Carrying the hopes for low-cost access to orbit, Space-X's Falcon 1 is scheduled to fly sometime in the days following July 29, according to Smithsonian's Air&Space. Unlike the first two tries, "Flight 003" will carry paying cargo, including, in another first, one very small satellite that will deploy a technology straight out of science fiction, solar sails.

On board cameras filmed Flight 002, above, and watching stage separation and the Earth gradually getting smaller is, for me, a real thrill.

Should all go well, fourteen Falcon flights are scheduled through 2011. But saying in a recent speech before the Royal Aeronautical Society that "it is difficult to predict how long that window will remain open," Musk, who has sunk $100 million of his own money into Space-X, has much bigger goals in mind for the nascent commercial space transportation sector - sending humans to Mars.

There are a couple of ways to think about "that window."

First, the risk/reward ratio in the rocket business is very, very high, so spending a mind boggling fortune is easily done.

Princeton astrophysicist J. Richard Gott, who will be speaking at the IdeaFestival in September, will address the second.