Protecting the Earth from killer asteroids

Near-Earth asteroids are interlopers from the asteroid belt between Mars and Jupiter composed of rocky debris left over when our solar system formed. Planet Earth is at risk of colliding with an asteroid with possible disastrous consequences.

The last major collision occurred about 66 million years ago when a 12km-wide asteroid slammed into Earth, killing 70 per cent of biological species, including the non-avian dinosaurs. Throughout our history, mankind has had no means of defending itself against asteroids on a collision course with Earth. But now, for the first time, we are beginning to take steps to protect ourselves. Sarah Scoles tells the story in Scientific American (June 2021).

The moon has been a close companion to Earth for 4.5 billion years. Looking at the moon, we see a planet pockmarked with thousands of craters resulting from impacts with asteroids and meteorites. But impact craters are not very obvious on Earth – only about 128 are logged. This is because the surface crust of the Earth is cracked into several giant plates that move about (plate tectonics) on a molten underlayer. Earth also has an active atmosphere of winds, rain, snow, ice and flowing water that weather and erode the surface of the Earth and, together with plate tectonics, erase our earthly record of craters.

On the other hand, the moon is airless and has no place tectonics or erosional capabilities. Craters formed on the moon last for an extremely long time, presenting a relatively untouched geological archive and filling in gaps in the Earth’s impact history.

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Impact probability

But, you may say, isn't the probability of an asteroid colliding with Earth vanishingly small? While it is true that the probability of an asteroid hitting Earth tomorrow is very small, there is good evidence that we now live in an era of relatively high impact probability. The rate of asteroid impacts with the Earth increased dramatically about 290 million years ago and probably remains high to the present day (Robin Andrews, National Geographic, January 17th, 2019). The most recent asteroid incident happened on February 15th, 2013, when a house-sized asteroid entered our atmosphere and exploded over the city of Chelyabinsk, Russia, injuring 1,500 people.

Nasa and other organisations run asteroid data-gathering programmes. The aim is to detect 90 per cent of near-Earth objects with diameters over 140m that would be problematic on impact. There are about 25,000 such objects and 40 per cent have so far been detected. Globally about 30 space organisations participate in an international asteroid-warning network

Unfortunately, Earth's defences against incoming asteroids suffered a serious setback when the Puerto Rican Arecibo Observatory radio telescope fatally crashed in December 2020. Arecibo had been scanning the skies since 1963 and had the world's most powerful and sensitive planetary radar system.

Shape and composition

When a rogue asteroid was detected, Arecibo could determine its distance from Earth, compute its orbit and calculate its position far into the future, paying particular attention to whether or not it would collide with Earth. Arecibo could also calculate the shape and composition of asteroids. The loss of Arecibo leaves a significant hole in Earth’s defences against asteroids, a hole that must be filled as quickly as possible.

If we discover a killer-asteroid heading towards earth, our only hope for survival is to launch a counter offensive to rendezvous with the asteroid and either deflect it from its earthbound path or blow it into harmless fragments. An asteroid could theoretically be deflected from its path by, for example, sending out a spacecraft to collide with it, pushing it off course, or to train lasers on one side of the asteroid evaporating off material and thereby affecting the flight course of the asteroid.

The double asteroid redirection test (Dart) was launched last November, the world’s first planetary defence test mission to demonstrate that we can change an asteroid’s path. On September 26th next, the Dart spacecraft will intentionally collide with Dimorphos, an asteroid that orbits the sun and is 11 million km away from Earth.

The collision, it is hoped, will slightly change the direction of the asteroid’s motion in space. Dimorphos is of a size (160m diameter) that would endanger cities if it slammed into Earth. If Dart is successful, it will demonstrate our ability to navigate to an impact on a relatively small target asteroid and that this is a viable technique to deflect a genuinely dangerous asteroid.

William Reville is an emeritus professor of biochemistry at UCC