Why aren’t earth-crossing asteroids a priority?

Risk management processes generally rank risks in priority order according to the product of their probability and their impact. High probability risks that would have very high impact if they occurred go to the top of the list – perfectly reasonable. But relatively low probability risks which would have truly catastrophic consequences if they occurred usually rank fairly high as well, which is why so many redundant systems are built into spacecraft and airplanes.

Now given the various crises which the world faces these days, from economic meltdowns to terrorism to wars to pandemics, I can understand why politicians put asteroid tracking well down the list.

However, we may be deluding ourselves as to how low the risk of an asteroid impact really is. A few days ago (Monday, March 2) asteroid 2009 DD45 missed the earth by just about 50,000 miles. It was about 100 feet in diameter, and at the speed it was travelling would have completely leveled 500-1000 square miles if it had hit. And we spotted it coming only about a week before it passed.

We already know that asteroid 99942 Apophis will (we hope – 99.7% probability) miss earth by only about 20,000 miles in April of 2029, and if it passes just right, earth’s gravity might deflect its orbit so that it may return and impact earth in 2036 (current estimate, about 1 chance in 5000). An impact from this 25 million ton asteroid, travelling at about 28,000 mph, would really cause a stir, though it wouldn’t be quite enough to cause another mass extinction. Those of you who studied physics may recall that the kinetic energy KE = 1/2 (mv**2) – the effect of velocity v is squared, so even a small object going 28,000 mph has enormous kinetic energy. A 25 million ton object going that fast has a gargantuan amount of kinetic energy.

As of March 1, 2009, we have found 1031 potentially hazardous earth-crossing asteroids, and the evidence suggests we have only found a small fraction of them. One estimate is that the entire population of Earth-crossing asteroids includes some 1500 larger than one kilometer (mass extinction size) and 135,000 larger than 100 meters in diameter (enough to devastate an entire state or small country).

Now it is well within our technical capacity to nudge most of these asteroids enough to shift their orbits so they miss the earth, if we can find them far enough ahead of time. For example, the orbit of 99942 Apophis discussed above could be shifted enough by hitting it with a 1-ton object at 5000 miles per hour – well within the capabilities of our current interplanetary spacecraft – so long as we did it some years before its impact.

We also know from fossil evidence that mass extinction-sized objects (typically assumed to be 1 kilometer or larger in diameter) hit the earth on average about once every 65 million years, but of course these are random events, so the next one could hit tomorrow or in 100 million years. Smaller ones, say 30 meters or so in diameter (city-killing size) hit about once every 100 years.

Given all this, one might assume the government might want to budget a reasonable amount of money to fund search telescopes on earth and in orbit to find as many of these dangerous asteroids as possible as soon as possible. In fact, the government has provided minuscule amount of funding for this activity. At a time when we are dumping trillions of dollars into bad banks and bankrupt auto companies, we are budgeting only a few tens of millions of dollars on surveys of near-earth orbiting asteroids – far less money than American’s spend on chewing gum or the government spends on paper clips and staples.

Maybe we have our priorities wrong!