<blockquote><font class="small">In reply to:</font><hr /><p>As we know that entering the earth's atmosphere generates lot of heat due to friction of air.<p><hr /></p></p></blockquote><br /><br />Actually, that's not true. It's a common misunderstanding, but the heat is not generated by friction. It's generated by compression. The sheer force of the Space Shuttle (or any other entering object) pushing through the atmosphere at fantastic speed causes the air to be compressed ahead of it, creating a shockwave. This shockwave is so compressed and so hot that it becomes a plasma -- the fourth state of matter. Plasma is electrically conductive, and in fact radio waves have a hard time penetrating it; this is why spacecraft can lose contact with the ground during reentry. (The Shuttle cheats; it doesn't talk directly to the ground, but rather to the TDRS satellites in geosynchronous orbit and thus not obscured by the plasma. But it's line-of-sight to the satellites isn't always good, so even the Shuttle's signal may drop out periodically during entry.)<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>What if we make the vehicle enter earth's atmosphere very very slow ? This will greatly reduce heat produced due to friction and thus prevent any damage to the shuttle.<p><hr /></p></p></blockquote><br /><br />There are a couple of problems. The first is that the vehicle is going the equivalent of about Mach 17 when its in orbit. It has to; your orbit is defined by your forward velocity, so if you go to slow, you actually fall and hit the Earth. (Orbiting means falling while you're going forward so fast that you actually miss the Earth and just fall around it forever. Isaac Newton's famous thought experiment for this concerned a canonball shot at greater and greater velocities. I can elaborate on that if you want.) No matter what, you have to slow down from Mach 17 to 0 MPH in order to land. That's a hell of a lot of energy. You have to do something with it.<br /><br />On <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>