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If this asteroid had hit Mars, it would have had about the same energy as the Siberian Tunguska impact on Earth in 1908. But since Mars has a much thinner atmosphere, this one would’ve plunged right into the surface and made a crater, a crater roughly the size of Meteor Crater in Arizona, which is a little less than a mile across. For Mars scientists that would’ve been a tremendously exciting opportunity, because so many of the questions that we ask about Mars require us to understand what is below the surface. We think about building drills to reach the subsurface, but it’s very difficult to do. Here we had the possibility that nature would excavate a hole for us and we could have looked at what was inside, looked at the ejecta, and perhaps seen a water lake form on the bottom of the crater. It would’ve been a unique opportunity.

NTB: Would that collision have had any effect on the planet Earth?

Dr. Morrison: No. The collisions that we think about – even something as big as the one 65 million years ago – are miniscule compared to the geology and astronomy of the planets. They have no effect on orbit, on spin, on axis tilt, on magnetic field, or anything else. A little one, like this one that might have hit Mars, wouldn’t even have an effect on the planet beyond a few tens-of-miles from the impact.

NTB: Scientists are currently studying a 300-meter diameter asteroid, called Apophis, that will pass within a few thousand kilometers of Earth on April 13, 2029, and could theoretically, depending on its trajectory, collide with the Earth on April 13, 2036. Is this something we should be seriously concerned about?

Dr. Morrison: We should seriously be concerned about the long-term risk from asteroid impact. Apophis is the example that we have in front of us now. It is not posing any danger on the close flyby on April 13, 2029, and there is only a very, very small chance that, subsequent to that flyby, it will find itself in another orbit, which brings it right back to hit the Earth seven years later. That is a chance that is not zero, and it is worth considering seriously. But it’s not an imminent threat. We are more at risk today from the objects we have not discovered than from the objects we have found that come very close to the Earth.

NTB: If such a collision did occur, what would be the consequences for life on Earth?

Dr. Morrison: Apophis is about 300 meters in diameter. That is below the threshold, we think, for a global disaster. That is, the problem would be confined near the area of impact and it wouldn’t plunge the whole Earth into some sort of impact winter as it’s sometimes called.

However, if you were anywhere within hundreds of miles of the impact point, you would have a very bad day. It would dig a hole several miles in diameter, and throw ejecta, and create hurricane-force winds and earthquakes… It would be enough to pretty much wipe out a small country or a small U.S. state.

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