A BLIND SPOT OF COSMIC PROPORTIONS From The Age, 13 January 2002
Phew, that was close! When an asteroid 300 metres wide misses Earth by 600,000 kilometres, most people only jokingly speak of a near-miss. Astronomers, however, take a much more serious view of what happened at 6.37pm on Monday. They warn that Hollywood blockbusters such as Armageddon and Deep Impact have a basis in discomforting fact: asteroids have devastated life on Earth in the past, and could do so again. Indeed, not only cinema, but the biblical book of Revelation presents images of earthly catastrophe that correspond disturbingly to asteroid impact. Last year, when Arthur C. Clarke, author of 2001: A Space Odyssey, was asked to look ahead to 3001, he cautioned that one couldn't simply assume civilisation would still exist. His main reason was the threat of asteroids and comets.
To contemplate Earth's place in the universe is a humbling experience: our small planet travels through a cosmic shooting gallery, in which the bullets can be kilometres across and 10 times as fast. The vast emptiness of space dramatically improves our chances, but Earth has suffered four major impacts in the past century, fortunately in uninhabited parts. When a relatively modest asteroid, 60 metres wide, exploded 10 kilometres above Siberia in 1908, thousands of square kilometres of forest were razed. As recently as May 1995, Perth residents were jolted awake by a sonic boom. An asteroid, incongruously likened to a basketball court, had plunged through the atmosphere before exploding 20 kilometres up. "It nearly got through to Earth," said a local astronomer. "We like them a bit further out." They have got through before. In Australia, they left craters such as the 600 million-year-old Lake Acraman, 100 kilometres across, and a similar giant in the north-west from at least 200 million years ago. The one- kilometre Wolfe Creek crater is only 300,000 years old.
When nuclear physicist Edward Teller first proposed a tracking system and missile arsenal to divert asteroids, he was ridiculed. Now for some people his status as "father of the hydrogen bomb" may not recommend his plan as rational but, as scientists and historians alike can attest, the world is sufficiently mysterious that, frequently, what appears absurd later turns out to be sensible. "Nobody believed Chicken Little when he said that the sky was falling," the late American scientist Eugene Shoemaker wryly observed, "but occasionally the sky does fall, and with horrendous effects."
By July 1994, when the Shoemaker-Levy 9 comet hit Jupiter, blasting an area four times the size of Earth, many experts had been persuaded. It is asteroids bigger than a kilometre across that threaten us with extinction. Among astronomers, the pessimists say there is a one-in-1000 chance such an asteroid will hit within 100 years; the optimists say one in 5000. That is not reason enough to lie awake at night, but should prompt us to take some action - after all, a one- in-a-million chance of victory is enough to persuade millions of people to buy lottery tickets. When it comes to being hit by a big asteroid, the scientific consensus is that in time it will happen -- unless we use technology to defend the planet.
Planning has begun. The US military and NASA have worked together on the problem for a decade. Europe and Japan have followed the US lead. Russia and America have even cooperated on schemes to use nuclear missiles against asteroids. The irony is inescapable: those who were once seen as sowing the seeds of self-destruction may ensure our preservation. A British call for a global early- warning system has been backed by the United Nations and OECD. The biggest gap now is in the southern hemisphere. Since 1996, when the Howard Government withdrew funding from what was a world-leading tracking project, no facility has been dedicated to keeping watch from the south. Australia is, as physicist Paul Davies observes, letting down the world for the sake of a few million dollars. An Australian, David Steel, a leader in the field who moved to Britain after 1996, says he would "challenge anyone to find a project with a better cost-benefit ratio".
NASA has subsidised some Australian programs to further its goal for the decade of identifying most of the 1000 or so asteroids thought to pose a threat. But our political leaders have been slow to recognise the risk, or the damage done to Australian science, which had a global reputation in asteroid research. The asteroid that went past on Monday was not detected until last month. US experts estimate we need at least 10 years' warning to have a realistic chance of diverting an asteroid. At least we have some idea of what's out there. When a giant asteroid hit what is now the Gulf of Mexico 65 million years ago, the soon-to-be-extinct dinosaurs never saw it coming. Arthur C. Clarke quotes a colleague's observation: "The dinosaurs became extinct because they didn't have a space program." Humans do, and can use it to avoid the dinosaurs' fate.
Copyright 2002, The Age
From The Age, 13 January 2002
By Paul Davies
After the initial shock, mankind reacted with a determination and a unity that no earlier age could have shown. Such a disaster, it was realised, might not occur again for a thousand years - but it might occur tomorrow ... So began Project Spaceguard.
Thus wrote the famous futurist Arthur C. Clarke in his 1973 science fiction novel Rendezvous With Rama. Clarke was describing the aftermath of the worst foreseeable ecological catastrophe - impact by an asteroid packing enough punch to wipe out an entire country.
A few days ago, our planet narrowly escaped just such a fate when a 300-metre-wide chunk of rock hurtled past us on its way around the sun. It was but one among thousands of rogue asteroids that constantly menace life on Earth as they flash by. Alarmingly, scientists have very little idea where most of these objects are, and sooner or later one of them will slam directly into us, causing massive destruction.
Computer simulations suggest that a one-kilometre rock hitting Earth at 30 kilometres per second would explode with the power of a million-megaton bomb, blasting a crater 20 kilometres across. Billions of tonnes of rock would spew forth, some of it crashing down in molten lumps around the globe, igniting widespread fires.
The blast and seismic shock might devastate millions of square kilometres. If the object plunged into the ocean, tsunamis looming higher than skyscrapers would pulverise coastal cities. Vast quantities of dust would blanket the planet, blotting out the sun for weeks, causing crops to fail on a huge scale. Worldwide starvation and economic collapse would follow. The final death toll from the resulting mayhem is estimated to be upward of one billion.
Admittedly, a cosmic impact of this severity is extremely rare; there is roughly one chance in a million it will happen in the next month or two. But a one-in-a-million chance of a billion deaths implies an expectation of death of a thousand people per month, greater than many familiar disasters like plane crashes. Translated into personal terms, you are more likely to be killed by an asteroid than, say, a bolt of lightning.
When it comes to risk assessment, humans are remarkably irrational. Many people shrug aside a million-to-one risk. At the same time, the uncontrolled descent of the Mir space station, in which the threat to human life was minuscule, was greeted by near hysteria in some quarters.
That asteroids, comets and large meteorites do collide with Earth from time to time is evident from even a casual survey. At Wolfe Creek in Western Australia you can see a one kilometre-wide 300,000-year-old impact crater. A much larger feature, Lake Acraman in South Australia, is the remnant of a 600 million-year-old crater 100 kilometres across.
A collection of 100-metre holes at Henbury, near Alice Springs, has become a well-known tourist attraction. In all, a couple of dozen impact sites have been identified in Australia, and many more in other parts of the world.
The solar system is swarming with rocks, most of them rubble remaining from the formation of the solar system 4.5 billion years ago. In the far past, the bombardment of the planets by this debris was intense. A glance at the moon reveals a surface pockmarked with ancient impact craters, some of them enormous. On Earth, these surface scars have been mostly obliterated by geological processes, but our planet will certainly have taken its share of punishment over the aeons.
Although the fierce early bombardment abated about 3.8 billion years ago, it never entirely ceased. Throughout geological history there have been several episodes of sudden, massive species extinctions, which many scientists now attribute to colossal impacts. The most severe occurred 250 million years ago, when 90 per cent of species, including the famous trilobites (extinct animals), suddenly disappeared, probably from the combined effects of the impact and the extensive volcanic eruptions it triggered. It also seems likely that the dinosaurs met their end 65 million years ago in similar fashion, when a 16-kilometre- wide object struck what is now the Yucatan Peninsula in Mexico, excavating a crater over 200 kilometres across.
Today, almost all the large asteroids are safely confined to a belt between Mars and Jupiter. However, smaller bodies cross our orbit all the time, threatening catastrophe. Part of the problem in quantifying that threat is that we have only a rough idea how many near- Earth objects are out there. In astronomical terms a one- kilometre rock is pretty small and inconspicuous. Most of them could hit us with only a few seconds warning.
Even much smaller rocks still have awesome destructive power, and they are far more numerous. In 1908, 2000 square kilometres of forest were flattened when an object the size of an office block came out of the blue and exploded over the remote Tunguska River area of Siberia. This sort of event can be expected every few decades. Indeed, a similar explosion is believed to have taken place in the South American jungle in the 1930s.
Prompted by Clarke's apocalyptic scenario, scientists have proposed Project Spaceguard to deal with the near-Earth objects threat. It consists of two distinct stages. First is a systematic search program designed to find as many near-Earth objects as possible, and compute their orbits. The observations, to be carried out over many years, should determine when the next impact will occur. The second stage is to develop the technology needed to deflect the approaching object from its threatening path.
What can be done about an asteroid coming our way? Blasting it with nuclear warheads is not a sensible option, for it would serve only to turn a bullet into buckshot. A better strategy is to alter the orbit. Moving a billion-tonne rock isn't easy, and nobody is quite sure how to do it, but one idea is to detonate nuclear bombs in a controlled manner near the surface, vaporising a layer of material to propel the rock slightly to one side. Another proposal is to attach a giant sail and let the solar wind blow the asteroid off course.
Unfortunately, not much is known about the make-up of asteroids. A year ago the US space agency NASA landed a space probe on Eros, an irregular-shaped rock 35 kilometres long. Mission scientists concluded that Eros is indeed a solid object likely to hold together if nudged. But the same probe had earlier visited a bigger asteroid named Mathilde, which looked more like a collection of boulders held together only by its own feeble gravity.
Whatever deflection strategy is adopted, the sooner it is done the better. A tiny change in orbit decades ahead of projected impact will be more effective than a big push when time is running out. Hence the need for efficient early warning, which only a comprehensive search program can provide.
Last year, the British Government pledged £35 million ($A96 million) to planetary defence. Now the United Nations is establishing a taskforce to review the danger.
Most of the bigger objects could be found and tracked using a global network of cheap telescopes equipped with some fancy electronics and data analysis. Already the United States has several research groups dedicated to finding near-Earth objects this way. Expanding the search over 10 or 20 years and involving countries such as Australia would deal with the major threat, at relatively modest cost.
But what to do about the smaller, more frequent, yet still potentially deadly impacts? A rock the size of the one that just missed us last week could devastate an area as large as Tasmania. American scientists have called for a more ambitious detection system known as the Large- Aperture Synoptic Survey Telescope, a 6.5-metre instrument costing about $US150 million ($A287 million) and capable of finding 90 per cent of rocks down to 300 metres across. With such an early warning system, evacuation measures could be taken in the impact zone. Even deploying all this technology, there will remain a class of hazardous objects that we can do very little about. These are comets - loose aggregations of rock and ice that mostly inhabit a realm far beyond the planets. From time to time one gets diverted into the inner solar system on an elongated trajectory, whence it sprouts the distinctive tail.
As long ago as the seventeenth century, Edmund Halley warned of the peril of a comet striking a planet. The awesome consequences were dramatically evident in July 1994, when the core of comet Shoemaker-Levy 6 broke into fragments and ploughed into Jupiter, creating a series of titanic fireballs so big they could be seen through backyard telescopes.
Typically a new comet is spotted only months before it sweeps by. If one were to have our number on it, there would be far too little time to get out there and deflect it, even if a way could be found to move such a fragile object without breaking it into pieces.
Some critics of near-Earth objects research argue that it is a waste of money tracking down dangerous asteroids when killer comets will still go undetected, but that's rather like refusing to check the steering on your car because the brakes might fail. Though we are certainly in the firing line from comets, our astronomical location is actually rather favourable. In fact, we have Jupiter to thank for keeping Earth relatively free of impacts. Its immense gravitational field deflects many incoming comets and can even fling them right out of the solar system. In effect, this giant planet acts as a cosmic vacuum cleaner, sucking up or tossing out dangerous debris before it can penetrate the habitable zone.
In the last few years, dozens of planets have been discovered around other stars, but so far the configuration of the solar system looks to be unusual, even unique. If there are other Earths out there, chances are they will be pounded mercilessly for billions of years, preventing life from establishing more than a toe- hold.
It would be the supreme irony if on the rare planet that permitted life to evolve as far as intelligence, the resulting beings were still too stupid to protect their species from cosmic catastrophe.
Paul Davies is a physicist in the Australian Centre for Astrobiology at Macquarie University. His latest book is How to Build a Time Machine (Penguin).
Copyright 2002, The Age