James Cameron’s long-awaited Dec. 18 sci-fi adventure “Avatar” is going to be one of the hottest tickets at the movies this weekend — even though Fox is only showing 20 minutes of footage. My sci-fi pal Todd was prepared to go out to the far reaches of Brooklyn to see the clip, which is debuting at 6 o’clock tonight in theaters nationwide and tickets for which disappeared instantly. I’ll be there, of course. Check out the trailer here. The Sun recaps the plot briefly here.