Asteroid Eros
A close-up view of Eros, an asteroid with an orbit that takes it somewhat close to Earth, taken by NASA's Near Earth Asteroid Rendezvous mission in 2000. Credit: NASA/Johns Hopkins University Applied Physics Laboratory

The NASA inspector general (IG) recently excoriated the agency’s Near-Earth Object (NEO) Program, finding that the effort to locate potentially Earth-threatening asteroids and comets was poorly resourced and far behind its mandated detection goals. In 2005 Congress tasked NASA with locating 90 percent of NEOs 140 meters in size and larger, but currently only 10 percent of this population has been found. The IG concluded that program management and funding for the planetary defense effort were insufficient for the task at hand.

Unfortunately, the IG’s audit missed a much broader and far more important point: NASA is not looking for the right rocks.

Near-Earth objects are asteroids or comets that pass within 1.3 astronomical units — about 200 million kilometers — of the sun, displaced from the main asteroid belt, the Kuiper Belt or the Oort Cloud by gravitational perturbations or collisions with other objects. Over time, some of these objects strike Earth, as evidenced by the hundreds of impact craters that dot our planet. The Barringer crater in Arizona highlights the destructive forces in play: The 50-meter asteroid that struck there 50,000 years ago gouged a hole 1.2 kilometers in diameter and released the same amount of energy as a large thermonuclear bomb.

The size of asteroids is inversely proportional to the frequency in which they appear in nature. In our solar system there are fewer than a thousand NEOs greater than 1 kilometer in diameter, but more than 500,000 greater than 30 meters. While not civilization-killers, the smaller asteroids can wreak enormous regional devastation, as witnessed in the 1908 Tunguska air burst, which leveled more than 2,000 square kilometers of desolate Russian forest — all from the explosion of a body estimated between 30-70 meters in diameter. How many of the half-million Tunguska sized NEOs have been discovered? Half of 1 percent.

This is incredibly relevant since it is the smaller objects that hit Earth much more frequently than their larger cousins. Collisions from monster asteroids are exceedingly rare, with the average time between impacts measured in millions of years. However, a 10-meter-wide asteroid strikes Earth about once per decade and a 30-meter object every few hundred years. The probability that a Tunguska-sized asteroid will collide with Earth on any given day is five or six orders of magnitude more likely than the impact of a kilometer-wide space rock, and roughly 100 times that of a 140-meter object, which strike our planet about once every 30,000 years.

A strategy to search for the larger, “high consequence” NEOs would make sense if we could accept the damage caused by objects smaller than 140 meters. We can’t. In the century since the 10-megaton Tunguska blast, the world’s population has more than tripled, cities have spread into previously uninhabited areas, and coastal communities — vulnerable to tsunamis created by an ocean impact — have grown incessantly.

More troubling, mankind has assembled a massive arsenal of nuclear weapons, some of which are in the hands of unpredictable regimes. It is easy to see how a surprise air burst from a 20- or 30-meter asteroid could be mistaken for hostile military action, and horrifying to consider the consequences if such a natural event led to a retaliatory nuclear attack. The hair-trigger world of the 21st century is no place for an unexpected asteroid strike.

To reduce this risk, planetary defense activities should focus on finding all space objects that present a reasonable hazard, notionally anything larger than the 17-20-meter asteroid that exploded in 2013 over Chelyabinsk, Russia. The U.S. National Research Council and other advisory bodies have recommended enhanced efforts to find these smaller objects but neither Congress nor the Obama administration has adopted such a policy.

Policymakers need to set the bar high, with high meaning small. The good news is that searching for the smaller NEOs will yield discoveries of the bigger ones. The reverse is not true.

Today’s NEO search efforts are conducted primarily by terrestrial observatories, which are less effective in finding asteroids than telescopes stationed in space. The ideal is to orbit an infrared telescope around the sun, inside the trajectory of Earth and looking outward, to find the smaller space rocks that ground-based or Earth-orbiting telescopes will miss.

The B612 Foundation’s proposed Sentinel space telescope would be, in its Venus-like orbit, the most effective asteroid hunter ever employed. NASA’s developmental NEOCam telescope could be a close second, operating from the L1 Lagrange point. Unfortunately, neither spacecraft is fully funded and it remains to be seen if either will find its way into active service. We should hope they do.

It must be noted that NASA’s NEO Program has done commendable work with the extremely limited resources at its disposal. Through years of dedicated effort, it has located more than 11,000 NEOs, including 95 percent of the kilometer-sized asteroids that pass through the inner solar system. None of these mammoth rocks is expected to pose any threat to Earth over the next 200 years. It is reassuring to know that humanity will not be snuffed out anytime soon by a gargantuan asteroid like the one that eliminated the dinosaurs 65 million years ago.

What is deeply troubling, however, is the possibility that a smaller, undiscovered NEO could plow into our atmosphere tomorrow and rain destruction on an unsuspecting populace — and today there is little we are doing about it.

NASA leadership is culpable for not deeming the planetary defense mission a top priority: Placing responsibility over the entire effort in a niche component within the Science Mission Directorate — managed by a single program executive — dooms it to second-tier status, unable to harness all of the agency’s resources. Further, funding for asteroid detection, while having grown to $40 million per year, clearly is insufficient for the task at hand. Even more striking is the tiny amount of money allocated to the study of mitigation techniques for threatening asteroids, currently limited to only $1 million annually.

It’s been 106 years since Tunguska and the law of averages dictates that a similar-sized object will collide with Earth in the near future, perhaps in our lifetime. We have the technology to find these smaller objects and just need the political and policy wherewithal to make their detection and mitigation a priority.

An asteroid strike is a wholly preventable natural disaster, but only if we locate the full population of hazardous space bodies before one of them streaks unexpectedly from the depths of space and devastates part of our planet.

Now is the right time to find the right rocks.

Jim Howe is an independent policy analyst who works in the nuclear industry. He recently earned a graduate degree in Space Studies from American Military University.