The asteroid 9942 Apophis has been one of those celestial bodies that has captured the public's interest since it was discovered in 2004. It initially caused a brief period of concern that year because some initial observations of its trajectory indicated a significant probability, a 1-in-60 chance, that it would strike Earth some time in 2029.

Additional observations of asteroid Apophis, approximately the size of 21/2 football fields, ruled out any possibility of impact in 2029. However, the asteroid is expected to make a record-setting close approach to Earth on Friday, April 13, 2029, when it comes no closer than 18,300 miles above Earth's surface.

For comparison, geosynchronous satellites orbit at 22,300 miles. At closest approach, the asteroid will shine like a third-magnitude star, visible to the unaided eye from Africa, Europe and Asia. Close approaches by objects as large as Apophis are thought to occur at 1,000-year intervals, on average.

The asteroid's trajectory will bend approximately 28 degrees during the encounter in 2029, a result of Earth's gravitational pull. What happens next was uncertain. There was some early speculation that the asteroid might swing around and hit Earth after all in 2036.

Calculations in 2007 revealed there was a small chance (about 1 in 45,000) that the encounter with Apophis in 2029 would bend its orbit sufficiently, so that when it returns to Earth it actually hits it on April 13, 2036. Should such an impact arise, NASA estimates it could hit Earth with the equivalent energy of an 880 Megaton bomb!

NASA scientists have recently recalculated the path of Apophis. The refined path indicates a significantly reduced likelihood of a hazardous encounter with Earth in 2036. Updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036, for Apophis has dropped from 1-in-45,000 to about 4-in-1 million.

-- By Dan Malerbo,

Buhl Planetarium

and Observatory

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.