Heh. I was wondering, if the thing was five minutes from impact, why it wasn't already scraping the atmosphere. Or at least a heck of a lot closer than shown.
Depends on the speed, of course. Typically, objects in space go far faster than the speeds we're used to seeing on Earth. The Earth itself is moving around the Sun at over 100,000 kilometers per hour, faster than any human-built spacecraft has ever traveled under thrust (though some have been accelerated by gravity to faster speeds). If Mogo were orbiting at a comparable speed and the asteroid were coming head-on at the same speed, for a combined closing velocity of over 200,000 km/h, then five minutes before impact they'd be about 17,000 kilometers apart, or roughly one and a third times the diameter of the Earth. And of course if the asteroid were closing even faster, they'd be even farther apart at that time.
And the characters' priorities seemed really out of whack throughout the episode. Apart from the asinine "let's blow up the asteroid and REALLY mess up the biosphere", you've got Hal slowly trudging through the jungle trying to make small talk with a guy who may not even be able to talk. Apparently forgetting the urgency of the mission. Six hours is not very long.
Well, they were searching for the Green Lantern they beleived was on the planet (not realizing it was the planet). They weren't just making idle conversation.