If we are flying higher and higher it is meant to fly with greater and greater speeds. A speed, above a certain altitude, could be bigger than the circular orbital speed at that altitude.
So, at that altitude, it is better to use spacecraft than aircraft. Theodore von Kármán was the first to calculate this altitude. The Kármán line, lies at an altitude of 100 kilometers (62 mi) above the Earth’s sea level, and represents the boundary between the Sky and Space.
Image Credit: NASA