Your spacecraft is falling from the skies at an initial speed of Mach 25. Your reentry heat shield, that has to survive a 7,800 degrees Celsius (14,072° F) plasma shock, is a finely tuned hi-tech amalgam of refractory metals and carbides and reinforced carbon-carbon ablation materials. Care to replace your mighty heat shield with a balloon? Not likely! But that is exactly what NASA is considering.
Read the full article: NASA to test IRVE-3 inflatable reentry system