Near-Earth Asteroid (410777) 2009 FD is a potentially hazardous asteroid with possible (though unlikely) impacts on Earth at the end of the 22nd century. The astrometry collected during the 2019 apparition provides information on the trajectory of (410777) by constraining the Yarkovsky effect, which is the main source of uncertainty for future predictions, and informing the impact hazard assessment. We included the Yarkovsky effect in the force model and estimated its magnitude from the fit to the (410777) optical and radar astrometric data. We performed the (410777) hazard assessment over 200 years by using two independent approaches: the NEODyS group adopted a generalisation of the Line Of Variations method in a 7-dimensional space, whereas the JPL team resorted to the Multilayer Clustered Sampling technique. We obtain a 4-σ detection of the Yarkovsky effect acting on (410777), which corresponds to a semimajor axis drift of (3.8±0.9)×10 −3 au/Myr. As for the hazard results of both teams, the main impact possibility in 2185 is ruled out and the only remaining one is in 2190, but with a probability smaller than 10 −8 .
We propose a densification algorithm to improve the Line Of Variations (LOV) method for impact monitoring, which can fail when the information is too little, as it may happen in difficult cases. The LOV method uses a 1-dimensional sampling to explore the uncertainty region of an asteroid. The close approaches of the sample orbits are grouped by time and LOV index, to form the so-called returns, and each return is analysed to search for local minima of the distance from the Earth along the LOV. The strong non-linearity of the problem causes the occurrence of returns with so few points that a successful analysis can be prevented. Our densification algorithm tries to convert returns with length at most 3 in returns with 5 points, properly adding new points to the original return. Due to the complex evolution of the LOV, this operation is not necessarily achieved all at once: in this case the information about the LOV geometry derived from the first attempt is exploited for a further attempt. Finally, we present some examples showing that the application of our method can have remarkable consequences on impact monitoring results, in particular about the completeness of the virtual impactors search.
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