The path of the small asteroid (2011MD) which will just miss hitting Earth Monday. Notice how Earth’s gravity sharply alters the asteroid’s trajectory. See also fly-by animation from (NASA)

Asteroids coming this close cross through the zone of geosynchronous satellites (such as the GOES series), but the chances of a collision are extremely small, though not zero.

Small asteroids such as these are difficult to discover and, indeed, each of these latest occurrences was detected only within a week of the closest approach. Once identified, they can be readily tracked. According to NASA. there was never a chance either of recent near-misses was headed directly towards Earth

If they did make contact with Earth, it would not be especially unusual. Objects of this size would likely just produce a brilliant fireball with only small fragments landing in the ocean or crashing into the ground (hopefully not hitting anyone on the head). It’s estimated that there are nearly a billion objects this size or somewhat larger in near-Earth space. But on average such an object is expected to come this close to Earth about every 6 years.

Fortunately, asteroid strikes, especially those large enough to have disastrous consequences to life on earth (as, for example, the 112 mile wide asteroid which presumably led to the dinosaur’s demise of 65 million years ago), are few and far between. But no doubt they would be discovered and tracked years before any possible impact with Earth. Scientists consider asteroids as potentially hazardous only if their size is larger than 490 feet in width. A 900 foot asteroid is expected to pass within 18,000 miles of earth in 2029. Fear not, because the odds of it actually hitting the Earth are greater than 250,000 to 1.

Intermediate-size asteroids - larger than the latest two near misses - capable of serious damage to life and human dependent infrastructure, are a more frequent threat. But they too are likely to be observed and followed with long-lead times in the unlikely event they are aimed directly at Earth.

“Likely”, but not with complete certainty! On March 2, 2009 an asteroid about 200 feet wide buzzed by Earth only a few days after being discovered. Its closest approach was 40,000 miles, seemingly a safe distance, but a close call in astronomical terms. Had it struck the earth it would have exploded with the force of a large nuclear bomb on or just about the surface – with ONLY 3 days notice.

However, concern should not be overblown relative to, for example, the odds and consequences of a major geomagnetic storm. Even if an asteroid this size were headed directly towards earth with little warning, it’s more than likely it would explode somewhere over the large expanses of Earth’s oceans than some highly populated metropolitan area.

Post script: Although the chances of the Earth being struck by a large asteroid (or comet) are extremely small, the consequences would be disastrous. Last year a bill was introduced into Congress to establish a government-sponsored commission to study the danger of a collision with Earth and how ready we are to face the danger. The bill never became law. In a worst case scenario, I guess we can get Bruce Willis stand in to save the Earth as in the movie thriller, Armageddon.