By David Shiga AN ASTEROID that had initially been deemed harmless has turned out to have a slim chance of hitting Earth in 160 years. While that might seem a distant threat, there’s far less time available to deflect it off course. Asteroid 1999 RQ36 was discovered a decade ago, but it was not considered particularly worrisome since it has no chance of striking Earth in the next 100 years – the time frame astronomers routinely use to assess potential threats. Now, new calculations show a 1 in 1400 chance that it will strike Earth between 2169 and 2199,