Deep Spacecraft I Have Known

Deep Spacecraft I Have Known

Up close and personal

Contents

1  Voyager Mission (1977 to Present)
    1.1  Where are they now?
    1.2  Onboard computers
    1.3  Little known factoids
2  Galileo Mission (1989 to 2003)
3  What I Worked On (1981 to 1982)

1  Voyager Mission (1977 to Present)

These machines are the most distant man-made objects in the universe and are a testament to both NASA's awesome engineering capabilites and the role of robust robots vs. manned missions. Moreoever, they are still operating and now performing measurements that were not even conceived of during their original design, some 40 years ago.
Launched in 1977: Voyager 2 on August 20 followed by Voyager 1 on September 5.
Solar power is not an option because the distances from the sun are too great. Therefore, an onboard power system called an RTG (Radioisotope Thermoelectric Generator) is employed. The RTG's are contained in the 3 canisters at the lower left of the spacecraft.
The Voyager RTG's use a plutonium oxide nuclear reactor to produce heat which is continuously converted into electrical current via the Seebeck effect—essentially a semiconducting thermocouple. The Seeback material used in the Voyager spacecraft is a doped Si-Ge semiconducting alloy-not crystaline silicon but amorphous silicon similar to that used in solar panels. Original power output was about 400 watts. Think about it, that's equivalent to just ten 40-watt light bulbs! Today, it has probably degraded to something like 300 watts or less because of the precipitation effects described below.
Both spacecraft know how to communicate on Twitter (see below). Here's an example:
@NASAVoyager2 Voyager2
Adjusting my sun sensor bias to better point my High Gain Antenna at Earth: AACS:AC7SSB 4+170 SS BIAS CONTROL YAW
Voyager 2 is the more garrulous of the two, but Voyager 1 also tweets once daily at 1200 UTC on the same account.

1.1  Where are they now?

1.2  Onboard computers

The Voyager mission was officially approved in May 1972. At that time, hand-held calculators were the only computers available to the mass consumer market. A moment's reflection quickly leads to questions about the type of computer control used on the Voyager.
There are three types of computers on the Voyager spacecraft and there are two of each kind for redundancy.
  1. Computer Command System (CCS): 18-bit word, interrupt type processors (2) with 4096 words each of plated wire, non-volatile memory.

  2. Flight Data System (FDS): 16-bit word machine (2) with modular memories and 8198 words each

  3. Attitude and Articulation Control System (AACS): 18-bit word machines (2) with 4096 words each.

Each processor uses a 12-bit wide word with 64 instructions. They were built using custom TTL and discrete logic, manufactured by General Electric to JPL specifications. Remarks that they employed either of the RCA 1802 or the Intel 8008 microprocessor apparently have no factual basis.

1.3  Little known factoids

2  Galileo Mission (1989 to 2003)

Launched October 18, 1989 after being shelved for many years due to reductions in NASA funding.
The RTG power system employed a doped La-Cu-O thermoelectric semiconductor. The RTG's are located in the canisters on the left side of the spacecraft.
The Galileo spacecraft's 14-year odyssey ended on Sunday, Sept. 21, 2003 when it passed into Jupiter's shadow and disintegrated in the planet's dense atmosphere at 11:57 a.m. Pacific Daylight Time. The Deep Space Network tracking station in Goldstone, Calif., received the last signal at 12:43:14 PDT. The delay being due to the speed of light.

3  What I Worked On (1981 to 1982)

Syncal Corporation was a small consulting company located in Sunnyvale, California, that had contracts with NASA and JPL to develop the RTG thermoelectric materials for NASA's deep-space missions, like Galileo. It was later bought by Thermo Electron Inc. in 1982 (now Thermo Fisher Scientific). The first task I was given, was to analyze thermal stability data from the Voyager RTG. It was during that project that I discovered the stability of the Voyager Si-Ge thermoelectric material was controlled by a soliton precipitation. Here's the 1982 IEEE paper (PDF) that discusses the mechanism in detail. Over a very long period of time, the thermal gradient eventually drives the Ge out of solution. Remarkably, I was able to draw heavily on certain mathematical results from my Ph.D. thesis—which had nothing to do with either RTGs or the Seebeck effect.
Table 1: My 1981 presentation slides for JPL
This project also had a peculiar sense of time-travel about it, because I was trying to answer questions about the future operation of these spacecraft based on data captured serveral years before they were launched, while they were already traveling in deep space.
Anything that was discovered now (at that time) would be too late as far as changing the Voyager design was concerned. Put another way, it was like traveling in a car, where I was looking out the rear window with a telescope to help others, who were driving, to take the best path forward in a different car. Of course, the point was that any hindsight gained about the Voyager RTG characteristics could only be used in the design of the newer Galileo RTGs.
In case you are wondering, the data that I analyzed were not transmitted from the Voyager in real time. Rather, these data had been collected from lab ovens at JPL, where samples of thermoelectric material had been subjected to accelerated thermal cycling. JPL wanted to use any Voyager information as a foundation for selecting the next generation of Seebeck materials for the Galileo project.


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On 17 Nov 2011, 17:37.