Skip to main content

How does NEOfixer assign priority?

NEOfixer assigns quantitative values to five fundamental sub-scores, in order to calculate priorities for each target:

  • Importance: How important is the object, from a planetary defense perspective?  This can be estimated by combining its size (using H magnitude as a proxy), how closely it can approach Earth (using the Minimum Orbital Intersection Distance, or MOID), and its impact probability.  Large NEOs with small MOIDs and elevated impact probabilities have the highest Importance; small and distant NEOs with no chance of impact have the lowest Importance.  Objects that are conclusively not NEOs (q>1.3) have zero Importance to NEOfixer.
  • Confidence: How confident is NEOfixer that an object exists, and/or the observations are properly linked?  This principally affects new submissions to the NEOCP.  Confidence is determined by the discovery program’s prior history of NEOCP submissions, and may also be influenced by reports of negative detections from other observers.
  • Benefit: What will be the expected improvement to an object’s orbit, if new observations are obtained?  NEOfixer estimates this by comparing the current sky-plane uncertainties to the expected astrometric accuracy of the potential new observations.  A hundredfold reduction in sky-plane uncertainty is roughly equivalent to a 100x improvement of our understanding of the orbit.  For two hypothetical objects that receive the exact same benefit from new observations, NEOfixer places a higher value on improving the more poorly known orbit (the orbit with the higher U parameter). 
  • Urgency: What is the cost to image an object now, vs. some other time in the near future?  Urgency increases if the object is becoming more costly to observe (due to uncertainty, Galactic confusion, or required exposure time), or decreases if the object will be easier to observe after tonight.  Urgency will also decrease if other follow-up observers report interest or success in targeting a particular object. 
  • Cost: How much telescope time is required to successfully image this object?  NEOfixer estimates the subscribing site will need to image the object three times, to SNR=5, covering the full +/- 2-sigma uncertainty region.  Estimated exposure times account for the site’s instrumentation, the target’s V magnitude, plus the sky brightness, airmass, and stellar confusion at the time and location of the target.  The number of pointings required to tile an uncertainty area is determined by the instrument’s field of view.

Final priority is estimated by multiplying Importance * Confidence * Benefit * Urgency, and dividing by Cost.  For convenience the score is then translated into a linear scale, from zero to about 10.  An object’s priority may change throughout the night as Cost, Benefit, or other values change, for example when new astrometry reduces the sky-plane uncertainties.  The priorities listed on NEOfixer’s main table correspond to each object’s maximum priority for tonight, where “tonight” is defined as the night currently in progress at a site, or the upcoming night if NEOfixer is consulted during the daytime.

Please note that while NEOfixer is developed and maintained by Catalina Sky Survey personnel and collaborators, there are no survey-specific weightings applied to any object.