Complex sentence: NASA is known to have investigated the concept in case of an emergency situation on the shuttle where alternative methods of reentry are not available--however, such planning has not moved beyond the conceptual stage given the high energies involved in reentry from orbital speeds.
Split into simpler sentences: NASA is known to have investigated the concept in case of an emergency situation on the shuttle where alternative methods of reentry are not available. However, such planning has not moved beyond the conceptual stage given the high energies involved in reentry from orbital speeds.