It’s good to see asteroid deflection occasionally popping up in the news, thanks to the efforts of people like former astronaut Rusty Schweickart, whose efforts as co-chairman of the Task Force on Planetary Defense of the NASA Advisory Council are complemented by his work for non-profits like the B612 Foundation. Schweickart is worried about the potential consequences of even a small asteroid impact, pointing to the Tunguska event of 1908, in which 800 square miles of Siberian forest were flattened in the kind of strike that occurs every 200 to 300 years.

Bigger asteroids are, obviously, a far greater danger, and while they’re much rarer, they do have the capability of wiping out entire species, as may well have occurred some 65 million years ago in the destruction of the dinosaurs. In his recent New York Times article, Schweickart notes what we need to do:

With a readily achievable detection and deflection system we can avoid their same fate. Professional (and a few amateur) telescopes and radar already function as a nascent early warning system, working every night to discover and track those planet-killers. Happily, none of the 903 we’ve found so far seriously threaten an impact in the next 100 years.

Nonetheless, asteroids demand a constant vigilance. Schweickart continues:

Although catastrophic hits are rare, enough of these objects appear to be or are heading our way to require us to make deflection decisions every decade of so.

A deflection capacity is something NASA needs to be looking at, and the report of the Task Force on Planetary Defense urges that financing for it be added to the NASA budget. Schweickart believes that $250 to $300 million, added annually over the next ten years, would allow our inventory of near-Earth asteroids to be completed and a deflection capability to be developed, after which a maintenance budget ($50 to $75 million per year) would keep us tuned up for potential deployment.

Underscoring the need for a deflection capability is the work of Elisabetta Pierazzo (Planetary Science Institute), whose forthcoming paper in Earth and Planetary Science Letters focuses on two impact scenarios, 500-meter and 1-kilometer asteroids hitting a 4-kilometer deep ocean. What Pierazzo finds is that an ocean strike could deplete the Earth’s protective ozone layer for several years, resulting in a spike in ultraviolet radiation levels that would, among other things, make it more difficult to grow crops (not to mention its effects on other life forms).

Pierazzo and team’s atmospheric simulations show a global perturbation of upper atmosphere chemistry, as water vapor and compounds like chlorine and bromide alter the ozone layer to create a new ozone hole. Adds Pierazzo:

“The removal of a significant amount of ozone in the upper atmosphere for an extended period of time can have important biological repercussions at the Earth’s surface as a consequence of increase in surface UV-B irradiance. These include increased incidence of erythema (skin reddening), cortical cataracts, changes in plant growth and changes in molecular DNA.”

Ultraviolet radiation intensity can be expressed by the ultraviolet index (UVI), which indicates the intensity of UV radiation at the surface, with the higher numbers tending toward damage to skin and eyes. While a UVI of 10 is considered dangerous, resulting in burns to fair-skinned people after short exposure, values up to 18 are occasionally recorded at the equator. The highest recorded UVI is 20, recorded at a high-altitude desert in Puna de Atacama, Argentina.

Modeling a strike by an asteroid that hit at latitude 30 degrees north in the Pacific Ocean in January, Pierazzo’s simulations show that a 500-meter asteroid impact would result in a major ozone hole, boosting UVI values to over 20 for several months in the northern subtropics. A 1-kilometer asteroid would drive the UVI in certain areas to a sizzling 56, while boosting UVI values over 20 within a 50-degree latitude band north and south of the equator for about two years. The affected band’s northern end would include Seattle and Paris, while its southern end reached New Zealand and Argentina.

“A level of 56 has never been recorded before, so we are not sure what it is going to do,” adds Pierazzo. “It would produce major sunburn. We could stay inside to protect ourselves, but if you go outside during daylight hours you would burn. You would have to go outside at night, after sunset, to avoid major damage.”

We always tend to depict asteroid impacts in terms of their direst consequences as a way of illustrating the magnitude of the threat. But it’s chastening to learn that even a survivable impact like those Pierazzo and team have modeled would create serious environmental damage even if loss of life could be prevented. All this assumes, too, an asteroid that strikes in the ocean (the most likely scenario). There’s no question that building up our planetary defense against such impacts is the best insurance we could create, stopping potential impactors before they near our planet.

The paper is Pierazzo et al., “Ozone perturbation from medium-size asteroid impacts in the ocean,” in press at Earth and Planetary Science Letters (abstract). Jeremy Hsu’s article on this work in LiveScience is excellent.