This page describes the limits placed on the Near Earth Objects
close-approach tables
and discusses the uncertainties in those data.

### Characterizing the Uncertainties

Perhaps the most important issue to remember when interpreting our NEO
close-approach tables
is the inherent uncertainty in each NEO 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 observations.
Of these parameters, the time spanned by the observations generally has the greatest effect
on orbital uncertainty.

Even though NEO orbits are uncertain, it is possible to estimate the size of these
uncertainties and place corresponding limits on close-approach distance and time.
For example, we provide a nominal distance, and minimum distance (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 a given NEO can change
(often by large amounts) as new data become available and the orbit is updated.

### Limits Applied to NEO Close-Approach Data

In an effort to include only close-approach data that are reasonably well determined and
for which the uncertainties are not overly large, the data presented in our NEO close-approach
tables are selected using the following conservative limits.
Specifically:

- only Earth close-approaches whose close-approach time uncertainties are
within
**10 days (3-sigma)**, and
- only Earth close-approaches from the portion of the NEO’s orbit where the
position uncertainties are within
**0.1 AU (3-sigma)**.

Although these limits could be refined, they are sufficiently conservative to exclude
any highly uncertain Earth close-approaches.