If you had enough energy available you could descend and land vertically and not care anything about heating. The problem is that would take a tremendous amount of propellant. If you could slow well below orbital velocity will still above the atmosphere you would still generate significant heat entering the atmosphere, look at SS-1, at apogee it is not going all that fast, but gravity accelerates it on the way down.<br /><br />You would have to lift a lot of propellant to have a significant effect.<br /><br />The speed of sound is just a reference point, it varies to a certain point and then stays constant, outside the atmosphere it is irrelevant. The Shuttle scrubs off velocity by banking steeply as it descends, creating more drag. A forward firing engine to do the same thing would be prohibitive.<br /><br />At any altitude, above a body, it takes a specific speed to orbit at that altitude. You could orbit the Moon at a very low altitude, as long as you go fast enough and don't hit anything. In an atmosphere you have to counter drag and that can be affected by Solar conditions, the upper atmosphere expands. The ISS needs reboosting because it is fairly low, the Hubble not as much because it is higher.<br /><br />Skylab should have stayed in orbit longer, but increased solar activity caused it to re-enter before Shuttle could get to it. <div class="Discussion_UserSignature"> </div>