LOS ANGELES, Oct 21: The 3-D outer space thriller, “Gravity,” continued to rocket past its box office competition over the weekend, eclipsing newcomer “Carrie,” to maintain its tight grip on the US and Canadian box offices. “Gravity,” starring Sandra Bullock and George Clooney as astronauts stranded in space, grabbed $31 million in ticket sales at North American theaters to hold the top spot for the third consecutive weekend. It had overall ticket sales of more than $170.6 million. (agencies)