Planetary Defense Is Worth It

I will always be a strong supporter of a strong planetary defense against comets and asteroids. These "Black Swan" events are rare, but the consequences of even one of them striking Earth and doing great damage would be immense.

If they are headed towards Earth's surface and too large for our atmosphere to provide an adequate defense against, they are always the "bad guy."

There are estimated to be about 45 undiscovered mass extinction class near Earth asteroids, about about 13,750 undiscovered "city killer" sized near Earth asteroids (that would destroy everything in a whole metro area of a major city, if it hit one, killing millions of people), and about 214,000 Hiroshima blast sized near Earth asteroids out there. These asteroids could strike us with short notice. The asteroids wouldn't create the radiation blast and contamination of a nuclear bomb (or highly toxic chemical weapon), but would have comparable kinetic impact to a nuclear bomb.

We need both better asteroid detection technologies (which NASA's new NEO Surveyor mission which is scheduled to launch in Septemer of 2027, twenty-one years after it was proposed, will profoundly improve) and better asteroid interception technologies that can respond to incoming asteroids and comets on short notice (i.e. many hours to several days), which is not currently in place, although rudimentary and ad hoc ICBM responses could be devised that could mitigate the harm if done right (but could make things worse if it diverts a large fragment or cluster of fragments from an uninhabited area towards a city).

Image via Wikipedia.

The NEO Surveyor mission is projected to:

• Find 2⁄3 of Asteroids Larger than 140 Meters in Diameter
• Assess the Overall Threat Posed by Potential Earth Impactors
• Assess the Impact Threat Posed by Comets
• Determine Orbits and Physical Characteristics of Specific Discovered Objects

140 meters in diameter is a commonly used threshold for a "city killer" asteroid and would have an explosive energy twenty-times that of the Hiroshima A-bomb, and more in line with a never used H-bombs in the U.S. and Russian nuclear arsenal. 

If two-thirds of the 140 meter or larger near Earth asteroids were found, that would be about 5,000 of the estimated 13,750 undiscovered asteroids of that size in the first five years of the mission which is planned to run for twelve years.

It will also probably find at least a significant number of undiscovered global distinction sized asteroids of the estimated 45 that remain undiscovered, and a significant percentage of the estimated 214,000 undiscovered Hiroshima bomb impact class asteroids (of which only an estimated 7% of which are now discovered), although probably far less than two-thirds of them.

NEO Surveyor will also refine our estimates of how many large asteroids remain undiscovered despite this mission, and of the risk that these undiscovered asteroids could strike inhabited places on Earth. With more accurate estimates of known asteroid trajectories, more known asteroids which come close to Earth but are not on track to impact it could be ruled out as threats allowing surveillance of potential threats to be narrowed to those objects that really are something to worry about.

We are sufficiently technologically advanced at this point to build adequate planetary defenses, but organizing the planet to fund and deploy them is another matter. The NEO Surveyor mission cost $500 million to $600 million, which is less than a single major U.S. warship or Air Force long range bomber, or several start of the art U.S. Air Force fighter jets. Military missile defense technologies (like the "Golden Dome" plan for the U.S.) can be adapted for planetary defense purposes, to some extent, but aren't optimized for this task.

The good news is that the many millions or more smaller asteroids and space junk objects mostly burn up in the atmosphere before hitting Earth with enough damage to be much more dangerous than a random and rare lightning strike. So, we harm from not tracking or intercepting them is modest.

Also, a huge share of the Earth's surface is made up of oceans, large lakes, uninhabited ice scapes, thinly inhabited mountains and deserts, and thinly inhabited rural areas or wilderness. A "city killer" or smaller asteroid has a quite localized danger zone and isn't radioactive or toxic for the most part. 

So, while it might kill or injure a small number of people in the immediate vicinity of the impact, and might trigger a significant tsunami or landslide (urban areas are disproportionately coastal), about 71% of Earth's surface is water, about 28% of Earth's surface is nearly uninhabited or rural, and only about 1% of Earth's surface is urban. So, most "city killer" or smaller extraterrestrial impacts would lead to fairly modest casualties. But the risk of mass casualties from an extraterrestrial impact is much greater now than it was for most of human history, because the world's population has grown so much and is concentrated in cities.

This analysis assumes that all locations on Earth are equally at risk, which isn't strictly speaking true, but is a close enough approximation of the reality to be a useful starting point.

While there have been only a handful of attested really big extraterrestrial impacts in human history, and only a few more than we have clear evidence of in the last 300,000 years or so when the human species came into being, there have probably been hundreds, if not several thousand that went unnoticed because they landed in uninhabited areas, or left no survivors in thinly populated areas and didn't leave any traces clear enough to attested to their impacts geologically before their impacts were eroded away.

More than half of the “city killer” asteroids that might threaten Earth remain undiscovered. With an infrared eye, NASA’s NEO Surveyor aims to find them.

Image from the linked article in Science magazine.