More formally, the impact probabilities computed by the software behind this visualization tool are based on observation error statistics, assigned by means of an astrometric observations error model based upon Gaussian (normal) distributions. Because the number of individual observations for each object is very small, the law of large numbers does not apply. Thus the actual errors of the observations included in a single tracklet, normally between 3 and 5, may not be a representative sample of the corresponding random variable, which is normally distributed.
In simple words, the probability for a single tracklet to have large errors is small, but not as small as detecting an imminent impactor (less than 1 in a million tracklets). Thus some apparent detections of imminent impactors can be spurious and there is no way to avoid this, short of abandoning the statistical error model and resorting to a careful human inspection of the image to reveal possible causes of degradation of the data.
NEOScan is a system dedicated to the scan of the Minor Planet Center NEO Confirmation Page (NEOCP). The goal is to identify asteroids as NEOs, MBOs or distant objects to confirm or remove from the NEOCP and to give early warning of imminent impactors, to trigger immediately follow-up observations.
- Scanning of the NEOCP every 2 minutes, with the immediate run of new cases or old cases just updated
- Computation and sampling of the Admissible Region using a 2-dimensional representation in the range/range-rate plane with a grid or a spider web (used when there exists a reliable nominal solution, it samples a neighborhood of the nominal range/range-rate by following the level curves of the quadratic approximation of the target function)
- Computation of the Manifold Of Variations (MOV), obtaining a set of virtual asteroids
- Propagation of the virtual asteroids in the future (currently for 30 days)
- Projection on the Modified Target Plane, searching for virtual impactors
- If virtual impactors exist, computation of the impact probability
We assign to each NEOCP object an integer flag related to
the computation of its impact probability. It is
called impact flag, and it depends on the impact
probability value and on the arc curvature, as shown in the
table below. We say that an arc has significant curvature if
χ2 > 10, where χ is the chi-value of the
geodesic curvature and the acceleration. The impact flag can
take the integer values from 0 to 4: a 0 value indicates a
negligible chance of collision with the Earth, whereas the
maximum value 4 express an elevated impact risk. It is
conceived as a simple and direct communication tool to
assess the importance of collision predictions and to give
the priority for the follow-up activities.
|0||Negligible||IP ≤ 10-4|
|1||Very small||10-4 < IP ≤ 10-3|
|2||Small||10-3 < IP ≤ 10-2|
|3||Moderate||IP > 10-2 and no significant curvature|
|4||Elevated||IP > 10-2 and significant curvature|
The sampling of the Admissible Region and of the MOV allow the computation of some scores for each object. The scores give us a first insight into the nature of the object, even though the asteroid were not a potential impactor.
The systematic ranging allows one to express the object orbital elements as a function of the range and the range-rate. Thus if we identify a class of objects through a property involving its orbital elements (e.g. q < 1.3 au for NEOs), each MOV orbit can be matched to this class and thus the class of objects actually corresponds to a subset of the Admissible Region. A numerical computation of the probability integral over this subset gives the probability of the object to belong to the given class.
NEOScan currently computes five scores: the probability to belong to different classes (NEO, MBO, DO, and SO) or to be on a geocentric orbit. The definitions of the classes are listed below.
- Near-Earth Object (NEO): q < 1.3 au
- Main Belt Object (MBO): (1.7 au < a < 4.5 au and e < 0.4) or (4.5 au < a < 5.5 au and e < 0.3)
- Distant Object (DO): q > 28 au
- Scattered Object (SO): anything else