Uncertainty Parameter U

In order to quantify the uncertainty in a perturbed orbital solution for a minor planet in a concise fashion, the Minor Planet Center (MPC) has introduced the U parameter.^[1] This is an integer in the range 0 to 9, where 0 indicates a very small uncertainty and 9 an extremely large uncertainty.^[1] In practice, U is rarely larger than 6.^[1]

The JPL Small-Body Database Browser lists the MPC "Uncertainty Parameter U" as the "condition code".^[2]

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 the orbital uncertainty.^[3]

Objects such as 1995 SN₅₅ with a condition code (Uncertainty Parameter U) of E are considered lost. 2004 BX₁₅₉ with a very short observation of 3 days and an Uncertainty Parameter of 9 has a large uncertainty region that creates a cumulative probability of impact of approximately 1 in 2.1 billion.^[4] 1994 WR₁₂ has an Uncertainty Parameter of 8, and the next good chance to observe the asteroid may not be until November 2044 when the uncertainty will allow it to pass somewhere between 0.03–0.19AU from Earth.^[5]

References