The Case of the Menacing Asteroid: What Cost to Prevent a Collision?

The massive oil spill in the Gulf of Mexico reminds us about the possibility of unexpected, catastrophic events. How about the risk of asteroid collisions? What measures are being taken to protect ourselves against an extremely low probability, but truly catastrophic, risk of an asteroid hitting the Earth?

Asteroids in their millions whirl constantly through space, occasionally passing near the Earth. Very occasionally, one of them hits our planet. Asteroids are generally pieces of rock but the speeds at which they travel mean that the impact of a collision can generate forces comparable to that of a nuclear explosion. That’s the scientific basis for a 1998 Bruce Willis movie, “Armegeddon,” but it could actually happen, implausible as it sounds.

How do you calculate the rational level of effort to prevent an event of very low probability but catastrophic consequences? How much should society spend on the arrays of telescopes to identify approaching objects, and to prepare the means to destroy or deflect those that present threats?

An asteroid large enough to inflict serious damage would certainly be seen before impact, but the question is whether it would be seen in time to take action to deflect it. In the case of a large body, to do it efficiently would probably require more than a decade.

Remember the Dinosaurs

The best known example of a collision is the Chicxulub asteroid, evidently about 10 kilometers in diameter, which struck the Gulf of Mexico some 65.5 million years ago. It killed off half or more of the living species on the planet including, famously, the dinosaurs. That was a comfortingly long time ago but there have been reminders since then, on a much smaller scale, that it was not unique.

In 1908, an object fell into the atmosphere above the Tunguska region of Siberia. Its descent generated extreme temperatures that caused it to explode in the air, leveling the forest trees over an area of about 800 square miles. Because it happened over one of the world’s most remote regions, the human death toll was low—perhaps one person—and even the Russian government took little notice. Reconstructing the event much later, scientists at the National Aeronautics and Space Administration have estimated that the object was a rock about 40 meters in diameter and the energy released by its explosion was two orders of magnitude greater than that of the Hiroshima bomb.

Assessing the Risks

In his 2004 book, Catastrophe: Risk and Response, Richard A. Posner expresses astonishment that threats capable of wiping out the human race are hardly ever discussed in public. He takes the possibility of an asteroid impact as an example, and to illuminate the issue he offers a thumbnail formula: the investment in prevention ought to equal the cost of the disaster times the probability of its happening. His purpose is to suggest that the current effort is inadequate. (The United States is currently spending about $4 million a year on detection.) But putting an economic cost to the extinction of all humanity is not a simple calculation. Nor is there consensus on the probability of a collision.

NASA offers a register of what it terms Near Earth Objects (NEOs), and reassuringly comments that the “probability of any of these objects hitting Earth on these approaches is essentially zero…” (see NASA Fact Sheet in Further Reading). Not everyone is so sure. The Harvard-Smithsonian Center for Astrophysics keeps a list of asteroids it considers potentially hazardous, and can be found here.

A team of scientists led by Donald K. Yeomans at NASA’s Jet Propulsion Laboratory addressed some of these questions in a paper written for an international conference on protecting the Earth from asteroids, a meeting held in Spain in April 2009.

“Unless there are decades of warning time,” they wrote, “hazardous NEOs (near-Earth objects) larger than a few hundred meters in diameter may require large energies to deflect or fragment. In these cases, nuclear explosions, either stand-off or surface blasts, might provide a suitable response. For the far more numerous objects that are smaller than a few hundred meters in diameter, and provided sufficient warning time, a kinetic energy (KE) impactor spacecraft might be sufficient to deflect the hazardous NEO…”

But then the authors brought up another proposal requiring neither a nuclear explosion nor a collision. They suggested parking a spacecraft near the asteroid and, by the force of its gravitational pull, turning the asteroid’s course sufficiently to miss the Earth.

The Case of Apophis

“To illustrate some of these NEO mitigation issues,” they wrote, “we consider Apophis, a NEO that will make a very close Earth approach on April 13, 2029 (to within 5 Earth radii [about 32,000 kilometers] of the Earth’s surface). Although it is an extremely unlikely scenario, we will assume that Apophis will pass through a narrow 610 meter region in space (a “keyhole”), that would cause it to be perturbed by the Earth into a resonant return, complete 6 revolutions about the sun and collide with the Earth on April 13, 2036.”

Apophis has a mean diameter of around 167 meters, substantially larger than that of the Tunguska object although very much smaller than that of the Chicxulub asteroid.

“We then consider the deflection option,” the paper continued, “whereby a rendezvous spacecraft (S/C) is sent to Apophis several years in advance of the 2029 close Earth approach…. That is, the S/C could act as a gravity tractor (GT) in 2022 to slowly move Apophis away from the 2036 and secondary keyholes that are present at the time of the 2029 close Earth approach. At the same time, the tracking of the S/C would allow the orbit of Apophis to be refined to the sub-kilometer level so that a successful deflection via the GT could be verified.”

Yeomans and his colleagues calculated that the gravitational force of a spacecraft weighing half a ton, hovering near Apophis, could draw it out of the 2036 keyhole in only two months.

"By far the most important requirement of a successful mitigation campaign is a warning time sufficient to carry out the mitigation mission,” they concluded. “As a result, the most important aspect of mitigation is finding the hazardous objects many years in advance.”

Advance warning is equally important when it comes to funding mitigation efforts, according to Posner.  “Costs tend to be inverse to time. It would cost a great deal more to build an asteroid defense in one year than in 10 years because of the extra costs that would be required for a hasty reallocation of the required labor and capitol.”

Sensible preparation comes down to investment in telescopes, computers and, of course, astronomers. The rule is that the earlier the threat is apprehended, the less risky and less violent is the deflection strategy. There are many good reasons for humans to look deeply into space. Collision control is, as Posner says, the least widely discussed. But it is arguably the most urgent.

John W. Anderson is the journalist-in-residence at Resources For the Future. He is a former member of the Washington Post's editorial page staff.

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