Let the astronomers do their work and don't worry," says Dr Alessandro Morbidelli, a leading solar system formation expert from Lagrange Laboratory, Observatory of Nice in France.

"NASA funds asteroid search programs that use large optical telescopes to scan the night sky continuously, looking for asteroids and comets that could pose a hazard of impact with our planet. JPL's Center for Near-Earth Object Studies (CNEOS) is NASA's centre for using tracking observations to make an accurate assessment of the probability that any of these objects might impact our planet" says Dr Paul Chodas, chief of Center for Near Earth Object Studies at NASA-Jet Propulsion Laboratory, the US.

Meantime, astrophysicists all over the world are keeping vigil.

5 biggest impact craters

Vredefort Crater

Location: Free State in South Africa
Impact date: About 2 billion years ago 
Size: 190km in radius 
Trivia: Declared Unesco World Heritage Sight in 2005 

Chicxulub Crater

Location: Yucatán Peninsula in Mexico
Impact date: 65 million years ago
Size: Estimates of its diameter range from 170 to a whopping 300km
Trivia: Responsible for the extinction of dinosaurs

Sudbury Basin

Location: Ontario in Canada
Impact date: 1.8 billion years ago
Size: About 130km in diameter
Trivia: One of the largest and oldest known impact structures of the world

Acraman Crater

Location: South Australia
Impact date: 580 million years ago
Size: About 90km long
Trivia: It is now a huge lake 

Manicouagan Crater

Location: Quebec in Canada
Impact date: 215 million years ago
Size: A 100km in diameter
Trivia: Lake Manicouagan is one of the best preserved craters 

The writer is an Indian astrophysicist based at CEED, University of Oslo

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