With the shuttle in orbit, you've got 250,000 lbs going 17,500 mph (kinetic energy) at 200 miles (potential energy). To land it, all that energy has to go to zero.<br /><br />Esentially, friction gives you a zero cost way to bleed off all that energy - into heat. Any other way means adding weight to the orbiter - engins, fuel, sails, etc. Which means those things have to be boosted into orbit with the vehicle, which means a higher launch weight, which means even bigger rockets, and so it goes round and round.<br /><br />Dumping all that energy into heat is the most efficient way to go. All you have to worry about is not wrecking your heat protection system from external tank debris .... but that's another story.