Algorithms used in the NEO Coordination System

Algorithms used for the Priority List

The algorithms used for the Priority List were developed from May to October 1999. Most source information comes from the NEODyS database. A few testing campaigns were also conducted with a few observers in order to study:

i) the intrinsic difficulties to follow some objects;
ii) additional factors, even "psycological", that discourage observers from following targets under certain circumstances.

In order to build a prioritized list all these algorithms were put together and appropriately weighted in September 1999. A slight change occoured the following month, just before the DPS meeting in Abano Terme, Italy, where the Priority List was briefly presented.

The Priority List classifies the need to observe Near-Earth Asteroids into four valid categories: urgent, necessary, useful and low priority. This service focuses on newly discovered objects to ensure that they could be recovered at the next apparitions. Comets are not included at this time, but specific campaigns may be developed if there is a need for.

Objects brighter than magnitude 22.0 V and at least 40 degrees of elongation from the sun are eligible for this list according to this PLOT. It defines the End of Visibility (EoV) of the target.

The distinction of the category (CAT) is made according to the following scheme:

Priority CAT min CAT max
URGENT 16.00 >
NECESSARY 2.00 16.00
USEFUL 0.40 2.00
LOW PRIORITY 0.08 0.40
(not considered) < 0.08


* CAT is given by the following equation, consisting of five terms:

CAT = UN * OC * MUR * EV * VP


* UN is the 3-sigma ephemeris uncertainty and is expressed in arcsec ("). It usually varies from a fraction of arcsec to many arcminutes. UN has the potential to be the most heavy factor in the calculation of CAT. NEAs observed in the course of two ore more oppositions, but not numbered yet, will eventually be included when UN becomes significant.

* OC is a decreasing function of the Earth's Minimum Orbital Intersection Distance (MOID) and stands for Object Class. It usually varies from a minimum of 0.4 - 0.5 to a maximum of 3 and even of 5 on some rare cases.

* MUR is the Maximum Uncertainty Requirement . Using a relationship between ephemeris uncertainty and magnitude of the object, MUR indicates the difficulty in recovering it at the next opportunity. For example, MUR = 1 for an object of magnitude 19.0 V, corresponds to a UN of 1 degree. MUR varies from a minimum of 0.3, for objects with secured orbits, to a maximum of 5.0, in case of targets still considered lost. We imposed min and max values of MUR in order to prevent it from becoming too relevant in the final counting of CAT.

MUR can be represented by the following four plots: plot-1, plot-2, plot-3, plot-4.

* EV has a close relationship with the object's End of Visibility (EoV) . This is a very important factor for the determination of CAT and is istantaneously determined by the visibility conditions of the discovery apparition. It describes the intrinsic difficulties of the geometry of the apparition and the likelihood of getting sufficient follow-up coverage for the determination of a good orbital solution. It is mainly a function of the rate at which the object's magnitude and solar elongation decrease. EV varies from a minimum of about 0.3 to a max of 2 - 3.

* VP stands for the Visibility Period. This contribution was introduced in order to give more weight to those targets approaching EoV, because there is limited time left for further observing. We have written a simple function, VP, which varies from 0.3 to a maximum of 1.0. VP = 1.0 is reached, for every object, 6 days before its EoV. We consider 6 days a safe margin when the circumstances require to deal with bright moonlight. VP is often the least effective parameter in the calculation of CAT.

Additional notes

* From late October 1999 to early September 2000 we have been using an additional parameter, that was taking into account the influence of the Milky Way. Unfortunately, we have not been happy with that. We found very difficult to find an appropriate algorithm that could take into account the trouble caused by the Milky Way. This is due to the very irregular shape of the Milky Way, dark clouds and so forth. The current policy is to leave the observer to deal with that in order to find the best opportunities for observations.

* The size of the object is NOT counted in this calculation for a simple reason: although the hazard posed by bigger NEOs is much more serious than the one posed by smaller targets, the big objects are much easier to follow. The choice to ignore the object's size is simply a matter of compensation.

This document was updated on March 30, 2001 by Andrea Boattini and Germano D'Abramo