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Comment by guscost | Hacker News Reader

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?

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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.

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.

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