Humans have been riding rockets into space for more than 50 years, and for all that time, the cost of reaching orbit has remained astronomical—$5,000 to $50,000 per kilogram, depending on which rocket is used. The problem is that none of our rockets is very efficient. About 90 percent of a rocket's weight is fuel and propellant, leaving little room for payload. If it could lose some of that weight, a rocket could lift more cargo, reducing the cost of putting a given kilogram of stuff into orbit.