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Comment by jordanthoms | Hacker News Reader

It appears to be largely political issues that prevented this mission from using a Radioisotope thermoelectric generator , which would have eliminated this particular problem since solar panels would not be required. [1] This is how the Curiosity rover is powered.

If that is the case it's a massive shame - irrational fear of nuclear technology does a lot of damage.

1 - http://www.space4peace.org/ianus/npsm2.htm#2_3

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The first (failed) mission to Mars was attempted as early as 1960 by the USSR, continuing with 1964, 1965 Mariner by the US. Rovers came after a lot of previous work done, including a lot of unsuccessful attempts. And the rover weights 900 kg, probably 40 kg of it is the nuclear power generator. Philae is only 100 kg and the first on the comet.

It's certainly not "fear" of the nuclear technology in Europe that prevented something. See for example:

http://en.m.wikipedia.org/wiki/Nuclear_power_in_France

It's that the engineering tradeoffs have to be considered.

While it seems perhaps uncharacteristic of a space agency to make engineering decisions based on politics, if I had to guess I'd imagine that the team was well aware of the PR effect of the power supply chosen for their lander. I would also guess that the same issue was considered at NASA while planning the Mars Exploration Rover (solar) and Curiosity (RTG) missions. Are you claiming that this did not factor into their decision at all?

that it wasn't a significant factor, at least

Fair enough. I don't know nearly enough about Europe or the ESA to volunteer an over/under on that.

Space exploration was always very dependent on politics.

As Neil deGrasse Tyson says, NASA was founded on the fear factor of USSR's Sputnik which was launched using the intercontinental ballistic missile technology.

BTW: According to the comments here, the solar power generation on Philae weights at least 20 times less than the nuclear generator would.

Any hypothetical RTG for this mission would almost certainly weigh less than the one on the Curiosity rover. This lander does not need 100+ watts of power for 10 years.

Consider that the RTG from the Pioneer 10 mission weighed less than 16kg: http://en.wikipedia.org/wiki/Systems_for_Nuclear_Auxiliary_P...

Thanks. That still makes some 10 times less weight for solar panels. Still, once US makes a mission, I can imagine they would consider using that, launching much heavier probe etc. This was an European mission, not having access to the US resources.

Wow, I just looked into it and it turns out the ESA actually hasn't used RTGs before, but planned to use one with this very mission[0:p78]. The only ESA mission I can find which used an RTG is the Cassini-Huygens probe[1], a collaboration with NASA.

[0] http://www.ianus.tu-darmstadt.de/media/ianus/pdfs/arbeitspap...

[1] http://en.wikipedia.org/wiki/Cassini–Huygens

Where do you see that ESA actually planned to use a RTG for Rosetta mission?

On the p78 is only "The consortium formed of leading institutes of Europe includes the scientific organisations of Germany, Finland, Italy, Hungary, Great Britain and Austria, which have not the technological capacity to manufacture RTGs. It was therefore decided to develop new high-efficiency solar cell for this deep space mission."

Here's the passage as it appears on page 78 of the IANUS paper[0]:

"Up to now there exists a number of projects developed by ESA which plan to use radionuclide power or heat sources. One of the most far progressed is the 'Rosetta' project."

[Update: When this paper was written, it looks like they might have been only planning to use an RHU to heat the lander:

"'RoLand' includes not only the solar power generator but also RHU on plutonium-238 of the RHU 'Angel' type. By the opinion of the project experts, the RHU use is the most desirable, sensible and reliable means for achieving the main goal of the mission."]

[0] Energy supply for deep space missions: Risks of nuclear power in space and prospects for solar alternatives. From http://www.ianus.tu-darmstadt.de/media/ianus/pdfs/arbeitspap...

Yes it's only RHU (Radioisotope heater unit) is mentioned,

http://en.wikipedia.org/wiki/Radioisotope_heater_unit

never RTG (Radioisotope thermoelectric generator).

"While both RHUs and RTGs use the decay heat of a radioactive isotope (usually Pu-238), RHUs are generally much smaller as a result of omitting the thermocouples and heat sinks/radiators required to generate electricity from heat."

But then there seems to be a contradiction here. If the ESA was planning to use an RHU powered by plutonium-238, which resource (your word) or technological capacity (from the IANUS paper) were they missing that prevented them from using a plutonium-238 RTG? Money?

It would have made the lander heavier, thereby increasing the size and cost of everything else.

RHUs are in no way comparable to RTGs: They are basically very small amounts of radioactive material fully enclosed in a thermally conductive container. Think of it as an always warm pebble stone. They are unregulated sources of (almost) constant heat and have no other purpose.

In aerospace engineering, they are used in just a few circumstances to augment the active heaters in a spacecraft: Heater lines, which are resistive heaters in a foil with a PI controller. You need active thermal control since your environment changes a lot (launch, preliminary orbit, transit, orbit) and thus its thermal parameters (some parts of a spacecraft may experience temperature gradients of a few hundred Kelvin within a few minuts). Most spacecraft designs have issues with overheating and thus RHUs cannot be used here.

An RTG's primary objective is to produce electrical energy (at an abysmal efficiency). The excess heat generated by an RTG is seldomly used for thermal control since it can also not be regulated and it emits simply too much heat energy.

It seems highly unlikely that any scientists would make subpar choices that may compromise a long-term project on the off chance that it might cause some political discomfort. The latter isn't even particularly likely since there is a large difference between nuclear power as an energy source on Earth and nuclear technology for scientific use in space. In addition most of the public doesn't even know or is interested in the technical details of a mission like this. Therefore it is extremely unlikely that using RTG technology would have caused any kind of political problems which makes avoiding said technology on political grounds simply nonsensical when it has the potential to compromise the overall mission.

I think you are 1) overestimating the amount of independence that a typical space agency project actually enjoys and 2) underestimating both the amount and variety of PR that a high-profile space mission like this one generates.

The issue is that Europe never developed RTG technology because of politics, so scientists couldn't even consider it as an available choice.

(Aerospace engineer/student here)

While there were certainly some political aspects to the question, there were other more pressing reasons to not pursue an RTG design. First and foremost (as others have pointed out) is the lack of knowledge towards this technology within the EU/ESA member states. Also, there's little left of the fuel usually used for these generators (238-PuO2 in space applications), and while it can be bred, none was produced for centuries (last time I checked). It is expected that NASA can only launch a couple of addtional RTG-powered missions before its ressources are depleted.

An RTG also poses quite a few engineering challenges for the spacecraft itself. They are huge and heavy with low efficiency, and their design requires them to be positioned on the outside with sufficient surface area to radiate away their heat. One of their main benefits – almost constant power for a long time – is also their biggest flaw: Their power output cannot be regulated.

While these challenges are in no way unsolvable (they are used both in space and on Earth), they require a certain spacecraft design which makes them unsuitable for many missions. If you take a look at the design of Philae and where it was attached to Rosetta, there is no way an RTG would have fit the design and mission characteristics chosen.

They could have chosen a different design. Though the question is then: Would this have changed anything wrt science output and/or success of the mission? I personally don't think so: An RTG would have increased cost and complexity without a significant impact on science output. An RTG would not have helped with the harpoon and ADS (cold gas thruster) issues which are the primary reasons why Philae landed in the shadows.

Though certainly a malfunction has happened, I do not consider this mission a failure, quite the opposite is true! Due to the hopping, there's a slew of unexpected science data which awaits to be analyzed. Additionally, there's still the chance that Philae may regain enough power once the comet is closer to the sun. Lastly, the primary mission objective – analyzing a comet – is still well underway with Rosetta orbitting Tschurie.

Happily, Pu-238 is being manufactured again: http://news.discovery.com/space/nasa-plutonium-production-sp...

Another source I read at the time of the announcement said it will take a while to spin up, so there are some mission constraints over the next ten years. A Europa mission would wipe out existing supplies.

pound for pound, solar power is much cheaper. you cant disregard the weight considerations of a thermoelectric generator

Yes. I love nuclear, and think we should use a lot more of it both on earth and in space. But for things in the inner solar system not landing on a planet, solar is often the right answer.

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