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. This is an integer in the range 0 to 9, where 0 indicates a very small uncertainty and 9 an extremely large uncertainty. In practice, U is rarely larger than 6.
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 (observation 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.
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. 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.
- "Uncertainty Parameter U". Minor Planet Center. Retrieved 2011-11-15.
- "Definition/Description for SBDB Parameter/Field: condition code". JPL Solar System Dynamics. Retrieved 2011-11-15.
- "Near Earth Objects Close-Approach Uncertainties". NASA/JPL Near-Earth Object Program Office. 31 Aug 2005. Retrieved 2011-11-15.
- "Earth Impact Risk Summary: 2004 BX159". NASA/JPL Near-Earth Object Program Office. Retrieved 2011-11-16.
- "JPL Close-Approach Data: (1994 WR12)". 1994-12-31 last obs (arc=35 days). Retrieved 2011-04-07.