If the near-Earth asteroid 99942 Apophis ever strikes Earth, how big would the crater be?

Apophis and Earth in 2029
On Friday the 13th in April 2029, a 1,000-foot-wide asteroid named Apophis will pass close enough to Earth (within 20,000 miles) to briefly appear as a 3rd-magnitude star in the night sky.
Dan Durda

Apophis (formerly 2004 MN4) ranks at the top of the worrisome-asteroid list. On April 13, 2029 (yes, that will be a Friday the 13th), it will pass only 30,000 kilometers (18,600 miles) from Earth’s surface (S&T: May 2005, page 16). Dynamicists have noted that if the asteroid swings by in just the wrong way, Earth’s gravitational nudge could set it up for an impact in 2036. So what would happen if Apophis hits, say, Switzerland three decades from now? The dimensions of an impact crater depend on the impactor’s size, density, speed, and entry angle. S&T featured an article and BASIC program in the November 1996 issue (page 90) that explored crater sizes. Similar calculations can be made online at www.lpl.arizona.edu/impacteffects.

NASA’s Near Earth Object Program (http://neo.jpl.nasa.gov) offers characteristics of Apophis to use in the calculations: diameter of 320 meters, density of 2,600 kilograms per cubic meter, and speed of 12.6 km per second. Assuming an impact angle of 45°, we discover that the strike will create a crater about 2 km across and about 0.5 km deep.

— Stuart J. Goldman


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