Leibson's Law

Just two weeks before the next schedule repair mission, the Hubble space telescope has failed. It stopped phoning home. Although the instrument packages still work, the Side A Scientific Instrument and Data Handling Unit (SI DHU) has failed. Specifically, the Control Unit/Science Data Formatter within the SI DHU seems to no longer work. The SI DHU formats commands to and data from the Hubble’s instruments for communications with the ground. The formatting is required because ground commands use 27-bit words and on-board commands use 16-bit words.

There’s another side to the SI DHU, Side B, with redundant capabilities but there are two problems with activating Side B. First, five other modules must also be switched to Side B so that they feed data to the working channel. Second, the Side B channels haven’t been tested in space. In fact they haven’t been activated since the Hubble was launched in 1990. That’s nearly 20 years of space hibernation. No wonder there’s trepidation about flipping to Side B. Nevertheless, the mission team is working on the switchover because the repair mission is off until there’s resolution to this problem.

Meanwhile, this incident gave me the impetus to dig a little more into the SI DHU, which I’d never heard of before. Hey, it’s got a processor in it. One I’ve never heard of called the NASA Standard Spacecraft Computer 1 (NSSC-1). In fact, it’s got two of these computers in a dual-redundant configuration each equipped with four 8K-word memories. A little Googling turned up this paper, published in the Communications of the ACM in 1984. It’s got some archaic terms in it like DTL (diode-transistor logic) and MSI (medium-scale integration) that you’re not likely to know unless you’ve got grandkids. This architecture has been well used. It was standardized in 1974 and it’s designed to use core memory. Those NSSC-1 memories are made from core planes. That’s the memory with the little ferrite donuts that’s woven by Keebler elves when they’re not making cookies and that technology appears to have been running the Hubble for the last 18 years! Just imagine the mountain of astrophysical science that archaic technology has delivered. It’s awe-inspiring (“awesome,” for those of you who didn’t know what DTL and MSI were). I hope NASA gets Hubble working again using Side B. I think the elves are too busy baking cookies to weave more core planes in time for the rescue mission.