The object, designated 2004 FH, is roughly 30 meters (100 feet) in diameter and passed just 43,000 km (26,500 miles, or about 3.4 Earth diameters) above the Earth's surface on March 18th at 5:08 PM EST (2:08 PM PST, 22:08 UTC).

On average, objects about the size of 2004 FH pass within this distance roughly once every two years, but most of these small objects pass by undetected.

Clark Chapman of the Southwest Research Institute noted during Astrobiology Magazine's Impact Debates, that "estimates of the relative risks due to comets and asteroids of various sizes has changed little in the last decade.

Roughly 80 percent of the impact hazard is due to asteroids between 1 and several kilometers in diameter. About another 10 percent is due to an asteroid smaller than a kilometer striking the ocean and causing a tsunami. Roughly another 10 percent is due to comets, and less than 1 percent is due to small asteroids striking the land..."

JPL's manager of the Near-Earth Object Program Office, Don Yeoman, said "There are likely to be more than 300,000 that are about 100 meters long - larger in diameter than a football field. Because there are many more of them, they would be expected to hit Earth far more frequently than the near-Earth asteroids larger than 1 kilometer - once every few thousand years for the 100 meter asteroids, as opposed to every half million years for the 1 kilometer or greater asteroids...

"As more and more of these objects are found, the search for the missing ones gets tougher and tougher. ..it seems likely that we will have discovered 90 percent of the (near-earth asteroids) NEAs larger than 1 kilometer by sometime not much beyond the 2008 deadline."

This particular close approach was unusual only in the sense that scientists know about it. The fact that an object as small as asteroid 2004 FH had been discovered now is mostly a matter of perseverance by the LINEAR team, who are funded by NASA to search for larger kilometer-sized NEAs, but also routinely detect much smaller objects.

Fragments of Comet P/Shoemaker-Levy 9 colliding with Jupiter (July 16-24, 1994). Credit: NASA

The 30 meter object is about half that of the asteroid that struck Siberia in 1908. Alan Harris of Colorado-Boulder noted that, "The smallest impactor that can penetrate the atmosphere deep enough to cause any damage on the ground is not much smaller than the "Tunguska" bolide that flattened a couple thousand square miles of Siberian forest in 1908.

The area flattened is about equal to the area of the greater Washington DC area, inside the beltway. That asteroid was estimated to be about 50 to 70 meters in diameter."

Asteroid 2004 FH's point of closest approach with the Earth was over the South Atlantic Ocean. Using a good pair of binoculars, the object was expected to be bright enough to be seen during this close approach from areas of Europe, Asia and most of the Southern Hemisphere. Scientists looked forward to the flyby as it provided them an unprecedented opportunity to study a small NEA asteroid up close.

Harris concluded in the risk assessment from objects this size, "I have estimated that the frequency of Tunguska-type impacts worldwide is only about once in a thousand years. That's on the edge of implausible since one happened only a century ago, but I think anything more often than once a century is inconsistent both with historical records and with observations of NEAs in space."

As this is the current record holder for close approaches of this size, many might ask what kinds of mitigation remedies could be considered if rather than just passing between the Earth-Moon distance, it had a crossing orbit with the Earth itself. The mitigation of large objects is still speculative, according to scientists who have considered whether a 3-kilometer (giant) asteroid could be diverted or fragmented.

Cornell's Joe Viverka summarized some of the issues involved: "While a survey of objects 1 kilometer in diameter or larger can be carried out in a moderately short time, I think cataloging objects in the 100-meter category is much more important. First of all, these smaller objects have about a hundred times greater chance of causing mischief by interacting with Earth.

Also, it is more useful for us to worry about 100-meter objects, since we can imagine potentially effective and affordable defenses against such impactors. However, when it comes to bodies 1 kilometer in diameter - which on average will be one thousand times more massive - the idea of diverting them or blowing them up in the foreseeable future still borders on the fantastic."

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