In Defense of Earth[1]

The Tunguska event in 1908 (damage shown in the image at left) ranks among the most powerful explosions in recorded history. Luckily, the meteor exploded in the air over a remote region in Siberia.

Asteroids are multidimensional space attractions with facets that appeal to scientists, explorers, entrepreneurs, and the wider public. And among all these groups, much of the discussion of late comes from the crowd (of which Rusty Schweickart is a part) concerned with public safety—protection from asteroid impacts, or planetary defense.

Most of our focus has been on the long-term potential for impact prediction and deflection. This challenging but achievable capability depends on using powerful telescopes to find asteroids in space, calculate their future locations, and change their arrival time slightly if they are on a path that would intersect with Earth. We can literally prevent future impacts.

But more recently we discovered that even a set of small telescopes, like the Asteroid Terrestrial Impact Last Alert System (ATLAS), can see asteroids when they’re very close and about to hit. This first happened in October 2008 when a Catalina Sky Survey telescope picked up a small asteroid in the evening sky that actually hit Earth 19 hours later! Discovering it even that close to impact allowed NASA’s Near-Earth Object (NEO) Program to analyze its trajectory and predict precisely when and where it would hit.

It quickly became evident that a short-term (or last minute) warning system for asteroid impacts was possible. Planetary defense suddenly had two strategies; long-term prediction and prevention, and short-term civil defense. “Duck and cover” re-entered the lexicon—or, with just a few hours’ warning, evacuation

Interestingly, this short-term strategy to avoid impact threats to life (albeit not to property) suddenly put NEO programs on the radars not only of the civil defense systems of the world, but also of the general public. Unlike the long-term impact prevention aspect of planetary defense, where the public is a largely unwitting beneficiary, here the public is an active participant in evacuation and preparation. In fact, success depends on the public responding rationally to a threat completely outside their experience.

Who warns them? How are they warned? Duck and cover or evacuate? How does the identification of a moving spot in a small telescope’s field of view get out as news to real people in time to save lives? These questions and many more were addressed as part of Asteroid Day on June 30, 2015, an event whose goal was to familiarize the public with this unfamiliar threat and how to respond. (The next Asteroid Day will be June 30, 2017. See

It is truly amazing that with inexpensive technology available right now, we can prevent almost all of the potential loss of life from asteroid impacts, both long- and short-term. We are not dinosaurs, nor part of the 70 percent of life that was wiped out with them 66 million years ago. We have the tools and can act instead of merely observe. We can do this. (Or can we?)

[1] See Rusty Schweickart, “In Search of Death Plunge Asteroids,” Astronomy (43, 7, July 2015), pp. 28-33


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