Odds of asteroid 99942 Apophis striking Earth slightly higher than thought

An astrophysicist at the University of Western Ontario finds that the odds of the asteroid 99942 Apophis striking Earth are slightly higher than previously thought. In his study, published in The Planetary Science Journal, Paul Wiegert took a new look at the possibility of a smaller asteroid striking Apophis and changing its trajectory to a collision course with Earth.

In 2004, astronomers first observed an asteroid now called Apophis—the object was notable because of its high placement on the Torino impact hazard scale. After noting its size and trajectory, researchers ranked it as 4, making it the highest-rated asteroid.

Still, the rating was low enough to rule out the possibility that it would strike the Earth. Instead, scientists labeled it “one to watch” in the coming years. The asteroid is set to pass close to Earth in 2029 and then again in 2036 and 2068.

For this new study, Wiegert considered the possibility of a smaller object striking Apophis and changing its course slightly. What were the odds, he wondered, of such a strike putting Apophis on a collision course with Earth?

To find that answer, he began by noting that differently sized objects moving at different speeds would lead to different changes in course. He found that an object as small as 0.6 meters across could be big enough to knock the asteroid into a collision course sometime past 2029. He also found that an object just 3.4 meters across could strike with enough force to push Apophis into a collision course with Earth by 2029.

Wiegert then looked for a way to make an estimate of the number of objects that could potentially strike Apophis—he used the number of objects 3.4 meters across that strike the Earth each year as a reasonable guide. He then used that number to calculate the odds of such an object striking Apophis and found it to be approximately 10^–8, which is, of course, low.

Noting also that such a collision would have to be at just the right angle, he suggests the odds of just the right hit are likely 1 in 2 billion. And he estimates the chances of such a collision to result in a later impact to be 1 in 1 million.

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