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Of course, these calculations don't take into account the very slight changes in trajectory from each encounter with the Earth.

Actually, if you go the NEO website, and look under http://neo.jpl.nasa.gov/risk/doc/sentry.html you'll see that they do.

Not only do they account for the gravity of all planets, they account for the radiation pressure (solar/thermal) and uncertainties in albedo. Apparently the greatest uncertainty in future position is due to the effect of integrated radiation pressure! (One of the proposed schemes for deflecting asteroids is to dust them with talcum powder and modify their albedo! Not as dramatic as a thermonuclear blast, so no Bruce Willis movie, but likely more effective.)

I found it interesting that they use a Monte Carlo technique to assess risk. (Two thumbs way up!) This is expensive and slow and explains why it takes so long to produce results, but it's undoubtedly the most reliable way of making these calculations. [Where I work, we do real time numerical integration to propagate partial low earth orbits. For partial orbits we just use a "J2" (ellipsoidal) Earth gravity model. We propagate uncertainty by propagating the covariance matrix. Not as accurate as their technique. Comparison to "ground truth" indicates that this technique is far more accurate than real time orbital elements. (I.e., uncertainty is due to measurement errors, not model errors.)]

If you go to http://neo.jpl.nasa.gov/cgi-bin/db?name=2004+MN4 they provide an "orbit simulation" model. (You can pan/zoom, propagate and rotate viewing angle. It's way-kewl.) However this applet only propagates a Kelperian orbit, so it's kind worthless for "risk assessment". At the greatest zoom, the orbit of the moon is less than a pixel. It is good for visualization, however.

I couldn't help noticing that this object's orbit shadow's Venus' orbit around Heliocentric longitude 320 degrees (where the Earth is in middle August). You'll see it if you look. It's current orbital period is approximately 41/59 of Venus. If it's orbit were not constantly perturbed by the Earth, I would bet that it would "quickly" (within a few million years) be locked in a 2/3 resonance with Venus. Every time it gets close to the Earth that celestial harmony gets disrupted.

There is a chance that it is in a long term 2/3 resonance with Venus and that the Earth's perturbations merely cause an oscillation around the equilibrium point. Of course, if the asteroid and Earth swap paint at some point in the future, you can cancel the dance, to mix a metaphor. It never gets to close enough to Venus to pose a collision threat.