SOMETIME in the next 10,000 years an enormous rock will crash into an ocean, and 100-metre waves will radiate out to flood coastal cities and distant lowlands. The human toll could be tens of millions of lives, and the economic loss could be measured in trillions of dollars.
I am talking about the impending collapse of Cumbre Vieja, the unstable southern flank of the island of La Palma, part of the Canaries in the western Atlantic. The geological record shows that collapses of this magnitude occur about every 20,000 years, whereas asteroids or comets large enough to produce comparable waves strike Earth only a fifth as often. Indeed, the impact of a near-Earth object, or NEO 鈥� an asteroid or comet whose orbit comes close to Earth 鈥� represents just one of several low-probability, high-consequence calamities that can befall humankind. Topping the list may be warfare, plague and ecological collapse triggered by human disregard. So why, if these other threats are greater, should we care about hazardous NEOs?
Beyond their broad and novel appeal to space buffs and doomsayers alike, there are rational motives for designing strategies to deal with hazardous comets and asteroids. Quite unlike any other natural disaster, the NEO risk can be reduced almost to zero through achievable, feasible means. Indeed, the most dangerous NEOs could even benefit mankind. If we could move them to safe nearby orbits, they would be readily accessible outposts for human exploration and exploitation, science fiction made real.
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We cannot do much about volcanoes or earthquakes other than identify active regions and tell people not to build there. But cataclysmic asteroids are predictable: given sufficient lead time and a precisely determined trajectory, a tiny diversion applied decades in advance can change the future. Isaac Newton, inventor of the clockwork heavens, knew of gravitational perturbation, and argued that God had to occasionally nudge the orbits of comets to prevent them from eventually colliding with Earth. Perhaps it is our role to keep the wayward comets and asteroids on track.
Today, of the $4 million spent annually on investigating NEOs, almost all goes towards detection and orbit prediction. The process of asteroid detection has far outpaced the scientific understanding of what asteroids are, and what we can do about them. This is partly because no government agency deals with hazardous NEOs in any direct way, other than counting them and cataloguing their orbits. It is easy to list the national and worldwide agencies that deal with volcanic eruptions, earthquakes, tsunamis, disease, famine and warfare. Indeed, many agencies exist solely to deal with a particular hazard. But while many (including Hollywood) assume that NASA is in charge of NEO defence, NASA has no such mandate. Neither does the US Air Force or any other agency. NEOs fall into the cracks: they are a hazard without a home.
NASA and space agencies in Europe and Japan do, however, play a leading role in fundamental exploration of asteroids and comets. We now know that they are surprisingly complex entities. Indeed, we have figured out that we wouldn鈥檛 know how to divert one even if some agency were in charge. Until recently, the favoured NEO diversion scenario was a blast from a nuclear explosion (see 鈥淜illing it softly鈥�). But new data have shown that many asteroids are not solid rocks but loosely connected multi-component objects 鈥� rubble piles, in fact. This makes them very hard to disrupt or move predictably by impulsive force.
By doing no more than detecting asteroids, or treating them as geophysical curiosities, we are generating increasing concern without following through with a plan. Because of uncertainties in measuring their orbits, asteroids that have a finite initial chance of hitting Earth sometime down the road are frequently discovered. Collision is almost always ruled out by follow-up observations, but not before we read in the morning paper that we all are doomed. The painful irony is that these detection programmes should be making the public feel tangibly safer. Thanks to ceaseless efforts at a handful of small observatories, half of the kilometre-sized NEOs have been logged, and none is on a collision course with Earth: we can feel twice as safe as we did.
With sufficient effort in the form of larger telescopes, and significant funds for observations and follow-up research, we could soon have nearly all the hazardous rocks in the heavens tracked in a celestial version of air traffic control. Most likely (a thousand to one) this survey, when completed, would signal the all-clear for any object larger than 300 metres across headed for a collision within a century. But if we are gravely unlucky and we find something large is headed our way, we would want to do better than finding the biggest rocket available and strapping on the biggest nuclear warhead it could carry. This may not work, and it鈥檚 a scenario appropriate only to an unprepared civilisation scrambling for a last-minute solution.
The purpose of a detection effort is hopefully to signal the all-clear, and failing that, to buy us the long lead time required to engineer a reliable and cost-effective deflection strategy. It must be accompanied by a growing understanding of how to apply a small diversion to an asteroid. It is not going to be easy, unless you compare it with other space-related endeavours such as visiting the Moon. The basic research requires ambitious spacecraft missions to determine detailed geological characteristics for a wide range of comets and asteroids. This will not come cheap, unless you measure it in military metrics. For the cost of a single B2 bomber, we could launch a major NEO geophysical reconnaissance mission (at about $300 million per mission) every five years for the next 30 years. This would take us from this place of ignorance to one where clear decisions can be made 鈥� and where a future human presence in near-Earth space is all but certain.
We cannot afford spacecraft exploration if it means diminished asteroid surveillance. But we are clearly at the stage where surveillance alone will not do. With surveillance comes the public expectation that we have a plan for learning more about hazardous NEOs, including how to divert one. But we are scrambling for a plan, for funding, and for an agency to take on this difficult charge.