20 March 2010

It's a matter of time


We humans can avoid the fate of our dinosaur predecessors if we start to prepare now to detect and avoid the next major asteroid strike


We have all heard the stories about the reign of dinosaurs being ended by one or more impacts of asteroids. But that was millions of years ago -- is there really any chance that such a thing could happen to us? And surely, such an event is far in the future -- should we care now?
Scientists still debate what actually terminated the dinosaurs, but it is generally agreed that Earth, like our moon, has been struck many times in the past by asteroids and comets, with at least one large impact occurring late in the time of the dinosaurs and doubtless playing a substantial role in their demise.
Impacts of space rocks have not stopped. About a dozen small fragments survive atmospheric entry and impact daily, with most hitting the ocean. Occasionally, fragments puncture a roof or dent a car -- or injure a human.
Perhaps the best known recent impact is the Tunguska event, where an asteroid or comet estimated to be 30 to 40 metres in diameter entered the atmosphere over a remote area in Siberia and exploded a few kilometres above the surface. The event levelled more than 2,000 square kilometres of the forest below, devastating an area larger than that of Washington, D.C. The force of that explosion has been estimated to be between three and five megatons of TNT.
In 1998, NASA began a search for potentially threatening objects that are one kilometre in diameter and larger -- objects large enough to cause a global catastrophe should one impact Earth. Through early 2010, we've found 887 such objects -- approximately 90 per cent of those thought to exist -- plus more than 6,700 smaller objects. While none of the larger objects poses an immediate threat, the survey did find an object that may -- an asteroid 270 metres in diameter known as Apophis.
In 2029, Apophis will pass very close to Earth, closer than our geosynchronous satellites. Earth's gravity will bend its trajectory as it passes and if it bends the precise amount, Apophis could return and strike Earth in 2036. At present, the probability of such an event is approximately one in 250,000. It will be a very bad day for our planet should it hit; not bad enough to erase humanity, but bad enough to affect millions of people and cause considerable damage in a local area.
Currently, we aren't looking for objects in the size range of Apophis. A plan to look for objects as small as 140 metres in diameter has been developed, but it has not yet been funded. Predictions are that perhaps 25,000 more threatening objects will be found once the expanded survey begins.
The likelihood of surprise impact by an object in the size range of Apophis is small, but as the Tunguska event shows, impacts by objects smaller than 140 metres can also have devastating consequences if the impact occurs in a populated area. The likelihood of a surprise impact by such an object is not that small: about one in 10 this century.
Given that threats from asteroids and comets are inevitable, what should we do to minimize the risk of impacts?
First, we must find the threatening objects. While the effort to discover objects larger than 140 metres in diameter is obviously necessary and should begin soon, some believe that we should expand the search to include smaller, 50-metre-class objects. Of course, the difficulty and cost of the discovery effort increases as the size of the objects decreases.
Second, we must find the threatening objects early. The impulse required to deflect an object away from our planet increases as the object gets closer, so we want to intercept it years before impact. Factoring in the requirement that we must build and launch a deflection campaign and that the vehicles used require time to actually reach their target, it is clear that maximizing the time to act is critical.
Third, we need to have proven techniques to deflect an asteroid or comet. Proposals include both fast-acting techniques such as nuclear explosives and impacting the object with high-speed projectiles; or slowly tugging an object using the gravitational attraction between it and a nearby spacecraft, or using lasers or focused sunlight to impart a small force. The size of the threatening object, the time remaining and other factors will affect which techniques might be applicable in a particular case. And, since some of the launches and spacecraft may fail, we can't simply launch one interceptor. We're "betting the planet" on the success of the deflection mission, so we must include mission failures in the overall campaign design.
Fourth, we must decide when to act. This may be our most difficult challenge: since time is critical, we must begin to develop the deflection hardware before we are absolutely certain the object will impact. This means that elected officials must provide funding and allocate resources without certainty that the effort is required.
Finally, such a threat will be a global concern like none other our species has faced. Nations must share responsibilities for the decision to act, and nations with capabilities to respond must co-ordinate their activities.
So what do we do now? Clearly, we need to move forward aggressively to find threatening objects. Assuming that none will be a threat in the next 50 years, we should identify, develop and test the most promising deflection techniques; we should design deflection campaigns for threats of various sizes and characteristics; we should educate the public and elected officials on the nature of the threat; and we should develop national and international protocols for making a decision to act and for working together to eliminate the threat.
It's human nature to act when the threat is certain, but we can't wait until we are absolutely sure to move forward in this area. We humans can avoid the fate of our dinosaur predecessors if we accept the challenge and initiate a stable and ongoing effort to find threatening objects and build the plans, protocols, and technologies to deflect an asteroid or comet.
As we begin this effort, let's hope we have the most precious resource we'll need -- time.
William Ailor is co-chair of the 2nd IAA Planetary Defense Conference to be held in Bucharest, Romania in May, 2011

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