At 13:40 UT (that’s 7:40 a.m. CST) on March 2, the near-Earth asteroid 2009 DD45 zipped past Earth at a distance of only 0.00048 A.U. Considering that the average Earth-Sun distance is 1 Astronomical Unit, 0.00048 A.U. works out to be a mere 45,000 miles! That’s a close shave by anyone’s standards. The Moon’s average distance is 240,000 miles and the geosynchronous satellites monitoring our weather and blanketing Earth with a global positioning and communication network orbit the earth at 23,000 miles. Having an Apollo group asteroid careen past the planet at twice the distance of the geosynchronous satellites is certainly something to note.
This kind of celestial fly-by is not entirely unprecedented, however. In 1972 a small asteroid (or meteoroid, if you prefer) skimmed through Earth’s upper atmosphere as if it were a stone skipping off a pond. That object entered the atmosphere over the Northwest United States and exited somewhere over Canada. Estimated to be about the size of an SUV, an impact or airburst of an rock this size would have been very impressive. Not the kind of event that would spell doom for civilization, but enough to get your attention. The most recent event of this type was the Tunguska blast over a remote area of Siberia in 1908 when a loose-aggregate meteoroid, or perhaps a comet fragment, entered the atmosphere and exploded in the upper atmosphere. Trees were stripped of branches and felled for many square miles around the spot directly below the blast. On a bit larger scale, anyone standing in the desert Southwest 50,000 years ago might have been in for a shock when a 100-foot meteoroid slammed into the ground (becoming a meteorite in the process) and created the Barringer meteor crater. At over 3/4 mile in diameter and nearly 600 feet deep, Barringer crater is the best preserved example of what happens when big rocks cross Earth’s orbit.
Thought to be about the same size as the Tunguska event’s object and just bit smaller than the meteorite responsible for the Barringer crater, 2009 DD45 is over 100 feet in size and could have potentially unleashed many dozens of times the explosive force of the atomic bombs used at the end of World War II. Sleep well.
For further information: 2009 DD45 orbital parameters from NASA’s Jet Propulsion Laboratory (JPL) and a movie on YouTube showing the asteroid’s fly-by.