And finally tonight, we simply can't leave without a longer look at today's liftoff of a huge new rocket from SpaceX.Three, two, one.The world's most powerful rocket, the Falcon Heavy, took off from Kennedy Space Center in Florida.That's where NASA has long launched spacecraft. But, today, it was from the private company started by Elon Musk.At least two million people watched it on the company's livestream on YouTube.Our own science correspondent and resident space expert is here, Miles O'Brien.And it's the focus of our weekly segment on the Leading Edge of science and technology.Miles, welcome. Let's get the serious stuff out of the way. What is the goal of this effort?Well, Elon Musk wants to go to Mars and colonize Mars. That's the top-line goal.And this is one of the big, important steps along the way.But part of getting to Mars and making that a practical idea is making it more easy to get to space.And this is a lot cheaper. It's a fraction of the cost of anything NASA has built.We used to say in the shuttle days, it was about $10,000 a pound for any pound of anything you put into space.Elon Musk is down around $1,000 a pound.So, we're talking about an order of magnitude improvement on cost, and that has to do with the reusability of these boosters.