The British tabloids got it wrong. On Feb 25th, NASA revised its calculation (because it got new observations) and changed their estimate of a hit in 2017 from 1 in 250 million (what they said on Feb 2nd) to zero (what they said Feb 25th). They also say that this March, it won't pass closer than 15,000 miles, when they had said 11,000 earlier.
See, for instance, a site that reports on space, and so reports things like this better than British tabloids (a Google news search shows that it's only the British tabloids, and a few web-only sites, that are still quoting the 1 in 250 million chance):
Additional observations of asteroid 2013 TX68 have been obtained, refining its orbital path and moving the date of the asteroid's Earth flyby from March 5 to March 8.
The observations, from archived images provided by the NASA-funded Pan-STARRS asteroid survey, enabled scientists at NASA's Center for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory in Pasadena, California, to refine their earlier flyby and distance predictions, reconfirming that the asteroid poses no threat to Earth.
...
CNEOS's new prediction for 2013 TX68 is that it will fly by roughly 3 million miles (5 million kilometers) from our planet. There is still a chance that it could pass closer, but certainly no closer than 15,000 miles (24,000 kilometers) above Earth's surface.
The new observations also better constrain the path of 2013 TX68 in future years; CNEOS has determined that 2013 TX68 cannot impact Earth over the next century.
http://www.spacedaily.com/reports/Small_Asteroid_to_Pass_Close_to_Earth_March_8_999.htmlBut the thing about a probability is that you don't quote it with an uncertainty; any uncertainty just raises the improbability. Rather than saying "it's a 1 in a 1000 probability, plus or minus something", you say "it's a 1 in a 1,500 probability".
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