The scientific community has devised an "impact risk scale" called the Torino scale. Zero is nothing to worry about; ten is a certainty of a global catastrophe; no known object has heretofore exceeded a one. 2004 MN4 is presently a four.
The energy released in an impact depends on the composition, density and actual size of the object. (The present size estimate is based on brightness, but if the asteroid is unusually light or dark in color then it could be smaller or larger.) The estimates I've seen are all larger than 1000 MT, which is something like the total yield for all of the weapons in the U.S. nuclear arsenal. The effects depend on where it would strike, but they would most likely be regional rather than global.
I must confess to mixed feelings about this. On the one hand, the prospect of disaster (what better term, etymologically speaking?) is distressing. The horrifying earthquake in Indonesia this weekend, and the devastating tsunami that has killed so many in southeast Asia, show what such events can mean. But for those of us who are fascinated by space, and who have followed the discussions of the impact hazard over the last decade and a half, there is also something in us that says, "Cool."
Actually, the danger from 2004 MN4 need not be so great. It will probably miss the Earth, and we will probably know that it will within the next few months. By the time you read this, our estimate of the risk may be very much less. On the other hand, if it does hit the Earth, we will have a generation to be ready for it. We will very likely know its impact point rather exactly, well in advance, so that we can make necessary evacuations. There will be many opportunities between now and then to send probes to inspect it and have a better idea of the asteroid's physical properties, to predict its likely effects.
We may even wish to consider modifying its orbit. My back-of-the-envelope calculations tell me that a delta-vee of only a few centimeters per second, accomplished many years before the coming impact, would be enough to ensure a miss. With an asteroid mass of around a hundred million tons, a fairly ordinary large rocket could do the job. The Space Shuttle solid rocket boosters each produce about 15 million Newtons of thrust for over a hundred seconds; a couple of these, attached to the asteroid and fired, ought to be enough. (Getting a couple of SRB's to an asteroid, that's much more of a problem!)
So we might harbor a secret hope that something like 2004 MN4 will be just what the doctor ordered to summon the political will in our society to invest in a serious spacefaring capability. We children of the Apollo age have waited three decades for the next steps in exploration that we once thought were only a few years away. Maybe a real, tangible threat will persuade humanity to head for the stars. This is not a very admirable attitude, perhaps, but it does account for some of the eagerness and excitement that such news brings.
But maybe there is something deeper going on here. And that will be the subject of my next post.