Perhaps the most important issue to remember when interpreting our NEO
close-approach tables is the inherent uncertainty in each NEO's orbit.
This uncertainty is related to several parameters used in the orbit
determination process including the number of observations (measurements),
the time spanned by those observations (data arc), the quality of the
observations (e.g. radar vs optical), and the geometry of the
Of these parameters, the time spanned by the observations
generally has the greatest effect on the orbital uncertainty.
Even though the NEO orbits are uncertain, it is possible to estimate the
size of these uncertainties and place limits on the close-approach distances.
For example, we provide a nominal distance, and the minimum distance
(which is the minimum distance between the 3-sigma Earth target-plane error
ellipse and the Earth's surface).
We also note that Earth close-approach statistics for NEOs can change
(often by large amounts) as new data become available to update their orbits.