6 July 2011—For 30 years, the space shuttle fleet—Columbia, Challenger, Discovery, Atlantis, and Endeavour—strutted its stuff in low-Earth orbit. The spacecraft's missions included simple payload deployments, science module sorties, and the delicate assembly and servicing of the International Space Station. They were also used for in-flight repairs to themselves and to other satellites, hyperprecision orbits for radar mapping, tethered experiments, and gentle close-in maneuvering with smaller spacecraft. Those capabilities were originally unimagined by their designers, and they firmly refute the old maxim that "form follows function." Indeed, the shuttles performed functions beyond the dreams of their builders.
The current stable of heavy-launch vehicles can carry deployable satellites and rocket stages as big as or bigger than any that the shuttles have ever launched. With replacement vehicles already being designed for specific manned missions, such as Earth-to-orbit taxi services or for beyond-low-Earth-orbit sorties, the biggest engineering questions must be these: What operational capabilities are we giving up by retiring the shuttles? And are we sure we can dispense with them? Because if the answers are "Too many" and "No!" we need to start planning how to regain them with new vehicles.
Here's what we're losing.
Lost capability No. 1: Gentle delivery of large modules for attachment to existing complexes. Compared with other means, the shuttle provides an environment in its payload bay with relatively minimal acceleration, vibration, and noise, and that means very large components can be built a lot less expensively. The savings comes from several sources. The shuttle's own hardware delivers cargo close to its destination, after which robot manipulators can install it carefully. Without this capability, the items would have to be built with structural enhancements to survive the attendant stresses, making them significantly heavier. What's worse, without the shuttle, the module might need its own maneuvering capability—resulting in significant mechanical stress as the add-on connects to the existing structure. Such stress leads to design headaches: For example, the size of any connecting pressurized tunnels must be restricted. Furthermore, in the event of a mishap, the shuttle design is supposed to allow for intact retrieval of the payload for relaunch, mitigating the need to build expensive backup hardware.
Lost capability No. 2: Bringing cargo down gently. The shuttle payload bay can carry specialized laboratory modules into orbit and then back to Earth for reuse. It can also retrieve and return large spacecraft and their components for repair or redeployment. Entry stresses do not exceed 1.5 g's, and to demonstrate that, several astronauts have remained standing throughout most of the descent. Without the shuttle, the scale of returnable objects is greatly limited, and the stress and shock of descent is severe. To replace this large-scale capability, NASA might have to develop inflatable heat shields that could be scaled up in size as needed—but even with such shields, cargo would still be subject to significant entry and landing stresses.