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INTRODUCTION__

The menu on the left contains new items, including **Past
Impactors** and ** Imminent Impactors**. By past Impactors we
mean the cases so far of asteroids discovered by telescope surveys and
then impacted on the Earth. All the cases were small asteroids
(absolute magnitudes H between 30 and 31) discovered less than one day
before the impact upon Earth atmosphere.

Thus, at the time of discovery, these objects were Imminent Impactors: by this name we indicate objects which could impact a short time, at most a few weeks, after being first detected. The question is how to systematically search for these events, which are expected to occur an order of magnitude more often than the available detections (twice per year rather than once in five years).

We of the NEODyS consortium have decided to contribute to this search by making available a tool, characterized by a visual approach (that is, results are described also as plots, not just as numerical results). This tool automatically scans the Minor Planet Center NEOCP (NEO Confirmation Page) every 2 minutes and downloads the new/updated observation files of all objects posted there.

For each new/updated observations file, we compute the attributable and the Admissible Region (of Solar System orbits) in the plane with coordinates range and range-rate (from Earth). This plane is sampled with an adaptive algortihm, using either a square grid or a spider web, to represent all possible solutions compatible with the observational data. These solutions are fit to a Surface of Variations of alternate orbits, each one of them endowed with a probability density.

Leaving aside the technicalities, the fact is that by this method, for each portion of the Admissible Region, we can compute a probability that the object as observed belongs to the corresponding region in the space of orbits. This allows to identify the prioritary objects to be followed up by astrometric observation, either because they can be Imminent Impactors, or because they have a significant probability of being interesting, e.g., as NEO, or belonging to some other peculiar orbit type.

Besides the development of the mathematical theory, on which we have an especially long experience, the main problem was how to test this system. Indeed, the system has been under development for a long time, and only recently we could be convinced that our algorithms were complete at a satisfactory level of mathematical rigour: the first tests were perfomed on the Past Impactors 2008 TC3 and 2014 AA, and our algorithms performed well in both cases. However, this a posteriori verification was not considered enough and we had to wait for a real case, real time test.

Then we had the case of ZLAF9B2 (NEOCP name), later renamed 2018 LA. Although our system was not made public (considered to be in alpha-test), it worked very well, as you can see from the ZLAF9B2/2018 LA imminent impactor page.

Thus we have decided to make our Imminent Impactors system public, althouh it can and should still be improved (it is considered to be in beta-test). We intend to continue the development of this service, adding new information, again mostly in graphic form. In organizing our development work, we shall use also the suggestions and comments that the users will propose.

Reference: Spoto *et
al.* (2018).

There are few cases of asteroids with very good orbit detrmination for which even the minute push from non-gravitational perturbations is enough to change the impact risk, especially for long-term hazard analysis. To handle these cases, we have developed the software OrbFit version 5.0 and later. It handles the Yarkovsky non-gravitational effect as a model uncertainty, that is it solves for 7 parameters instead of 6. This applies both to the orbit determination (at the initial epoch) and to the propagations used in the impact monitoring procedure.

Currently there are four NEAs on our risk pages with this kind of more advanced impact probability computation. They are listed as Special cases, because of the somewhat different algorithms used.

The first is (99942) Apophis, which formerly had several possible impactors, but now has a main impactor in 2068 with a probability which cannot be properly computed unless the Yarkovsky non-gravitational effect is taken into account.

The second case is asteroid (410777) 2009 FD. Before the 2019 apparition it was not possible to determine the amount of Yarkovsky effect from the observational data and the orbital uncertainty was large enough to make possible impactors in year 2185 and later. The observations collected during the 2019 apparition allowed a 4-sigma detection of the Yarkovsky effect, which in turn ruled out the main 2185 possibility (see Del Vigna et al. (2019)).

There are two other cases on which the Yarkovsky effect is critical in making possible impacts: (101955) Bennu, for which the Yarkovsky effect is determined with a very large signal-to-noise ration (about 200), and (29075) 1950 DA, for which the impact possibility occurs in the year 2880.

We do not know if there are other possible impactors beyond the time limit of our current operational impact monitoring, which is now fixed at 100 years from the date of the computation. Progress in extending this time limit will need significant theoretical and computational advances.

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