If Philae had an RTG

A different power source could significantly extend the operation of the probe.

The success of the program is obvious: after 10 years of tracking the flight of the device, for the first time in history, a soft landing on the surface of the comet was made. This is an unequivocal triumph of space exploration. But it would be strange to expect everything to go perfectly smoothly.

The reader, who followed the landing news, knows that some of the Fila’s systems did not work as expected. Harpoons to hold the probe on the surface were not thrown away: the problem was 0.3 grams of nitrocellulose, which did not burn as expected. In 2013, Copenhagen Suborbitals showed that a system based on this substance in an airless space is unreliable.

The small rocket engine, which was supposed to press the probe to the surface in order to fix it with harpoons, did not work either. The team that managed the operation did everything possible. But the drilling of the surface did not help much either: the material of the comet is not so soft at all, it is hard as ice.

The biggest problem is that it is hardly possible to use the device: it needs electricity, and the sun barely shines on the Fila photocells, because it jumped off the surface and went to a different point from the ESA. The battery charge quickly faded away, but the probe managed to send home a lot of interesting information, including data on organic compounds in the atmosphere .
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60 hours - almost sounds like a failure, although in fact the mission was successful. We are accustomed to long periods of work of rovers: they plow the surface of the red planet for months and even years, providing social media services with beautiful photos and loud headlines. "Spirit" after 6 years of work stuck in the sand. Opportunity has been traveling on Mars for over 11 years. Its solar panels sweep the sand, but the latest third-generation rover Curiosity is not scary: its movement is possible thanks to a radioisotope thermoelectric generator (RTG).

Inside the RTG is placed fuel, which can be a radioactive isotope with a suitable half-life and other parameters, for example, the type of radiation. In the process of decay, heat is generated, which is used by the thermoelectric generator in the cylindrical shell of the container with fuel.

Over the years, the RTG power will weaken, so you need to do it with a margin. At some point, the energy released will fall below acceptable levels - this technology also has its limits of work. But still this is the best solution for powering autonomous spacecraft.

RTGs, unlike nuclear reactors, do not require control rods, a steam turbine, a capacitor and an electric generator. The principles of operation of RTGs and atomic reactors are somewhat different: not a chain reaction, but natural disintegration is used.

If a “RTG” stood on the “Fila”, then it could have worked for much longer than the 60 hours for which chemical batteries were enough to charge. But this is not the end.

The desired landing point should have been well lit. We had to choose not the point at which there was a better connection with the Earth, not the best geological part of the surface, not the flattest part of the comet. Sunlight in many ways added to the situation of compromise. If an “RTG” stood on the “Fila”, then the landing site could have been chosen better.

One should not think that the apparatus was not expected to work for a long time: on Earth, they wanted to see how the comet, when approaching the Sun, starts to lose mass due to the emission of dust and evaporation of ice and gases. Now we can hardly find out from the “Fila” what rocks lie under the surface, what is the structure of ice. In 60 hours of operation, the available data were collected and sent home, but the probe was capable of more.

What prevented the use of RTGs in the design?

Yes, "Filas" would have gained weight, but not so much. A kilogram of plutonium-238 emits about 500 watts of energy, most of which will remain warm, and has a half-life of 87.7 years, which means that its power will fall at least twice. Working on the basis of this isotope RTG SNAP-9A, which began to be used in space exploration in the sixties, it produces 25 watts of electricity and has a mass of 12.3 kilograms. Surely now there are much more elegant and easy solutions. For comparison: the mass of "Fila" is about 100 kg, it requires a comparable level of electrical energy.

The addition of an RTG could have increased the total cost of the mission, but it already amounted to billions, and the usefulness of the descent vehicle would increase by several times. For 10 years of the flight of the "Rosetta" to the comet, the power would drop slightly. Already several times " leaving " the solar system "Voyager-1" also has an RTG. It was launched in 1977, and then everything worked at full capacity - 470 watts. In 2001, it fell to 315. Partial operation will last until 2025.

Plutonium-238 is not a material for the production of atomic weapons. In decay, it gives alpha radiation, which does not require any special protection - it will stop the case. Therefore, RTGs with this isotope were used in the Apollo program during landings on the lunar surface.


The gray cylinder with a passive radiator is the RTAP SNAP-27 of the Appolon-14 mission

Unfortunately, there are not so many plutonium-238s in the world: after the end of the Cold War, nobody produces it anymore. Now the United States can begin to produce one and a half kilograms per year for the needs of NASA. Since 1993, the Russian side fully provided the US space program, 16.5 kilograms of plutonium-238 were sold. But stocks are decreasing on their own due to half-life. It is estimated that it will cost $ 75-90 million to start re-production of the United States, and this may take 5 years. In the above, for measuring RTGs, SNAP-9A was about a kilogram of this valuable material. If we want to explore space, then we definitely need a lot of this substance, this is the official position of NASA .

In addition, the European Space Agency could hardly use such materials and technologies in its space program. But, probably, these are more organizational issues, and cooperation with NASA or Russian companies could solve the problem.

Many plutonium-238 leaks are associated with failures in launching and operating spacecraft. “Ulysses” with RTGs on board was launched without incident, but a study was conducted, the result of which was that even if there was an explosion on the launch pad and a release of 11 kilograms of plutonium-238, the risk of cancer increase among the population would be negligible. The document literally says that with a probability of 0.0004% there could be no more than 3 deaths. Plutonium-238 is stored as a dioxide, but it does not dissolve in water.

But the public remembers the disaster of the Chernobyl nuclear power plant and Fukushima. The man in the street does not know how realistic nuclear power engineering is; therefore, in many respects, the reasons lie not in technical restrictions, but in politics. It was because of the politics and money that Fila worked for 60 hours, not years.

According to the materials of en.wikipedia.org, discussions at Hiktimes , Starts with a bang! and Quora .