My aunt was an excecutive at US Forest Service until retirement, for 20 years I had to listen to her dire warnings.
22 years from now emission may not matter:
http://impact.arc.nasa.gov/news_detail.cfm?ID=173
"The 270-meter diameter asteroid 99942 Apophis continues to be an object of interest, and the calculated probability of impact has varied as additional tracking data have been utilized to refine its orbit. Presenters discussed the predicted 2029 close Earth flyby and the associated “keyhole,” the small region in space during the 2029 approach where Earth’s gravity would perturb the asteroid’s trajectory such that the subsequent 2036 encounter would be an Earth impact. A deflection effort for Apophis prior to the 2029 keyhole would require more than four orders-of-magnitude less momentum transfer than after 2029, and good tracking data during the 2012-2013 apparition of Apophis is particularly critical for refining impact probabilities and deciding whether a deflection action is required before the 2029 close approach. As a note, there may be several deflection techniques, including slow-push techniques, that could be applied prior to 2029; after that date, deflection options narrow to more energetic techniques—probably nuclear explosives. The Arecibo radar, which is scheduled for possible closure, is the most powerful instrument available for improving orbital accuracy and physically characterizing many NEOs making close passages to Earth.
http://www.aero.org/conference...r.pdf