Do cargo-hauling astronauts really lift 15 tons?

James Oberg NBC News space analyst

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.

Precious 'down-cargo'
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.

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