The Bittersweet Taste of Philae’s Limited Success (Synopsis)

Its not clear to me that the Europeans have the same phobia (or had the same phobia, when the mission was launched) of nuclear power that many Americans do - and this was an ESA mission, not a US one. So I'm not sure we can really blame the lack of a thermonuclear power source on American irrationality regarding TMI and other accidents. Maybe it was a PR decision, or maybe it was a science/engineering decision that we don't have all the details about.

Second comment; isn't it ironic that NIMBY fears impact our use of these generators on spacecraft, even though this use is pretty much the textbook definition of not being in anyone's back yard. :) Oh, I get that the complaint is really about potential liftoff disaster and the stuff falling out of the sky. But the complaint paints a curious superficial picture, of someone complaining about an attempt to remove nuclear material from our terran back yard.

Ethan, You may want to make a small correction in your description of the lander equipment malfunctions. Philae doesn't have "descent thrusters". The thruster that failed was not intended to reduce the descent velocity but to push the lander down against the surface after touchdown to counteract any rebound tendency.

I've been upset for a while now about our dwindling supply of Plutonium-238. Can't we get somebody on this? Elon Musk maybe?

I must confess I feel a bit queezy when I read stuff like "this is only likely to result in additional 3 deaths" etc, pitting the value of a mission against lives of people. Kind of "lose one probe at launch, world population -= 4 people".
It also leaves out of sight those who develop diseases not immediately resulting in death, yet connected with suffering.
I think this merits a bit more commentary.

After all, it is an estimate of risk, and the risk is carried by every individual of a larger community, including the scientists who want to shoot that rocket with Pu-238 in the tip to the sky. Given that people take on much bigger risks, e.g. high mobility at the price of traffic accidents, I do agree with you that this "additional risk" goes under in background noise and is worth the scientific knowledge one gains.

I am not with you on the subject of fission as a source of energy though. You say that fission energy has an unparalleled safety record in spite of the major accidents you cited. Yet, those accidents would have turned out much worse were it not for people to risk their lives, and in the case of Tchernobyl a significant number of people to effectively commit suicide, so as to contain these accidents which would otherwise have turned out much worse than they did. And I'm not speaking about a 25 km exclusion zone around Tchernobyl, but half of Europe becoming uninhabitable.
I don't know whether you have ever heard of this, but to this day, even in Germany, you have meat from wild pigs you cannot eat because they have fed on contaminated mushroom.

Now with Germany in its nuclear phaseout another factor is coming into play: the enormous costs of removal of old power plants. In fact, the power companies are lobbying hard to make the tax payer pay for any costs in demolition that exceed the surplus funds these companies are required to keep aside for exactly this purpose. And these costs are estimated to be double as high.

So in essence, short term cost savings come at a heavy price in the future, that will have to be paid for by the next generation. As usual.

There's something that strikes me as odd. Since Philae operates off of a battery, and the PV charges the battery, shouldn't there still be enough power to operate a quarter as much as planned for? Or is the power also needed to warm the lander to operating temperature?

Maybe Rosetta can reflect some sunlight onto Philae ?

Hi Ethan,

Great points ... coworkers and I have been discussing exactly this issue at work.

One important correction to your original post is that your (500 W / kg) is not specific and off by a factor of about ~165 for electric power per kg of 238Pu. RTGs with 238Pu generatoe ~14 W_thermal / kg but only ~3 W_electric / kg. It's really only the electric power that matters since this is what power the electronics. You can find a list of RTG parameters here:

http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Mode…

This correction makes an enormous difference since ~30 W_electric for Philae requires ~10 kg, which is about 10% of Philae's total weight! Still a very reasonable number, but the correct value demonstrates there are serious engineering tradeoffs to be made here.

...powered by Plutonium-238 instead of by solar panels.

Um, Ethan, I know I'm being pedantic, but there's no such thing as Plutonium-238. Uranium-238 is struck by a neutron and transformed into Plutonium-239.

*sigh* In the first place, RTG's are very heavy and very expensive... Philae probably wouldn't even have existed had it been powered by RTG's. They also have a huge impact on spacecraft design because they have to be located outside the main body so they can radiate away heat. The need to radiate heat away to keep one leg of the thermocouple cold also means their efficiency would have gone to hell when it got near the sun. (I.E. this is why we used them so rarely, and mostly for missions going way the hell out there.)

Also, because of their weight and expense, RTG's are a dang poor choice for a mission that's likely to be over in a few months when the comet approaches perihelion and starts to outgas.

On top of all, the idea the NIMBY's are keeping us from using RTG's is utter hogwash. Not one vehicle that needs/needed them has ever been stopped by NIMBY's. (And what NIMBY's there are have decreased over time - hence the essentially complete absence of protest over the RTG's on New Horizons.)

The biggest thing keeping us from using RTG's more often (after weight and expense) is the fact that the best fuel (PU-238*) was essentially a by product of nuclear weapons production - and nobody is producing nuclear weapons anymore. There are alternative processes, but they're hideously expensive.

Omega, yes. The power is needed to heat the lander electronics.

*Yes Julian, PU-238 exists. Google it.

The political cultists most actively involved in anti-nuclear energy scaremongering were never savaged by scientists with anything like the same level of enthusiasm that Dawkins exercised against creationists for one simple reason.

Scientists and anti-nuclear activists belong, for the most part, to the same political cult.

Rule number one in cultism: Never attack your own (except for heresy against the cult).

@ Julian
sure there is... http://en.wikipedia.org/wiki/Plutonium-238

@Omega
yes, it uses power for heating as well. It's quite chilly out there ;)
.....
IMO there were several factors to take into consideration back in late 90's when rosetta was on the drawing board. (speculation begins now...)
One that is likely is PR. Cassini launch triggered quite a number of "greens" to protest to it, both in general public and science community. Maybe ESA didn't want to have to tackle the same issue.
Second is that ESA doesn't have RTG's or Pu238, and either due to cost or just in spirit of cooperation, opted to use technology that can be developed within EU instead of purchasing RTG from US or Russia.
And thirdly, it might be that it was considered an overkill. After all, philea's lifetime was never intended to be decades but a year or so.

In hindsight, I'm sure everyone today (who was present then) would say.. hell yeah.. stick a RTG and sleep tight, and I'm sure that any future missions into interstellar space will probably go with RTG instead of solar, but given what was going on in the late 90's... can understand the choice for solar at that time. And in some way it is good, since I know the solar cells they use on rosetta were discovered for that purpose. There weren't solar cells available that would function is such low temperatures before then. So now we have them :)

There are several reasons not to use a RTG:

1) It is expensive to get a hold off. It is scarce and most owners wont part with it. ESA is also not in the position to do a Doc Brown stunt and steal it from terrorists and build a probe instead of a bomb.
2) It is heavy and can't be as easily integrated into the hull.
3) RTGs are best for continual long time use. It is not suited to start when the probe arrives as the decay of the fuel just happens all the time.
4) There aren't years to come. In a few months the probe will be destroyed by the sun. There were years when the power wasn't needed.
5) RTGs produce a lot of thermal power (10% efficiency in theory, 3-7% real efficiency). That's not an unsolvable problem in flight, but the target is a block of ice and will change its characteristics with a 'hot' probe.

In short, the access is limited, the price is high and it really doesn't match the mission profile.

The simple reality is that RTG technology was not available during the design and build phase of Rosetta/Philae.

Thus ESA had to use conventional batteries and solar panels to power the probes - or postpone the whole thing by at least a decade while ramping up RTG production. Buying a RTG from the US or Russia was also not a realistic option due to the short supply in both countries.

BTW: ESA currently is developing it's own RTG based on Americium-241. But it will take years before it becomes mission ready.

It won't take incredible serendipity for Philae to get enough sunlight to operate in the future...it's at 3 AU from the sun, approaching 1.24 AU. It'll get nearly 6 times as much power at perihelion, in addition to warming that will reduce the heating requirements (overheating being what was originally expected to limit its lifetime). It merely has to survive the cold until then, and it sounds like they expect it to do so.

As for RTGs, they would have been overkill for a simple stationary surface lander which by nature would have a short operating lifetime. They might have allowed a more ambitious long-term probe, but that would also have been riskier given the uncertainties involved...better suited for a followup mission where we know more about the target (even if just little things like the fact that the surface may be solid ice).

This is not to say that the article is wrong. The radiophobic hysteria adds greatly to the cost of missions that use RTGs and interferes in decisions on launch vehicles:
http://www.spacenews.com/article/civil-space/42588epa-finds-no-show-sto…

Also, to show it as the irrational reaction it is, look at the development of space fission reactors. Nuclear fuel is barely radioactive before it's been used, making vehicle fabrication, assembly, and launch even safer, and a reactor can alter its power output to suit demand, rather than producing its peak power on the ground and during flight. Despite this, apart from a few early and primitive devices, the potential of space reactors has been ignored.

Quick sanity check here.

Plutonium has low power production density. Therefore to produce enough power, how much would an appropriate size of reactor have to increase in weight to accommodate the needs of the probe?

You could alternatively use the wasted power produced whilst in transit by storing in a battery to offset that, but either this requires less plutonium or more weight for the battery to store the production.

Or it could be designed to a different power draw to fit that which a nuclear reactor on the probe could manage.

And what extra weight would be needed to harden the system against radiation produced by the power supply?

Of course, we COULD just pretend that we only need to think about one change. Quite how well that would work I don't really know at the moment.

Scientists and anti-nuclear activists belong, for the most part, to the same political cult.

Rule number one in cultism: Never attack your own (except for heresy against the cult).

Someone who proclaims that diverse groups of others come to their conclusions, not out of cogent rational thought, but through unthinking reactionary or emotional proclamations is far more likely to be the one who turned off the lights in their own head before posting....

@14:

the potential of space reactors has been ignored

Ignored may be the wrong word. AFAIK fission-powered spacecraft were seriously considered by a number of countries in the '50s and '60s (in the US, Project Orion even inspired a series of sci-fi novels by Pohl Anderson). But with the cold war on, the US and Soviets agreed not to put anything even remotely bomb-like in orbit, for fear of the arms race repercussions, and so in the early '60s they were banned by one of the test ban treaties.

Geopolitically there may be some residual reasons to keep up that ban (some country orbiting an EMP device is probably a more realistic threat than some missile launch platform), but the threat is so much lower than it was that I think the international community could certainly revisit the subject, and remove the ban if they think its no longer relevant for global security.

Ack, yeah I know RTGs run on fission. I mean spacecraft relying on the use of at-critical or over-critical fission reactions (such as a traditional nuclear reactor, or the wacky 1950s idea of detonating nuclear bombs behind a spacecraft to give it forward thrust)

Two points:
1. You have given no rational for the chosen design. You make a good case for the use of Pu-238 but I suspect that this was considered by the ESA and rejected. Why?
2. Your wording suggests that those who dislike nuclear power are uninformed or just plain stupid. I am neither. Human intuition does poorly in assessing events with very low probability of occurrence but potential for devastating consequences. I think that you are glossing over the known costs and potential destructive consequences for this reason.

I thought about weight first as a reson against RTG, but discarded it later since the data available on RTG's and Philea in terms of weight are pretty similar. This is all from wiki:

- Philea's power source weighs 12.2kg and generates 32W electrical at 3AU
- for that output we can look i.e. SNAP 19 RTG (powered Pioneer 10&11).. generates 40W electrical and weigh's 13.3 kg (of that less than 1kg is in Pu238)

so from that... they look on same footing in terms of weight and power production.

If the greatest need is to get the electronics warm, RTGs start sounding better, i.e. you want the "waste" heat. In fact I could easily imagine a hybrid system, a small RTG used to keep critical components warm, and PV to produce electricity.

In that case we would probably change RTG to RT.

Except the problem is precisely the opposite, OC. The end of life was expected because it would soon be too hot to operate and cooling would not be possible.

Plus to get any reasonable power out of an RTG you need at least ONE potential to be kept much cooler than the one nearest the pile, else there's no thermoelectric current.

So the "if" clause of the conditionary statement, being false, renders the rest of the query/conclusion incorrect.

– for that output we can look i.e. SNAP 19 RTG (powered Pioneer 10&11).. generates 40W electrical and weigh’s 13.3 kg (of that less than 1kg is in Pu238)

SNAP-19 at 12.2 kg would produce 25W of electrical power and would be expected to be at the end of a long conveyor arm to avoid interfering with the instrumentation. And shielding/protection (as well as a moveable arm) would have been extra weight.

Also, you can't buy these off the shelf,and the ESA could not buy from Russia, and the USA, the only other producer of such RTGs, had none to spare.

The cost of a battery is much less than an RTG.

One of the jobs I applied for was with the ESA.

@eric: Reactors are nothing at all like bombs and are not banned by any treaty, and RTGs run on radioactive decay, not fission.

@Omega: it's not the greatest need, though it is a significant one...apparently around 60 watt-hours per "day", which works out to about 5 watts average. A small radioisotope heater could provide that and allow it to charge even with the brief periods of sunlight it's getting now, but would probably shorten its mission by making the overheating problem worse if it landed in full sunlight as planned.

However, just swapping out solar cells for an RTG misses some of the opportunities available in a target with accessible ice. Radiators become unnecessary and the achievable RTG efficiency increases greatly if a heat exchanger can be pressed into the ice below the dust.

Wow. I was proposing to not use the radioactive generator for power just heat. The panels are better suited to generating power. Since the lander is sitting in a near vacuum, the flow of heat can be controlled by something as simple as a sheet of aluminum foil. Add some mechanism for moving the foil and you have some control over where the heat goes.

The solar panels have to be in sunlight, obviously, but in Philae's case, the probe is covered in them. You'd need to add solar "wings" or otherwise separate them from the spacecraft so your movable foil bits can regulate the heat from them, and this all adds to the mass and possible failure points.

Well, Philae is out there, teaching us something for the next time. There is nothing we can do about her circumstance. Hindsight is, seemingly, a great platform to expound individual knowlege on such matters, so, let's put heads & ideas together for the next mission to make it more successful. Look at the successes of this mission so far; there may be more to come in the not too distant future from this craft.

Oh yes, having an RTG with some tens of watts of thermal power, to reap some watts of electric power, on a celestial body consisting of stuff like frozen CO2... Good idea? No, it's not.
ESA is not fucking NASA.
We don't get fucking Pu-238.
But obviously Rosetta works quite well with solar panels only. Get that, red state senators!

Does the hipster armchair expert have a source for his assertion that the decision to reject an RTG was based on fear?

Because I'm hearing - from people who actually know what they're talking about - that there were sound practical reasons for the decision, e.g. weight, availability, cost...

PJ, ESA are working on an RTG using Americum, but it's a long and expensive project. And a long way from being ready.

Since the lander is sitting in a near vacuum, the flow of heat can be controlled by something as simple as a sheet of aluminum foil.

Unfortunately a vacuum is a damn good insulator and, since the thermal temperature of the craft is necessarily so low, heat loss by radiation is likewise slow.

But we now have a battery AND an RTG and the solar panels (fairly light), and the weight has to be offset by a reduction in the payload, whilst the expense has to be offset by a cutback in the mission payload for another cut in the science done.

ONLY WITH 100% accurate hindsight would we have seen that it would have been a better idea to use an RTG (since the lower science payload would operate longer and *maybe* result in more science done, but not guaranteed), but unless you have a TARDIS, we're SOL on using that idea...

"SNAP-19 at 12.2 kg would produce 25W of electrical power and would be expected to be at the end of a long conveyor arm to avoid interfering with the instrumentation. And shielding/protection (as well as a moveable arm) would have been extra weight."

And putting a heavy thing at the end of a long arm might create difficulties given the uncertainties of the landing. It wouldn't particularly help if the thermal generator tipped the lander over.

Well, hindsight would indicate that we already had problems with trajectory control (contact jets failing, etc), which could only have made things worse with a complex body shape...

All we can say now is that if we could go back in time, we'd do it differently.

IMO it would be better to look at how to test and plan a backup case in case things go wonky next time.

And maybe look at putting more effort in to an EU RTG.

If you watch some of the press conferences which were held (you can find them on youtube) Stephan Ulamec specifically says that there are political reasons why Philae does not have an RTG.

And one of them is because the usa won't sell it. They're running out themselves.

And the purchase would increase the budget (with some going outside the EU, against the corporate role of the ESA), decrease the payload (again, against the corporate role of the ESA), or require quid-pro-quo with the Americans to get their science on board (yet again,...).

It';s political for the same reason that Dogs for the Blind having too much money is political: the charity has to spend their money on dogs for the blind, and there are more generous donors for dogs for blind people than there is need for dogs for blind people.

And laws (written for political reasons: i.e. the politics of the citizenry) against having too much spare cash on hand.

Philae woke up! :))

http://www.bbc.com/news/science-environment-33126885

operating temperature -35C .. and 24 watts of power :))

Hooray for Philae ! ! More good science to come.