Red isotopesby Dwayne Day
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| It seems fairly clear that the lander and rover are powered by solar panels, and have RHUs only to keep them warm during the cold lunar night—in other words, the RHUs are heaters but not power sources. |
So far China has released a fair amount of information about Chang’e-3, including numerous photos of the lander and rover. The lander is big: 2.5 meters on a side, and 4.76 meters from one landing gear to the opposite one. Fully fueled its mass is 3800 kilograms and it is capable of delivering a 1700-kilogram payload to the Moon’s surface. It is not really big enough to carry humans, but it can carry much more payload than the relatively small rover it is hauling on this mission.
There are still many unanswered questions about the mission. One of the more interesting ones is how the Chinese plan on keeping the lander and rover warm during the long lunar night. According to a paper by Sun ZeZhou, Jia Yan, and Zhang He in Science China (“Technological advancements and promotion roles of Chang’e-3 lunar probe mission”), the lander and rover are equipped with radioisotope heating units, or RHUs, using a two-phase heat transfer system.
The article does not indicate what isotope the Chinese are using in the RHUs. According to an article by Yang Jian on Xinhua, the rover will be “powered” by plutonium-238 (Pu-238.) Another article by Craig Covault in Aerospace America claims that both the lander and rover will be powered by Pu-238, but also have solar panels. Thus, none of the sources that discuss the use of radioisotopes are consistent with each other, but the Science China paper was written by engineers responsible for developing the spacecraft and seems fairly clear that the lander and rover are powered by solar panels, and have RHUs only to keep them warm during the cold lunar night—in other words, the RHUs are heaters but not power sources.
It is possible to use other isotopes besides Pu-238 in RHUs, such as polonium-210 (Po-210.) The Soviet Lunokhod rovers used Po-210 to keep warm during the lunar night. But Po-210, in addition to being extremely toxic, also has a short half-life, meaning that it decays relatively quickly, limiting its lifetime. The European Space Agency is considering use of americium-241, primarily because they believe they can manufacture it in Europe. But other isotopes have short half-lives, generate nasty radiation, or have other negatives. Pu-238 has drawbacks as well, but it is ultimately the least-worse option for a long-term heat supply for space missions.
The United States has long used plutonium-238 for radioisotope thermoelectric generators (RTG), which produce electricity, to power spacecraft such as the Pioneers, Voyagers, Vikings, Galileo, Cassini, and more recently New Horizons and the Curiosity rover. NASA has also used RHUs for solar-powered landers on Mars. Pu-238 decays naturally, producing heat, and the RHUs, which can be as small as marbles, keep the spacecraft warm.
| China apparently has invested significant resources into developing a capability to manufacture radioactive isotopes, which have many uses, such as medical imaging. |
Pu-238 does not occur naturally. It has to be manufactured using a laborious process. It is not the same kind of plutonium used in nuclear weapons, which is Pu-239. The manufacture of weapons-grade Pu-239 produces a silvery-gray metal known as neptunium. If slugs of neptunium are suspended next to a specially designed reactor core, some of the neptunium is converted to Pu-238. The slugs are then processed and the Pu-238 is removed and refined and concentrated. This is a time-consuming and complicated process—any process to produce isotopes produces tiny amounts at a time—and it requires dedicated specialized equipment. The United States still has that equipment. Russia may still have it, although the equipment has probably fallen into disrepair.
So where did China get the Pu-238? They probably manufactured it themselves. And that’s no minor accomplishment.
According to Dr. Ralph McNutt, a space physicist at the Johns Hopkins University’s Applied Physics Laboratory who co-chaired a 2009 National Research Council study on the supply of Pu-238 for American spacecraft, China apparently has invested significant resources into developing a capability to manufacture radioactive isotopes, which have many uses, such as medical imaging.
Dr. McNutt notes that there is considerable literature—most of it in Chinese—about the activities of the China Institute of Atomic Energy in developing various isotopes, including working with neptunium, which can be processed to make Pu-238. He points to a 2007 annual report produced by the CIAE (an English version is available here) that refers to the development of Pu-238 for radioisotope heater units for spacecraft, even discussing the difference in temperature between the center and surface of a chunk of Pu-238. They also analyzed safety issues, such as how the fuel pellets would behave under high pressure, seawater corrosion, impact from an exploding booster or hitting the ground after falling from high altitude—all of the factors that NASA has long taken into account with its own radioisotope fuel sources (see this image of the relevant section of the report.)
After 2007 the CIAE annual reports do not mention work with Pu-238, although apparently some Chinese associated with Chang’e-3 have publicly discussed it. Event though the CIAE reports do not clearly indicate that it has manufactured Pu-238, their publicly acknowledged activities demonstrate impressive capability and infrastructure, and an interest in using Pu-238. Therefore, manufacturing Pu-238 seems to be something that they have likely done, even if their public documents are less than clear.
Chang’e-3 is an intriguing spacecraft in other ways. Efforts by some Americans to downplay the spacecraft are primarily posturing. Yes, the United States did land spacecraft on the Moon many decades ago, but the United States is not landing spacecraft on the Moon now, so Chang’e-3, if successful, will represent a technological capability that the United States does not currently possess. NASA has studied robotic lunar landers, and the planetary science community has highly ranked both a sample return mission that would return material from the Moon’s South Pole-Aitken Basin, as well as a network of seismic sensors. JPL has studied a large lander, but NASA is not actively developing one at the moment. Chang’e-3 is a large spacecraft, much larger than required to carry its rover (not to mention larger than any lander currently contemplated by NASA), leading planetary scientists to speculate that it is a prototype for a future Chinese lunar sample return spacecraft.
| Although Chang’e-3 is doing something that the United States did decades ago, it does include some new capabilities and technologies that NASA does not currently have, at least not in flight readiness. |
Another intriguing aspect of Chang’e-3 is that, according to the Science China paper, the lander has an autonomous hazard avoidance system that will guide it down in a spot presumed to be Sinus Iridium, known as the Bay of Rainbows. Autonomous hazard avoidance is a capability that NASA has tested on Earth, but has not yet flown on any lander spacecraft. NASA has essentially aimed its spacecraft at relatively flat areas on the Moon and more recently Mars and hoped for the best. NASA currently has no plans to integrate it into the Mars 2020 rover. China may beat the United States in the development of this useful technology. A minor spaceflight first, but an important technology nonetheless.
Besides Chang’e-3’s technological capabilities, it will also go to a part of the Moon not yet visited by humans or robots. One of the lessons of planetary science is that every time you visit a new location, you learn something new. Spirit and Opportunity touched down on very different parts of Mars, and both locations surprised their science teams. Chang’e-3 will produce interesting scientific results, and will give China the basis to mount even more ambitious robotic missions in the future.
Thus, although Chang’e-3 is doing something that the United States did decades ago, it does include some new capabilities and technologies that NASA does not currently have, at least not in flight readiness. And unlike NASA, China has plans for setting down on the lunar surface. Perhaps they will even share their scientific data even if they won’t share their technology.