Observations

International Asteroid Day 2018

Black and white photograph showing a forest of trees which have been flattened.
Aftermath of the Tunguska impact, showing trees flattened by the explosion. Photo: Leonid Kulik/NASA.

On 30 June 1908 a massive explosion occurred over Siberia flattening 2000 square kilometres of forest. The event, now known as the Tunguska impact is believed to be result of the air burst from a meteoroid disintegrating about 5-10 km above the Earth’s surface. It is the largest impact event in recorded history.

Meanwhile, catastrophic asteroid collisions have become a mainstay of science fiction, forming the premise of films like Armageddon, Deep Impact and Asteroid vs Earth, to name but a few. And while the movies might not quite get the facts right, the risk of a major collision is real nonetheless. Every child in school knows about the devastating impact which wiped out the dinosaurs and many people fear the possibility of the Earth being hit by another asteroid that size.

Image of the Earth from Space with two asteroids in the foreground, one large and one small, which are implied to be heading towards Earth.
Artist’s impression of asteroids heading towards Earth. Image: ESA.

In 2016 the United Nations designated 30 June as International Asteroid Day, using the anniversary of the Tunguska impact to raise awareness about the risk posed by asteroids, and the work being done by organisations around the world avoid such an event in the future. The same year, NASA formed the Planetary Defence Coordination Office which continually monitors and updates the risks of so-called Near Earth Objects, and would be responsible for coordinating the response to an actual impact threat.

Near Earth Objects, or NEOs, are comets or asteroids whose orbit brings them within the Earth’s neighbourhood (defined as anywhere within 1.3 times the distance between the Earth and the Sun, or approximately 200 million km). As of 2018, more than 18,000 NEOs have been discovered, with about 30 new NEOs found every week. Luckily many of these are small, and would likely burn up in our atmosphere before they reach the ground. However, NEOs more than 140 m across, and which will pass within 7.5 million km of Earth, are considered Potentially Hazardous Objects (PHOs). To date just over 8000 NEOs larger than 140 m and almost 900 larger than 1 km have been found.

Greyscale image depicting three asteroids of various sizes. The asteroids have irregular shapes and visible craters.
Asteroids Mathilde, Gaspra and Ida (from left to right) shown to scale. These asteroids reside in the asteroid belt between Mars and Jupiter, where most NEOs originate. Mathilde is the largest of the three, at approximately 66 km across. Image: NEAR Project, Galileo Project, NASA

So, the big question is – what would we do if one of these were found to be on a collision course with Earth? How we respond to an impact threat would depend on the size of the asteroid and time left until collision. The proposed strategies fall broadly into two categories: deflection and destruction. Popular in Hollywood films, destruction involves detonating an explosive which would break the asteroid apart, into small enough pieces that they would burn up upon entering the Earth’s atmosphere. Deflection meanwhile, aims to change the path of the asteroid just enough for it to miss Earth. Luckily the relative orbits of the asteroid and Earth through space mean that only a small change in direction or speed are necessary to avoid a collision.

The technology to carry out these manoeuvres is still being developed, and NASA has warned that we are currently unprepared to deal with a serious impact threat. The good news is that so far no NEO is known to be on a collision course with Earth. And while it may seem scary to hear that we’re discovering more asteroids with every passing week, these objects are out there whether we know about them or not. Asteroid search programs like those run by NASA and ESA (the European Space Agency) simply mean that we will be armed with the information we need to pick up any threats before it’s too late.

Written by Sarah Reeves, Assistant Curator

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