South Jersey Skies: The odds of the asteroids

Keith Johnson By Keith Johnson South Jersey Newspapers
on March 03, 2013 at 6:00 AM, updated March 03, 2013 at 6:08 AM
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It's been more than two weeks since that amazing fireball flashed across the sky in Chelyabinsk, and its larger cousin coasted past our world later the same day. This might be a good time to reflect on the probabilities involved with these events.

It's certainly not the first time we've experienced celestial impacts. The first object we were able to discover and track before it arrived, a rock around 13 feet across, exploded over the Nubian desert in 2008 with an energy release of two kilotons of TNT.

Another rock of similar size blew up over Indonesia in 2009 at a power level of 50 kilotons. Neither of these events were well-reported by Western media.

The legendary object of 1908 — either an asteroid or a comet nucleus — leveled trees for miles around over Siberia. Like many asteroid impacts, it was an air-burst, with a power of 30 kilotons. It may have been five miles in diameter.

It's well-known that the dinosaurs, and much of the other life of our planet, were likely wiped out by an asteroid a few miles across 65 million years in the past.

The likelihood of our being hit with an asteroid in, say, the next hundred years depends very strongly on how big the object is that we're considering. All our probability estimates for big impacts are imprecise, simply because we can only judge by past events, and there have not been all that many of those for which we have accurate information.

We are getting better at watching for such threats. There are several asteroid-monitoring programs in operation now, including the SPACEWATCH program at Kitt Peak in southern Arizona ( and MIT's LINEAR (Lincoln Near-Earth Asteroid Research, program in New Mexico. We think we now understand the orbits of most of the so-called near-Earth asteroids (NEAs) that pass close to the Earth's orbit.

Most of these are small enough not to be too troubling. As of last August, we knew of just 848 NEAs with diameters greater than 1 kilometer (a little less than 2/3 of a mile.) Moreover, only 154 will approach our planet close enough to be classed as potentially hazardous (PHAs) at any foreseeable future time. We estimate that we've pinned down more than 90% of the objects that have even a faint chance of causing damage over the next 50 years, and none of these is a serious threat.

These are, however, just short-term predictions. Over the long run we know we can't escape destructive impacts from sizable objects. Here are some rough estimates of what to expect.

First, let's define what we mean by “destructive.” The biggest human-caused explosion was a Soviet test that took place in Novaya Zemlya in 1961, equal to 60 megatons of TNT. Wooden houses hundreds of miles distant were destroyed, and the mushroom cloud reached up to seven times the altitude of Mount Everest. We'll call that 1 NZ unit.

Our studies of asteroid sizes and orbits lead to the conclusion we can expect a space rock with a diameter of 130 feet or more to hit the Earth every thousand years on the average. This amounts to about one-sixth of an NZ unit. It would be sufficiently powerful to wipe out an average city.

A 600-foot asteroid would produce 4 NZs of destruction. Such an event is likely every 100,000 years; it would devastate any large city and a lot of surrounding countryside.

An asteroid half a mile across would generate 170 NZs of power, and can be expected to occur once every million years or so. Even though an object of that size is far smaller than the Earth, it would probably destroy human civilization, and perhaps kill most living things on our planet.

The impact would raise a thick cloud of dust that would migrate around the world, partially blocking sunlight and causing endless acid rain. Enormous fires would result from white-hot debris raining down onto the surface.

If it struck the ocean, it would raise monstrous, recurring tsunamis that would cause more damage than an impact on land.

Now that we have at least some knowledge of the danger, what can we do about it? We don't have any methods in place to protect us at present, but there are some things we might be able to do in the near future.

One thing we won't do is blow one up! A nuclear weapon used on an incoming asteroid would, to a large extent, simply split it up, converting it from one large predictable bomb into numerous smaller fragments that would strike the Earth over a wider area, probably causing more damage than the single rock.

We could fasten powerful rocket motors to the asteroid and shove it to one side. We'd have to start years before it arrived, and this would be an expensive and difficult project, but in theory it could be done in the future.

Some scientists have suggested other methods. A huge, thin mirror placed to one side of the asteroid would reflect sunlight onto one side of it; the pressure of the light would push it slightly to one side. Such a process would take years to cause sufficient orbital change, but it would probably be safer and cheaper than using a nuclear bomb.

Perhaps one of the most exotic methods is to spray the asteroid with paint! (Details are at

Two factors make this possible. 1. Asteroids spin around, as do all celestial objects, at various speeds. 2. When an asteroid is heated by the Sun, it gives off bits of infrared light (called photons) back into space.

More of these photons are emitted from the dusk side of the object, where the Sun is setting, than from the dawn side, as the dusk side has had all “day” to be heated by the Sun, while the opposite side has been in the dark. As a result, the emitted photons tend to exert a gentle push that is stronger in one direction than the other. The force is very small, but it's constantly working.

If we discover an asteroid that is heading for a collision with our planet, and we can measure how strongly its path is being influenced by this photon pressure, we might be able to alter that pressure, and save ourselves.

Suppose we were to spray silver paint all over the object. This would reflect sunlight, preventing the asteroid from being heated so strongly by the Sun; the asymmetrical pressure of the infrared photons would be reduced almost to zero, and the asteroid would fly by the Earth without striking it.

We don't have the capability of performing any of these tasks yet. But someday we may be saved from a cosmic collision by a giant can of celestial spray paint!
