Nuclear Powered Space Missions - Past and Future by Regina Hagen 11/8/98
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2. Technology

2.1 The U.S. - RTG, Nuclear Reactor, And RHU

2.1.1 Radioisotope Thermoelectric Generators (RTGs)
2.1.2 Nuclear Reactors
2.1.3 Radioisotope Heater Units (RHUs)

2.2 The USSR/Russia - RORSAT, Topaz, And RTG

2.2.1 RORSAT Nuclear Reactors
2.2.2 TOPAZ
2.2.3 Radioisotope Thermoelectric Generators (RTGs)
2.2.4 Radioisotope Heater Units (RHUs)

2.3 Other Nations - "RTG Technology Is Not Available"

2.1 The U.S. - RTG, Nuclear Reactor, And RHU

2.1.1 Radioisotope Thermoelectric Generators (RTGs)

All but one of the nuclear powered space missions launched by the U.S. used RTGs. In a document about the Ulysses mission, ESA/ESTEC (European Space Agency/European Space Research and Technology Center) explains the RTG technology as follows:

"What Are RTGs?
RTGs are lightweight, compact spacecraft power systems that are highly reliable. RTGs are not nuclear reactors and have no moving parts. They use neither fission nor fusion processes to produce energy. Instead, they provide power through the natural radioactive decay of plutonium (mostly Pu-238, a non-weaponsgrade isotope). The heat generated by this natural process is changed into electricity by solid-state thermoelectric converters. RTGs enable spacecraft to operate at significant distances from the Sun or in other areas where solar power systems would not be feasible. In this context, they remain unmatched for power output, reliability and durability.

Safety Design
More than 30 years have been invested in the engineering, safety analysis and testing of RTGs. Safety features are incorporated into the RTG's design, and extensive testing has demonstrated that they can withstand physical conditions more severe than those expected from most accidents.

First, the fuel is in the heat-resistant, ceramic form of plutonium dioxide, which reduces its chance of vaporizing in fire or reentry environments. This ceramic-form fuel is also highly insoluble, has a low chemical reactivity, and primarily fractures into large, non-respirable particles and chunks. These characteristics help to mitigate the potential health effects from accidents involving the release of this fuel.

Second, the fuel is divided among 18 small, independent modular units, each with its own heat shield and impact shell. This design reduces the chances of fuel release in an accident because all modules would not be equally impacted in an accident.

Third, multiple layers of protective materials, including iridium capsules and high-strength graphite blocks, are used to protect the fuel and prevent its accidental release. Iridium is a metal that has a very high melting point and is strong, corrosion resistant and chemically compatible with plutonium dioxide. These characteristics make iridium useful for protecting and containing each fuel pellet. Graphite is used because it is lightweight and highly heat-resistant." [ESTEC/b]4

On its web page "Cassini RTG Information", NASA’s Jet Propulsion Laboratory gives additional technical information:

"Each RTG NASA uses on recent planetary spacecraft contains approximately 10.9 kg (24 lb.) of plutonium dioxide fuel. On Galileo's two RTGs, that amounted to a total of about 48 lb. On Cassini, which has three RTGs, it's about 72 lb. ...

RTGs have been used on 23 U.S. space missions including Voyager, Pioneer, Viking, Apollo, and more recently the Galileo and Ulysses missions5. As in the past, Cassini's RTGs are to be provided by the U.S. Department of Energy (DoE). Heat source technology pursued by DoE has resulted in several models of an RTG power system, evolving from the Systems for Nuclear Auxiliary Power (SNAP)-RTG to the Multi-Hundred Watt (MHW)-RTG, to the currently used General Purpose Heat Source (GPHS)-RTG used on Galileo, Ulysses and Cassini spacecraft. The GPHS technology is the culmination of almost 25 years of design evolution.

A GPHS-RTG assembly weighs 56 kg (123.5 lb), is approximately 113 cm (44.5 in) long and 43 cm (16.8 in) in diameter and contains 10.9 kg (24 lb) of plutonium dioxide fuel. At launch, the three RTGs will provide a total of 888 watts of electrical power from 13,182 watts of heat. By the end of the mission the power output will be 628 watts." [JPL/c]

A specific aspect of RTG usage is pointed out by Canadian journalist Michael Bein: "Although the American planners have obviously been concerned enough about safety to draft general criteria and institute a three-step, multi-agency review process that must be completed before each launch, there are a number of weaknesses in the U.S. regulatory system vis a vis NPS [Nuclear Powered Satellites]. First of all, there is no licensing by an independent authority like the Nuclear Regulatory Commission, the watchdog of America’s commercial nuclear power industry. All the nuclear missions flown to date have been classed as research devices and have therefore been exempted from licensing under a provision of the Atomic Energy Act. DoE, meanwhile, reserves the right to approve deviations from the published safety criteria. And, perhaps most importantly, there is no provision for public participation in the safety review process." [BEIN]

2.1.2 Nuclear Reactors

Although the U.S. has also worked on nuclear reactors for space missions, they launched but one spacecraft equipped with a reactor: the SNAPSHOT mission of 1965 (see Section 3, Past Missions – a Chronology for details.) The funding to build or test space nuclear reactor systems was stopped in 1972. After the end of the Cold War, U.S. nuclear laboratories purchased Russian Topaz II reactors and tested them thoroughly (British, French, and Russian scientists were part of the research team.) However, plans for a test space mission were not pursued for various reasons.

2.1.3 Radioisotope Heater Units (RHUs)

Many of NASA’s scientific space missions are not (only) equipped with RTGs but also with nuclear heaters, the RHUs. "The Cassini spacecraft and the Huygens Probe will use approximately 117 lightweight radioisotope heater units (RHUs) to regulate temperatures on the spacecraft and on the probe. Each RHU provides about one watt of heat, derived from the radioactive decay of 2.7 gm (0.1 oz) of non-weapons grade radioisotope, plutonium 238-dioxide, contained in a platinum-30 rhodium alloy clad. The exterior dimensions of an RHU are 2.6 cm (1 in) by 3.2 cm (1.3 in) long, weighing about 40 gm (0.09 lb.)" [USDOE/a]

RHUs are used to keep instruments warm during cold Moon and Mars nights as well as during deep space missions. RHUs were for example used for the Apollo 11-17 missions, for Galileo, as well as for Cassini. This article focuses on RTGs and reactors, therefore the use of RHUs is not listed in the chronology of past missions.

2.2 The USSR/Russia - RORSAT, Topaz, And RTG

2.2.1 RORSAT Nuclear Reactors

Generally, little information was found about the RORSAT missions. A few pages in the book "Der rote Orbit" (The Red Orbit) by journalist Harro Zimmer (published in 1996) deal with Soviet nuclear powered space missions. Some additional information was found at the Federation of American Scientists’ Internet homepage [FAS].

Harro Zimmer describes the RORSAT missions as follows [ZIMMER, pages 110- 112]6:

"Additional details became known about a ‘dirty’ side of Moscow’s military spaceflight program. From December 1967 to March 1988, the USSR orbited 33 radar satellites with nuclear reactors. They functioned at orbits of appr. 255 km altitude with an average lifetime of two to three months. Usually, the RORSATs - the acronym for RADAR OCEAN RECONNAISSANCE SATELLITE - were launched to coincide with major naval maneuvers of NATO and US Navy.

The characteristic feature were their large radar antennae the signals of which were sent to the surface of the ocean in order to locate the ships. Ideal objects were aircraft carriers with their large and flat surfaces which made particularly good reflectors. When planning these satellites, the Soviets had to compromise between various requirements. On the one hand, the orbit of a reconnaissance satellite must be low enough to receive the weakly reflected signal. On the other hand, the orbit must be high enough to cover a maximum area. Considerable electronic deficiencies enforced simple but power-consuming solutions, as a result of which only a small nuclear reactor could be used.

A RORSAT consists of three major components: the payload and propulsion section, the nuclear reactor, and the disposal stage, which is used to maneuver the reactor to an orbit of 900 to 1,000 km of altitude at the end of the mission. The satellite is 1.3 m in diameter and 10 m in length. A RORSAT weighs 3,800 kg, of which 1,250 kg are made up by the reactor and the disposal stage. These two components are 5.3 m long. The reactor core consists of 37 cylindrical fuel elements with 31.1 kg of highly enriched (90%) uranium-2357 embedded in a beryllium casing.

The cooling liquid for the reactor is liquid sodium-potassium. The thermo-ionic converter uses the dissipated heat to create electrical energy with an efficiency as low as 2 to 4%.

For the radar equipment, a RORSAT requires appr. 2 kilowatt electrical power. The technical structure of the system was extremely simple. Shielding was omitted unless absolutely required. Therefore, these satellites were a flying source of radiation which severely impacted the operation e.g. of science satellites equipped with gamma ray detectors.

As mentioned before, the active RORSAT lifetime was fairly short. The ‘record’ was 134 days. How should the radioactive payload - which was not limited to the reactor alone - be dealt with? From the relatively low orbit, the satellites would have entered the denser spheres of Earth’s atmosphere after one year at the latest, and they would have burned up only partially. The purpose of the disposal stage was to avoid this happening by injecting the reactor into a higher orbit between 900 and 1,000 km of altitude.

The lifetime of a fairly massive object at this altitude should be appr. 600 years, while uranium-235 and uranium-238 have a half-life of more than one billion years."

After describing the two major accidents (Kosmos 954 and Kosmos 1402, see Chapter 3, Past Missions – a Chronology), Harry Zimmer continues with the following conclusion [ZIMMER, page 113]:

"The heritage of this program: at appr. 900 to 1,000 km of altitude about 940 kg highly enriched uranium as well as more than 15 metric tons of radioactive material orbit with an inclination of 65o. In addition, recent radar observation indicates that several ten thousand ‘drops’ 0.6 to 2 cm in diameter circle on this orbit. The drops consist of liquid sodium-potassium, the reactor coolant."8

2.2.2 TOPAZ

About the Topaz program, Harro Zimmer writes as follows: "These [RORSAT] missions were not discontinued because the risks of using nuclear power in the orbit might be too high as compared to the advantages. Rather, ocean surveillance should have been continued by means of satellites at a higher altitude, equipped with larger and more powerful reactors. As soon as February 1, 1987, Kosmos 1818 was launched into an orbit at 800 km altitude. On board the large satellite was a reactor of the Type Topaz, weighing appr. 1,000 kg, which produced electrical power of about 5 to 6 kilowatt with an improved efficiency between 5 and 10% for six months. ... On July 10, 1987, Kosmos 1867 followed, equipped identically. This reactor operated about one year. At their altitude, these satellites might be safely stored at least for the next three hundred years." [ZIMMER, page 114]9

The Russian Institute of Physics & Power Engineering describes the development process and results as follows: "In 1958 comprehensive research was started to develop a reactor-converter with the advanced thermionic principle of direct energy conversion. As compared to thermoelectric conversion, thermionic conversion makes it possible to increase efficiency, to prolong the life-time, and to improve the overall dimensions of the power system and the spacecraft as a whole.

The investigations performed at the IPPE in the field of small-sized reactors and shadow radiation shielding, the solution of the problems concerning collector and emitter materials selection and elaboration, investigations of the processes of electron emission and diffusion in cesium plasma, heat and mass exchange and liquid metal coolant technology (Na, K-alloy) provided creation of the first in the world intermediate neutrons thermionic reactor-converter which was called ‘TOPAZ’.

Between 1970-1984 seven power systems with reactors of this type were tested on the ground at the special IPPE test site. ‘TOPAZ’-units were tested twice in space as an electric power source for the ‘COSMOS’ satellites. Thermionic fuel elements (TFE's) for ‘TOPAZ’ reactors were designed, fabricated, and in-pile tested in the IPPE." [IPPE]

Whereas funding for space nuclear reactors was stopped in 1972 in the U.S. [USAF], research continued in the USSR and led to the development of a follow-up version of Topaz I.

"TOPAZ-2 small-sized nuclear power system with a thermionic converter represents a power source developed around a nuclear reactor and a thermionic heat-to-electricity converter.

Advantages: high power and reliability; long lifetime; small overall dimensions; complete radiation safety; the possibility to fully discharge the fuel and to store/ship it separately from the system; the possibility of final fuel loading directly during the system pre-flight preparation.

Application: space power systems. ... Characteristics of the reactor core: height, mm 375; diameter, mm 260; uranium charge, kg up to 27; guaranteed lifetime, yr over 3." [KURCHATOV]

In the mid-90s, a "program managed by the Ballistic Missile Defense Organization" resulted in the purchase of six Topaz II reactors from Russia by the U.S. A joint team of U.S., British, French, and Russian engineers tested the space reactors "to evaluate the Russian technology and to find peaceful civilian applications". [USAF] None of the Topaz II reactors have actually been used for space missions, however.

2.2.3 Radioisotope Thermoelectric Generators (RTGs)

Extremely little information could be found about the use of RTGs in the Soviet and Russian space program. Information that RTGs are used at all was made public by the media in their reports about the accident of the Russian Mars-96 probe in November 1996. This mission got out of control soon after launch and decayed over South America (see Chapter 3, Past Missions – a Chronology for further details.)

2.2.4 Radioisotope Heater Units (RHUs)

RHUs are also used in the Soviet/Russian space program. For example, the Moon missions Luna 17 (1970) and Luna 21 (1973) used polonium-210 isotopic heat sources to keep the Lunokhod rovers warm during the lunar nights. No further details are known.

2.3 Other Nations - "RTG Technology Is Not Available"

Up to date, no other nations launched nuclear powered space missions and little information is available about corresponding research programs. The American Institute of Aeronautics and Astronautics (AIAA) sums the status up as follows:

"During the 1960s and early 1970s several other nations, including France, Germany, and the United Kingdom (U.K.) examined space nuclear reactor power systems. In the 1980s some studies were done by Japan and the U.K. The French government assembled a design team that worked on a reactor concept employing a Brayton cycle to convert reactor heat into electrical power. The French, Japanese, and Chinese now have small programs to explore the use of space nuclear technologies.

Currently the U.S. is not producing plutonium-238 for space use, so DoE has been buying some plutonium-238 from Russia to supplement the existing inventory." [AIAA]

The non-availability of RTG technology has quite an impact on space mission planning outside the U.S. and Russia. The most striking examples are ESA‘s Rosetta mission to comet Wirtanen and ESA plans for the EuroMoon 2000 mission.

Wirtanen is a comet at approximately the same distance from the Sun as planet Jupiter. This means that the brightness of the Sun at Jupiter reaches about 5% of the brightness at Earth. According to NASA, current solar energy technology is not yet advanced enough to provide enough power for the spacecraft instruments at that distance. ESA, on the other side, had to look for an alternative for their Rosetta mission.

"ESA's next cometary mission takes its name from the Rosetta Stone. Just as the Rosetta Stone deciphered the hieroglyphics of ancient Egypt, so the Rosetta spacecraft will help to decode the messages of atoms and molecules that help us to make sense of our cosmic origins.

The Rosetta spacecraft will rendezvous with comet 46 P/Wirtanen as it makes one of its periodic visits to the Sun. The spacecraft will map the comet's surface in fine detail and land a package of instruments (the Rosetta Lander) on it. Waltzing around the comet for many months, Rosetta will be able to watch its surface erupting in the warmth of the Sun. On-board instruments will analyse the effusions of dust and gas.

Scheduled for launch by Ariane-5 in January 2003, Rosetta will take eight years to reach its target. On the way it will inspect two asteroids (planned targets currently Mimistrobell and Rodari) at close quarters." [ESTEC/c]

In a Press Release from 1994, ESA explains why RTG technology could not be used:

"New solar cells with record efficiency
Under contract with ESA, European industry has recently developed high efficiency solar cells for use in future demanding deep-space missions such as the recently approved ROSETTA mission. The new solar cells reach a 25% efficiency under deep space conditions. ...

Until now, deep space probes had to use thermonuclear power generators, like the so called RTGs (Radioisotope Thermoelectric Generators). As RTG’s technology is not available in Europe, ESA therefore attempted to develop a power source based on very high-efficiency solar cells. ...

ESA expects that the new high performance Silicon solar cells could profitably be used in deep space missions for Europe and that this technology could also be of interest for near-Earth orbit space applications as well as for Earth based ones." [ESA/a]10

Similarly, ESA had to be inventive for EuroMoon 2000 to be launched in autumn 2000.

"What is EuroMoon 2000?
The EuroMoon 2000 mission consists of a Lander and an Orbiter with a total mass in lunar transfer orbit of at least 2900 kg. The composite spacecraft would be placed into a circular polar orbit of 200 km altitude with a dedicated Ariane-4 launch. After about one month of observations, mainly for establishing preliminary gravitational data, the composite's altitude would be lowered to 100 km, where the Orbiter (weighing about 300 kg) would be separated from the Lander.

The Orbiter's task would be to make a detailed topographic map using a stereoscopic camera and to establish the lunar gravitational potential more accurately with the help of a small subsatellite, in order to assist the subsequent landing operation. The Orbiter's payload (approx. 50 kg) would also address a large proportion of the MORO mission's objectives, including geochemical science.

The Lander would set down (to within ±100 m) on the highest point of the rim of the South Pole crater, in order to take advantage of the permanent sunlight there. The landed mass of 1000 kg would include more than 250 kg of payload, the primary objective of which would be to study the soil composition, heat flow and possibly seismic activity in the neighbourhood of the intended landing site, which lies inside the largest lunar crater, the Aitken Basin.

In addition to the ESA element, more than half of the Lander's payload capacity would be allocated to three or four 'Millennium Challenge' experiments. These would be the winners of a contest involving Universities and European Industry. Their 'challenge' would be to devise various robotic devices to investigate the inside of the South Pole crater (20 km in diameter and approximately 3000 m deep, with temperatures on the order of 200 deg C), hopefully reaching the South Pole itself." [ESTEC/a]

This design has two advantages: ESA’s lander can use solar panels as it will not descend into the deep crater where no sunlight is available but remain on the rim of the South Pole crater. "This location enjoys almost continuous sunlight thus missions can rely on solar power instead of bulky batteries or costly and potentially hazardous nuclear power generation." [ESA/b] And ESA leaves it to the participating universities and industry enterprises to find a solution for robotic devices’ power supply - knowing they can not use nuclear power.

In addition to having found alternatives to nuclear power for board instruments, ESA was also successful in solving another problem. As described above, Radioisotope Heater Units (RHUs) are used to keep the sensitive instruments warm during the cold space nights. ESA managed to develop "a thermal control system securing operation without the use of radioactive heaters" [JPL/q] for the Rosetta Lander. "The design of the thermal control subsystem is challenging, because the lander has to operate on a comet nucleus with unknown rotation period, in distances between 3 and 1 AU from the sun with temperatures of the environment in the range between 120 K and 350 K. Special effort has to be taken for thermal insulation and heat storage to keep the temperature inside the lander in a range between -55oC and +70oC throughout the mission." [JPL/q]

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