Do cargo-hauling astronauts really lift 15 tons?

Miscellaneous cargo
Other items are also being left at the station. Quibbling over precise values shouldn’t divert attention from the tremendous importance of what has been brought up.

The biggest item is a 6,400-pound (2,900-kilogram) tool caddy to be mounted on the station's exterior, near the U.S.-built Quest airlock. It will provide electrical power and data hookups for other spare parts to be brought up later, ready to be installed as needed in place of future malfunctioning mechanisms.

About 1,400 pounds (640 kilograms) of loose items were stashed in lockers in the shuttle’s mid-deck, from which they mostly have already been passed through the tight docking tunnel into the station. Once installed on the side of the station, Raffaello can open an interconnecting doorway more than twice as wide as the tunnel, allowing refrigerator-sized units to pass in and out.

Conveniently, the space shuttle’s power system uses fuel cells for electricity, and the waste product is pure water. That water is being loaded into collapsible plastic containers and hauled aboard the station. By the end of the mission, perhaps half a ton of the precious fluid will have been delivered — and it didn’t even count as station-bound payload.

So the total amount of material to be left aboard the station comes to about 13,000 pounds (5,900 kilograms), an impressive and vital cargo to be sure. But it’s far from the 15 tons often quoted by NASA and news outlets.

Coming back to Earth is about 1,300 pounds of stuff loaded into the empty lockers on the shuttle’s mid-deck, and 5,000 more pounds stashed into the Raffaello cargo hold, and the 600-pound gyroscope in the payload bay open frame. That’s about 7,000 pounds (3,200 kilograms) in all, predominantly but by no means exclusively "garbage."

The mass brought up to space, and then brought back to Earth, also includes the 19,728 pounds (8,967 kilograms) called Raffaello’s "cargo element" — the structure used to hold the up-cargo, and then the down-cargo. This is the "double-bookkept" 10 tons that the oft-published figures demand underserved credit for.

Pound-for-pound comparisons
With an accurate estimate for the deliverable cargo, the practical payload capacity of the space shuttle acting as a freight hauler can then be compared with alternatives. It turns out to be not very different from a single flight of the European Space Agency’s robot freighter, the Automated Transfer Vehicle or ATV, whose first mission is slated for the middle of next year.

The same mass of supplies could be sent up on about two and a half launchings of Russia’s old reliable Progress system, although large cargo items would need specialized exterior-mounting attach points on the Progress. At a mission cost in the $40 million range, this suggests that the cargo delivery of a billion-dollar shuttle flight could be matched by $100 million worth of Progress missions.

As NASA shops around for shuttle replacement capabilities, it needs to keep these and other options in mind. And it needs to use authentic figures for the mass of the desired cargo to be transferred.

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