2013 TV135 update: no longer in the impact risk list - 2013 TV135 update: no longer in the impact risk list
On 8 November 2013 asteroid 2013 TV135 has been removed from the Risk Page. The non-zero impact probability spotted by monitoring systems just after its discovery (on 8 October 2013) dropped essentially to zero. The bulk of astrometric data collected by astronomers during exactly one month of observations helped to improve our knowledge of its orbit, that eventually turned out to be a safe one.
With respect to the impact monitoring analysis, the case of 2013 TV135 is a very typical one. Statistically speaking, almost every NEA initially found to have non-zero impact probability will eventually turn out to be actually safe for the Earth. And this is even more true for the bigger ones.
Every time new astrometric data become available from observations, the characterization of the asteroid orbit improves and the estimated impact probability is re-computed. This may happen in the days, weeks, months or even years following the discovery date, depending on the availability of observational data.
A typical pattern is that as the orbit becomes more precisely determined, impact probability often increases initially, or shows a quite erratic behaviour (see, for instance, the table in "2013TV135, A newly discovered object at the top of the impact risk list"), but then decreases until it falls to zero, or some very low number. This is exactly what happened with 2013 TV135.