Meteor and asteroid impacts

30 June is the first Asteroid Day – part of a new movement to raise awareness of the threat from asteroids and how we can protect ourselves against it.

Asteroid impacts
credit: NASA

Although serious impacts are rare, they are thought to have played a big role in shaping the evolution of life on Earth.

According to some versions of the theory of “panspermia”, meteorites are responsible for bringing the first primordial life to Earth.

The theory holds that either very simple forms of life or the materials necessary for it to form are carried to Earth on comets or fragments of asteroids. These survive their journey through the atmosphere and ultimately evolve into the species we see around us today – including the very scientists hypothesising about their origins.

Certain amino acids – the “building blocks” of life, necessary to build proteins ­­– have been found by NASA on comets, and analysis of some meteorites has suggested that amino acids and other organic compounds may have formed in space, rather than being present as a result of contamination of the samples.

However it is difficult to test panspermia theories fully without exploring or surveying a huge amount of space. Critics point out that it is uncommon for the molecules required for life to develop are only rarely found together at sufficiently high densities for chemical reactions to occur.

Similarly, comet impacts have been credited as the source of Earth’s water.

Impacts from celestial bodies are not only thought to have brought life to Earth, but are also implicated in ending much of it.

The extinction of the dinosaurs in the Cretaceous-Tertiary event 65m years ago is famously believed to have been caused by a massive asteroid impact.

A geological layer dating from the time of the event was found in 1980 to be high in iridium, which is rare in the Earth’s crust but common in asteroids. The Chicxulub Crater in Mexico is the most commonly accepted point of impact, with the responsible body thought to be around 10 km in diameter.

When an impact event happens, the enormous kinetic energy of the asteroid or meteorite – a massive body travelling at high speed – is partly transferred to the Earth but largely converted into heat and sound, creating pressure waves travelling radially outwards from its centre, similar to that of an atom bomb.

As well as this blast created on impact, large amounts of dust can be kicked up into the atmosphere – like a nuclear winter – blocking out sunlight and preventing plants from photosynthesising, which has a knock-on effect to much of the rest of the ecosystem.

Meteors with a radius of between five and ten metres collide with the Earth’s atmosphere about once a year, releasing about as much energy as the atom bomb dropped on Hiroshima, but are too small to reach the surface.

Impacts from bigger bodies are less common – the largest during recorded human history is the Tunguska event, in Siberia in 1908, in which a body estimated at a few 10s of metres across exploded just a few miles above the Earth’s surface, releasing as much energy as 10–15 megatons of TNT. (By way of comparison, the comet Shoemaker-Levy 9 slamming into Jupiter in 1994 released the equivalent of more than 6 000 000 megatons of TNT.)

In wiping out the dinosaurs, however, the meteorite created the opportunity for mammals to become the dominant form of life on Earth – so it worked out quite well for us. More luckily still, there hasn’t yet been another meteorite impact big enough to cause that kind of large-scale extinction, and now we may be able to actually avert Armageddon (the apocalyptic event, not the Bruce Willis film… unfortunately).

Asteroids are detected by surveying the sky with telescopes, both from on the ground and in space, predominantly by the US, Japan, Canada and Italy.

NASA had originally aimed to have catalogued 90% of near-Earth objects larger than 1 km in diameter – large enough to cause a mass extinction – by 2008. Although this is behind schedule it is expected to be completed within the next few years. However the discovery in 2009 of an object 2–3 km in diameter shows that we are still some distance from being fully protected from asteroids.

Such surveys have succeeded in predicting the time and location of small impacts, and ruling out some larger ones.

Even if asteroids on a collision course are detected, their presence must be picked up early to allow enough to time to build a device to move it out of our path, and to plan the mission to carry out that task.

Attempts to avert a catastrophic impact might be based on either destroying an inbound asteroid, or deflecting it from its collision course.

Destruction risks creating a shower of secondary fragments that could still do a great deal of damage, including potentially making the planet uninhabitable, while deflection of a body of such large mass could take an enormous amount of energy.

A proposed European Space Agency probe will test the viability of deflecting an asteroid by ramming a spacecraft into it. It is named Don Quijote for the section of the novel by Miguel de Cervantes in which the eponymous protagonist attacks a windmill, mistaking it for a giant. If this is attempted far from Earth, an asteroid would only need to be deflected through a small angle to avoid a collision.

Other possibilities include propulsion by conventional rockets or an ion engine, use of a mass driver, or nuclear pulse propulsion – harnessing a weapon of mass destruction to avoid the annihilation of civilisation by the same type of body that might have originally brought life to the Earth.

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