The Success of Spaceguard

For years, many scientists warned of the dangers that asteroids pose to life on Earth, but for many years they weren’t listened to. Even after it was first proposed, in 1980, that the dinosaurs were killed off by a huge asteroid striking the Yucatán Peninsula in Mexico,1 there was, in the words of leading astronomer Clark R. Chapman, a “giggle factor” associated with the risk from asteroids.2

This all changed in 1994 when comet Shoemaker-Levy 9 thudded into the side of Jupiter with the force of three hundred billion tonnes of TNT, equivalent to 125 times the world’s nuclear arsenal.3 One of the Shoemaker-Levy fragments left a scar on Jupiter twelve thousand kilometres across, about the size of Earth.4 David Levy noted that the comet that he codiscovered “killed off the giggle factor.”5 The impact made headlines across the world.6 In 1998, two blockbuster films, Deep Impact and Armageddon, explored how the people of Earth might respond to a huge approaching asteroid. Scientists commended Deep Impact for its understanding of the impact threat and the realism of its special effects, which reflected the input of a fleet of technical advisers that included Gene Shoemaker, whom the comet Shoemaker-Levy was named after.7 (Armageddon, in contrast, was described by Clark Chapman as “scientifically and technologically preposterous in almost every respect.”8)

Due to increasing interest from the public and advocacy from scientists, in 1998 Congress tasked NASA with finding 90 percent of all near-Earth asteroids and comets larger than one kilometre within a decade.9 The effort would, with due acknowledgement to Arthur C. Clarke, be called Spaceguard.10

Spaceguard has been a huge success. We have now tracked 93 percent of asteroids larger than one kilometre and found more than 98 percent of the extinction-threatening asteroids, which measure at least ten kilometres across.11 Prior to Spaceguard, the estimated risk that Earth would be hit by an extinction-level asteroid was around one in two hundred million per year.12 We know now that the risk is less than one in fifteen billion—one hundred times lower.13