Earth’s defences against asteroids
IF you watched the 1998 movies Armageddon or Deep Impact, you’d have the impression that the way to deal with a big asteroid hurling towards earth would be to send some astronauts to land on it and place some bombs on it. Using bombs to deflect an asteroid’s trajectory is a technique that’s feasible but the mission wouldn’t involve any astronauts. “That’s something relegated to the movies — it makes a good movie, but we do not see in our studies any technique that would require the involvement of astronauts,” said NASA’s planetary defence officer, Lindley Johnson, in June after releasing a report entitled “National NearEarth Object Preparedness Strategy and Action Plan”. NASA is the USA’s National Aeronautics and Space Administration, arguably the leading space agency in the world.
The report outlines the steps that NASA will take over the next decade to prevent dangerous asteroids from hitting Earth. “This plan is an outline not only to enhance the hunt for hazardous asteroids, but also to better predict their chances of being an impact threat well into the future and the potential effects that it could have on Earth,” said Johnson, adding that the plan will help NASA “...step up our efforts to demonstrate possible asteroid deflection and other mitigation techniques”.
ASTEROIDS CAN CAUSE EXTINCTION
An asteroid is a rocky or metallic object orbiting the sun. They’re now defined as being larger than one metre in diameter with objects smaller than that being called meteoroids. The largest asteroid impact in recorded history is known as the Tunguska event, where an asteroid exploded over Siberia on June 30, 1908.
That explosion knocked down some 80 million trees over 2000 square kilometres of forest. Remarkably there was no known human casualty because it was such a sparsely populated part of Siberia. Scientists classify it as an impact event even though the asteroid — estimated to be between 60 and 190 metres — is believed to have disintegrated at an altitude of 5 to 10km above the earth and didn’t actually hit the earth. The 15-megaton impact of the explosion though is equal to about 1,000 times that of the atomic bomb dropped on Hiroshima during World War II.
The Tunguska event, as devastating as it was, is nothing compared to the one that scientists believe hit the Earth 65 million years ago. Estimated to be between 10 and 15km in diameter, that asteroid’s impact threw so much dust into the air that it would have cut off sunlight all over the world and thus prevented photosynthesis as well as lowered temperatures considerably.
The impact would have also caused mega-tsunamis while ejected melted rock would have caused widespread forest fires. Scientists believed that that event resulted in the extinction of some 70 per cent of the species on Earth.
To prevent another asteroid like that — or worse still, one that’s even bigger — from ever impacting Earth, scientists have developed various mitigation techniques, most of which involve deflecting the asteroid so that it steers off course.
The way they did it in Deep Impact and Armageddon was to use some kind of explosion to deflect the asteroid. NASA has determined that a series of standoff nuclear explosions could push an asteroid off course. The idea isn’t to explode a nuclear warhead on the asteroid itself — because that would risk it breaking into several smaller pieces which could still be just as deadly — but to detonate it near the asteroid. The heat from the explosion would sear one side of the asteroid and as material vaporises from its surface, the asteroid would accelerate in a direction away from Earth.
NASA’s preferred option, however, is a kinetic impactor called Double Asteroid Redirection Test (DART) which is expected to launch in 2021. DART would be “...our first technology demonstration of the kinetic impact technique to deflect an asteroid,” said Johnson. The DART spacecraft, just about the size of a refrigerator, will encounter the Didymos asteroid which is expected to fly by Earth in 2022. Didymos is a binary system consisting of a larger object, 780 metres, and a smaller one, 160 metres, which is orbiting the larger asteroid. DART is programmed to strike the smaller object while travelling at the speed of six kilometres per second. The impact of this hit should disrupt the orbit of smaller asteroid and provide crucial data for such attempts on a larger scale.
Another approach also favoured by NASA is something called a gravity tractor. This approach doesn’t require any spacecraft to impact the asteroid. Rather, it utilises the force of gravity to do this. A spacecraft flying alongside an asteroid for years would have enough gravitational pull to change its path. Over the long haul, it would then be able to guide the asteroid away from earth. However, the technique has never been tried in practice and would require literally decades of testing, according to NASA.
OTHER EXOTIC APPROACHES
As they say, there’s more than one way to skin a cat. Over the years, scientists have proposed many different options for deflecting asteroids. These include painting the asteroids white to change the amount of solar radiation. Apparently this would affect its trajectory. Other approaches include attaching solar sails to asteroids and using robotic landers to mount thrusters to gradually change the direction of the asteroid’s flight path.
GLOBAL COOPERATION NEEDED
Whatever the approach adopted, NASA can’t do it alone. The US, as the world’s leading superpower, would naturally lead the way but it would need the help of other nations with strong space programmes.
To help the rest of the world prepare for an asteroid strike, NASA’s Planetary Defense Coordination Office is working with the United Nations Committee on the Peaceful Uses of Outer Space to look at what should be the international response should a dangerous asteroid be found to be heading towards earth.
NO IMMEDIATE THREAT
In Hollywood movies about asteroids, they appear rather suddenly, requiring a rapid response by space agencies. In real life, scientists are able to see and project the trajectory of dangerous asteroids not just decades and but centuries in advance. NASA predicts that there’s less than a 0.01 per cent chance of a potentially hazardous asteroid making an impact in the next 100 hundred years.
Scientists have projected that in 2135, an asteroid called Bennu will fly pretty close to the Earth.
Although there’s no danger of a collision, its flyby will actually be within the moon’s orbit, which could change its path such that there is a 0.037 per cent (or 1 in 2,700) chance that it could hit the earth sometime between 2175 and 2199. Between now and then, there will be generations of scientists from NASA and other nations’ space agencies working on effective deflection systems. In any event, the 500-metre-wide Bennu isn’t considered an extinction-level asteroid.
Of course, if you look long term enough, perhaps a few centuries or thousands of years from now, an extinction-level asteroid might indeed head towards the Earth. Fortunately for humankind, today’s scientists have already started working on solutions to this problem and would have surely perfected it by the time the Earth actually needs it.