Ask the Astronaut: Why not build and launch spacecraft from the ISS?

Tiny CubeSats are launched from the space station, but not Mars ships.

Q: For deep space exploration, instead of launching Orion on the Space Launch System rocket, why not build a new spacecraft at the ISS and depart from there for trips to the Moon, asteroids, or Mars? And why is NASA building three new ships: Orion, Crew Dragon, and the CST-100 Starliner? (John Pitchlynn, Norman, Oklahoma)

There are several parts to this question. First, the questioner asks why we’re building three new ships: Orion, Crew Dragon, and CST-100 Starliner? Crew Dragon and Starliner (from SpaceX and Boeing, respectively) are transport ships from Earth to low Earth orbit destinations, including the ISS. Their life support systems and heat shields cannot handle extended deep space conditions or survive reentry at reentry speeds of 25,000+ mph (vs. 17,500 mph from low Earth orbit). NASA is contracting to use these ships for astronaut trips to ISS, but SpaceX and Boeing want other customers, like tourists and industrial firms, too. The companies are contributing around half of the development costs in partnership with NASA.

Orion and the Space Launch System are designed for taking four-person crews to the Moon’s vicinity. Orion has three times the internal living space of Apollo, can support the crew during a month-long mission in deep space, and has a heat shield capable of interplanetary reentry, where temperatures are 5,000 degrees F vs. the 3,000 degrees experienced by low Earth orbital craft. The SLS requires more takeoff power than the Saturn V moon rocket because it must launch the heavier, larger Orion, with its life support supplies and rocket propellants, to the Moon or beyond.

We won’t use the ISS as a departure point for cost reasons. First, the ISS today is a microgravity research lab, not a spacecraft assembly hangar. Modifying it for assembly, checkout, and propellant storage would cost billions of dollars NASA does not have. A second, more serious problem is that the ISS orbit is inclined to the equator at 51.6 degrees, as opposed to a 28.5-degree orbit reached by launching straight east from Kennedy Space Center. (We chose the ISS orbit so the Russians could reach it from their launch sites farther north.) To haul spacecraft parts and propellant to ISS for assembly in that high-inclination orbit, we would lose about 20 percent of each rocket’s payload capacity, since we can’t use as much of the Earth’s eastward rotation to give us a free boost to orbital velocity. That payload penalty would add billions to the costs of any deep space expedition assembled at ISS (e.g., a Mars expedition will need many hundreds of tons of propellant for Earth departure).

We will need more than Orion and SLS to get to deep space and stay there. We’ll need habitat modules, extra propellant and rocket engines, radiation shielding, a nuclear reactor or giant solar arrays, and a lunar or Mars lander. Several SLS launches will be required along with assembly in a 28.5-degree orbit to put a lunar, asteroid, or Mars expedition together. The scale of such expeditions will be far beyond Apollo’s, and will require international cooperation as well as commercial involvement to become reality.

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