Flying in space requires reliable, uninterrupted, stable electrical power, not only for engines to maneuver and navigate but for systems on spacecraft performing a range of functions. As a result, one of the critical components of any satellite either in Earth orbit or dispatched elsewhere is the power system that allows the operation of its many systems.
I have been interested in this subject for many years and intend to write a history of the subject at some point in the future. I have already undertaken one part of the story, that of the use of nuclear power systems for long-duration deep space missions far removed from the Sun and the solar power that can be obtained through photovoltaics. In the paragraphs that follow I will lay out the basics of the power systems available at present as the beginning point for discussions of the histories of each system.
There are arguably only four methods of providing the electrical power needed for spacecraft, all of them with positives and negatives. The first method, and the one used on the first spacecraft launched into orbit, was batteries. Their wattage was limited, but even more limited was their longevity. Within a few weeks they always ran down and the spacecraft’s systems no longer operated. For example, about three weeks after the launch of Sputnik 1 on October 4, 1957, its batteries ran down and it ceased to broadcast telemetry although it remained in orbit for about ninety days after launch.
Second, to help resolve that problem NASA helped to pioneer in the 1960s fuel cell technology, which generated more electricity for the size of the cell and had a longer effective life. Even so, fuel cells have an effective life of less than two months. Of course, this may change in the future as NASA pursues more efficient fuel cells for its future space systems that could have remarkably long lives.
Third, photovoltaic solar cells emerged in the 1960s as a useful alternative to batteries and fuel cells. They have a long life measured in years rather than weeks or months, and with additional refinement they have become the critical power generation technology for most spacecraft.
Solar power has one important drawback; it requires the Sun’s powerful light source to be effective. For spacecraft traveling into deep space beyond Mars, where the Sun becomes much less intense, photovoltaic systems up to this point have proven insufficient. This may change in the future as new technologies increase the efficiency of energy collection and power management but past and present capabilities have thus far prohibited their use for deep space missions beyond Mars.
Accordingly, when requirements are for short mission times or do not require high power, chemical and/or solar energy may be used effectively to make electricity for spacecraft power. But for the generation of high power levels over longer periods of time, especially farther away from the Sun, nuclear energy has thus far been the only way to satisfy mission requirements.
For this reason, as well as others of a more sublime nature, many spacecraft designers have adopted nuclear power technology as a means of powering spacecraft on long deep space missions. As NASA’s chief of its nuclear electric power programs remarked in 1962:
Basically, radioisotopes are of interest because they represent a compact source of power. The energy available in radioisotopes is many orders of magnitude larger that that available in batteries, and thus they constitute a unique, concentrated energy source that may be used for space purposes if design requirements are met. Radioisotope power is inherently reliable. It cannot be turned on or off. There are no moving parts of oriented arrays. It will provide heat energy in accordance with the fixed laws of radioactive decay. This heat is absorbed in a device that converts the heat directly into electricity.
Allowing the natural decay of an isotope and harnessing its heat to generate electricity with Radioisotope Thermoelectric Generator (RTG) has become the preferred method for supplying the power needs of American deep space probes but it has also been used on some Earth-orbital and lunar spacecraft. It operates by releasing heat during the decay process of a suitable radioactive material that is then converted into electricity through means of an array of thermocouples, with the outer end of each thermocouple connected to a heat sink.
Radioactive decay of the fuel produces heat which flows through the thermocouples to the heat sink; generating electricity in the process. The thermocouples are then connected through a closed loop that feeds an electrical current to the power management system of the spacecraft.
In addition to its longevity, space nuclear power offers a significant saving in terms of mass associated with an individual mission compared to the other possibilities. One policy analyst commented:
Nuclear power has been used for deep space vehicle for over 40 years. RTGs have been used for spacecraft electrical power since 1961. All RTGs have operated as designed, both in normal operations and accident conditions. RTGs were designed carefully with consideration for the accident environments that might be experienced during every phase of the launch. The design requirement is to protect public and worker health and safety during all phases of operations during launch and accident conditions.
Each of the four types of spacecraft power systems have their uses and their drawbacks. Providing on-board power for spacecraft is one of the key challenges facing the designers of all missions and their hardware. How they have met this challenge over time will be the subject of several forthcoming blog posts. Stay tuned.