Spacecraft I Have Known

Spacecraft I Have Known

Up close and personal — yet from afar

From 1981 to 1982, I worked on the thermoelectric power systems for NASA's Voyager and Galileo deep-space missions. These spacecraft operated too far away from the Sun to employ solar power.

Contents

1  Voyager Mission (1977 to Present)
    1.1  RTGs and the Seebeck Effect
    1.2  Where are they now?
    1.3  Onboard computers
2  Little Known Facts
3  Galileo Mission (1989 to 2003)
4  What I Worked On (1981 to 1982)
    4.1  My 1981 JPL slides
    4.2  Time Travel

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 instead of manned spaceflight. Moreoever, they are still operating and now performing measurements that were not even conceived of during their original design, some 40 years ago. My association with them came well after they had launched. I was required to create a model of the degradation of their on-board power systems based on JPL thermal cycling data (see Section 4). The overall objective was to create a similar model for the planned Galileo mission (see Section 3).
Launched in 1977: Voyager 2 on August 20 followed by Voyager 1 on September 5.



1.1  RTGs and the Seebeck Effect

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 RTGs are contained in three canisters that can be seen on the end of the boom arm 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 single-crystal silicon but amorphous silicon, similar to that used in solar panels.
The RTG is actually like a tiny solar panel with its own Lilliputian `sun' provided by the mini nuclear reactor. The original power output was about 500 watts. Think about it, that's equivalent to just ten 50-watt light bulbs!
How do you communicate with 10 light bulbs that are now about 20 billion km from Earth? The answer is, you need a hella big EYE. That special eye is called the NASA Deep Space Network (DSN). Find out its bit-serial transmission rate in Section 2. You'll be underwhelmed.
By January 2015, the available electrical power on Voyager 1 had dropped to 255 watts and to 258 watts for Voyager 2 because of the precipitation effects. As described below, this is close the 300 watt value that my model predicted. In addition, JPL has been conserving power consumption in recent years, realizing the Voyagers could now do many unplanned measurements as they leave our solar system.
The later generation of RTG power systems, like those on the Galileo misson (see Section 3), used lanthanum chalcogenides as the thermoelectric semiconductor.

1.2  Where are they now?

1.3  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.

2  Little Known Facts

3  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 two 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.

4  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, Voyager and Galileo. Syncal was later bought by Thermo Electron Inc. in 1982 (now Thermo Fisher Scientific).
The first task I was given at Syncal, 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.

4.1  My 1981 JPL slides

These are the slides I used in a presentation to JPL. Clearly, we're talking pre-Powerpoint days. Me cajoling Syncal into purchasing an IBM PC, rather than continuing to buy VAX 11-780 time, was considered to be something of an exotic gamble. The payoff was that Fig. 7 (in a different form) could be used to predict the lifetime of Voyager RTGs. It came to about 35 years after launch. JPL, being very clever, have managed to extend the RTG operational lifetime even further by powering down any unnecessary data acquisition subsystems.
Table 1: My 1981 presentation slides for JPL

4.2  Time Travel

This project also had a peculiar sense of time-travel about it. I was trying to answer questions concerning the operation of a future spacecraft (the yet-to-be-built Galileo of Section 3) based on data related to the currently operational Voyager spacecraft, traveling irretrievably in deep space. Any issues that I might discover based on the Voyager data could not be used to correct that problem. It was simply too late for that. It would only be applicable to the new Galileo spacecraft under construction. It was like driving a car where you can only look at the scene in the rear-view mirror to decide how to steer the car forward.
In some sense, all modeling predictions based on historical data are like that but, the import here was far more momentous. The Voyagers and Galileo were very different spacecraft with very different power requirements. And, once launched, no physical corrections could be made to these multi-million dollar machines. So, the contrast between past, present and future seemed much more stark.
In case you are wondering, the Voyager data that I analyzed did not come from the spacecraft in real time. Rather, it had been collected from laboratory ovens at JPL well prior to the launch of the Voyager spacecraft. Samples of Voyager thermoelectric material had been subjected to accelerated thermal cycling. JPL wanted to use that thermal-cycling data in selecting the next generation of thermoelectric material for the Galileo RTGs. I was the first person to attempt to understand what those data were trying to tell us.



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On 30 Apr 2021, 13:01.