¶Reusing rocket upper stages. Blue Origin is considering repurposing the upper stages of their upcoming New Glenn rocket for use in orbit, possibly as habitats. (Development of New Glenn remains on schedule for launch in 2021.) A group consisting of NanoRacks, SSL, and ULA has also studied repurposing orbital rocket bodies as part of NASA's NextSTEP program. That proposal would involve an Atlas 5 upper stage refitted with life support and attached to the ISS or another station. Repurposing of upper stages in orbit makes sense when considering the energies involved with returning them to the ground for reuse. An upper stage may be moving at up to 8 km/s, compared to ~2 km/s for returning first stage boosters. Decelerating for reentry at those speeds requires a large amount of fuel, heat shielding, or both. The Space Shuttle and SpaceX's proposed Starship are examples of upper stages that return to Earth for reuse using heat shields (or hypothetically “transpirational cooling” in the case of Starship).