1. The 2027 encounter with 1999 AN₁₀

The asteroid 1999 AN₁₀ was discovered by the LINEAR telescope on 13 January 1999 [MPEC 1999-B03]. The discovery was somewhat unusual in that the declination was $+70^\circ$. The nominal orbit computed by our online information service [NEODyS], that is the solution of the least squares fit to 94 observations (with one outlier removed, RMS of the residuals 0.59 arc-sec), is as follows (J2000): a=1.458432 AU, e=0.562093, $I=39^\circ\!.932$, $\Omega=314^\circ\!.556$, $\omega=268^\circ\!.255$, $M=321^\circ\!.958$, for epoch $JD\; 2\,451\,206.805$. The absolute magnitude is estimated at $17.9\pm 0.5$; given that the albedo is unknown, this object could be between 0.5 and 2 km in diameter. The ascending node is only $0.000\,25$ AU closer to the Sun than a point where the Earth is in early August; the descending node is $0.004\,78$ AU inward from a position of the Earth in early February. This means that, whenever the asteroid and the Earth are in phase at each node, close approaches are possible. Indeed a close approach is possible in August 2027. To analyse the 2027 encounter, we need to consider not only the nominal solution, but also all the solutions compatible with the observations, that is resulting in residuals which are not much larger than the ones of the nominal orbit [Milani 1999]. The region of these compatible solutions can be approximated by an ellipsoid of confidence, which, for a $\sigma$ value up to 3, contains solutions with RMS of the residuals up to 0.63 arc-sec.

The nominal solution undergoes a close approach in August 2027 with a minimum distance from Earth's center of $0.025\,7$ AU. The plane normal to the geocentric velocity at closest approach is the Modified Target Plane [Milani & Valsecchi 1999]. The hypothetical objects filling the confidence region evolve along a bundle of orbits; their intersections with the MTP define the confidence region of the encounter. If the Earth is not touched by this confidence region, then a collision can be ruled out. The confidence regions are very thin, the width being only $2\,600$km, and 0.42 AU long. Thus the occurrence of a very close approach is not very likely: the true orbit could be anywhere along a very long line, including long stretches corresponding to very shallow encounters.

In conclusion, the 2027 encounter could be a shallow approach, or could be, with a low, non-negligible probability, very close. In any case it cannot result in an impact. But the case for a possible dangerous encounter is not closed after 2027; indeed, it is just opened.