In the weightless environment of space, keeping track of "weight" seems to be a big challenge for NASA. The most recent example of this difficulty has to do with the figures on how much useful payload, really, has been delivered to the international space station by the shuttle Discovery.
Miscounting such materials could contribute to public confusion over what sorts of future spacecraft need to be developed to replace the space shuttle fleet when it is retired in 2010. If the shuttle's performance capabilities are not portrayed accurately, it will be harder to assess the required attributes for the successors to the shuttle.
In press kits and on NASA Television, the figure “15 tons” keeps popping up as the amount of supplies being transferred from the shuttle to the station. Some reports say that's the weight of “items stowed in an Italian-made cargo unit.” After unloading all those supplies, the story goes, the shuttle will be filled up with an only slightly smaller amount of garbage: 13 tons.
Does the cargo really weigh that much? First of all, in scientifically precise terms, there actually is no “weight” in orbit, since all objects are falling freely through a gravity field that curves their path around the earth. So any reference to the weight of objects in orbit actually refers to their mass, which on Earth’s surface is typically the same number.
What's more, a closer examination of NASA documentation reveals that the actual mass of material being swapped between the shuttle and the station amounts to only a half to a third of the figures being bandied about. Most of the "15 tons" refers not to the transferred cargo but to the containers in which the cargo is transported. Those structures are then refilled and returned to Earth, not left aboard the station at all.
Counting the containers as payload makes about as much sense as the post office charging you postage for the weight of the mailbox, or an airline assigning the weight of your seat to your baggage allowance. It’s probably not a deliberate deception, but the figures do border on the bogus.
If you take stock of all the items that were packed inside Discovery's Italian-made Raffaello cargo module, you'll find that less than two tons' worth is actually delivered to the station and left behind. There are about 2,200 pounds (1,000 kilograms) of transferable cargo and a 1,600-pound (720-kilogram) medical science rack for the station's Destiny laboratory module.
Once emptied of this material, the Raffaello module will be loaded with about 5,000 pounds (2,270 kilograms) of "down-cargo," mostly but not entirely trash. (The trash is not simply tossed overboard because it might orbit around and recontact the station, damaging it). The down-cargo also includes some very valuable components destined to be repaired and reused.
Probably the most critical such items are guidance computers for the Russian spaceships in the manned Soyuz and unmanned Progress series. These suitcase-sized units are the heart of Russia’s "Kurs" ("Course") rendezvous controllers, and they are mounted in the front of every Russian ship that docks at the space station. Since that section of the visiting craft is always jettisoned and incinerated during descent into Earth’s atmosphere, for years the valuable electronics (and other hardware items) have been routinely removed and stored on the station for return to Earth aboard shuttle flights.
But the suspension of shuttle visits over the last two and a half years has meant the gradual accumulation on the station of a backlogged stash of guidance computers. It got so bad that the spacecraft factory in Moscow is close to running out of usable units completely, and the return of at least a few of the stranded computers is desperately needed.
Another piece of returning hardware is also hardly garbage, although it certainly is broken. This is the malfunctioning control gyroscope, due to be replaced on the shuttle crew’s second spacewalk Monday. A new one is bolted inside a carrier frame in the shuttle’s cargo bay. After it is installed in place of the broken one, that device will come back to Earth for diagnosis, repair and eventual reuse.
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.
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.