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Comment by jessriedel | Hacker News Reader

Does anyone know whether there was a good engineering reason to give Philae only solar-charged battery power rather than a nuclear generator? Or was is just regulatory/cost?

And what is the major power draw that leads to such a short battery life? The heater to keep the electronics warm?

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I didn't know this until a few years ago, but solar panels beat RTG on a power/mass ratio for most missions that are in space (as opposed to on a planetary body) out to the orbit of Jupiter or Saturn. It would have added mass to the mission to supply the same power.

There were a dozen ways that Philae could have gone wrong, and using an RTG would have only helped with one failure mode.

I'm sure they aways hoped it would live for weeks, but apparently most of the experiments have already run in the first day. That was the important part. I don't know the precise engineering trade-offs, but I suspect "get it to land and live for 24 hours" was the first goal, and anything that made that goal riskier was dismissed.

Good points I'll grant, but it naively seems to me that this issue with batteries running low is a perennial problem with spacecraft.

Unless the RTG/solar mass ration is very high, your points about having most of the experiments already ran in the first day, or about RTGs not helping with other failure modes, don't matter too much. Either way when you include a power source (RTG or solar) you are doing so for the purpose of getting data after the first day.

Besides avoiding the shading problem, I believe RTGs are also more robust to unfurling issues and to impact damage.

Looking at pictures, I believe Philae solar panels are on the external shell, they don't need to be extended.

You have a better physics background than me, but it seems like the Philae mission would be particularly concerned with weight because of the velocity it had to achieve. Considering it took 10 years to reach its destination, maybe the shielding required for an RTG would be prohibitively heavy.

Maybe the overhead in increased costs due to limited availability of PU238 and additional launch preparations/safety were better spent elsewhere.

I figure the ESA made the best decision for the mission, though, and were well aware of the tradeoffs involved, don't you?

What shielding? There's nothing live on it.

Electronics aren't particularly fond of radiation either.

All space-faring electronics need to be rad-hardened, and not because of an RTG source that emits only alpha particles.

This is actually very common with space instrumentation. We've been rather spoiled by NASA's Mars rovers lasting exceptionally long amounts of time. Opportunity only had a mission plan of 90 (Martian) days. Anything longer than that was deemed to be a bonus. It's been going strong for 10 and a bit years now.

Philae had a few specific goals: namely to land on the comet, take some data with each of the instruments and return the data to Earth. A single good measurement with each instrument would be probably classed as a completely successful mission.

Don't forget we've had plenty of missions where the lander was completely destroyed - e.g. probes into Jupiter or landers on Venus - and we expected them to be destroyed. The missions being: take data and hope we get it before the local environment obliterates the instruments. Some of the Venera landers lasted a mere 120 minutes before Venus killed them.

Obviously there's a significant amount of engineering work involved to make sure that things are as robust as they can possibly be, but it is normal to do the bare minimum to achieve specification simply due to weight or power restrictions.

A tweet from @elakdawalla to a similar question a few hours ago:

Technology to build and use Radioisotope Thermoelectric Generators in Europe has not been developed, largely for political reasons

https://twitter.com/elakdawalla/status/533256200088211456

"And what is the major power draw that leads to such a short battery life? The heater to keep the electronics warm?"

Yes, it's in the other post:

    "It is very unlikely right now. We have 1.5 hours [of
    sunlight] at less than 1 watt, and 20 minutes of 3 or 4
    watts. The lander needs 5 watts to boot….In order to charge
    the secondary battery, we have to heat it to 0 degrees
    Celsius. We need about 50-60 watt-hours a day in order to
    reach 0 degrees and still have daylight left to charge the
    battery."

http://elakdawalla.tumblr.com/post/102615327170/philae-updat...

I knew they are partially shaded, but are they literally only getting ~2% of the power they expected? Is this just a thin sunbeam or something?

I guess so. The rated power [0] is 32 watts at 3 AU (current distance from sun), and it's getting 10% of that, for 20 minutes a day. (I'm not sure what a "day" is; maybe it's the comet's rotational day, which is 12.4 hours).

[0] [PDF file] http://www.dlr.de/dlr/Portaldata/1/Resources/documents/Phila...

20 minutes per rotational period, does sound like a very narrow sunbeam!

"Asteroids and comets rotate, but not exactly like the Earth. Because Earth is a sphere, its mass is distributed relatively evenly, so it rotates smoothly. Asteroids and comets aren’t uniformly shaped, so their rotation can be more of a tumble. NASA equates their rotation to the spin you see on a badly thrown football." (1)

Now imagine that you target landing close to the wobbling equator but eventually end somewhere closer to the wobbling south pole, and maybe even in the shadow of something bigger (that helped you stay on the comet). Less sunshine and freezing gets much more certain.

67P/Churyumov–Gerasimenko has the rotation period of cca 12 hr. (2)

1) http://science.opposingviews.com/asteroids-comets-rotate-201...

2) http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimen...

In a press conference earlier today that very same question was put to the team. The answer was that due to a variety of (largely political) reasons there's been almost no development of RTGs within Europe at all.

I've read that in general space agencies are very hesitant to launch anything into space with a nuclear generator out of fear that if the rocket exploded on the way up it could scatter radioactive material across our atmosphere.

http://en.wikipedia.org/wiki/Kosmos_954

http://en.wikipedia.org/wiki/SNAP-10A

I think there's been others as well

Both of those are pre-Challenger and the first paragraph of Kosmos article reads:

"Kosmos 954 (Russian: Космос 954) was a reconnaissance satellite launched by the Soviet Union in 1977. A malfunction prevented safe separation of its onboard nuclear reactor; when the satellite reentered the Earth's atmosphere the following year it scattered radioactive debris over northern Canada, prompting an extensive cleanup operation."

so caution seems reasonable.

Challenger was not the first explosion of a spacecraft in flight. Far from it. Apollo 13 taught NASA far more about safekeeping of radioisotope generators than any explosion on the way to orbit.

I was wondering the same, an RTG like to one on Curiosity but smaller as a backup. My only guess would be weight, but the Curiosity RTG is ~5kg of plutonium and produces 2.5kWh/day, and Philae power consumption seems to be much smaller than that, so I don't know.

I realize this isn't the top issue being discussed right now, but I've also heard that the isotopes preferred for RTG's are in very short supply these days - I believe they were byproducts of the old nuclear weapons programs.

From 2013, "U.S. To Restart Plutonium Production for Deep Space Exploration" - http://www.universetoday.com/100875/u-s-to-restart-plutonium... .

"The problem is, plutonium production in the U.S. ceased in 1988 with the end of the Cold War. ... The plutonium that currently powers Curiosity across the surface of Mars was bought from the Russians, and that source ended in 2010. New Horizons is equipped with a spare MMRTG that was built for Cassini, which was launched in 1999. ... The current production run of Pu-238 will be carried out at the Oak Ridge National Laboratory (ORNL) using its High Flux Isotope Reactor (HFIR)."

Cost, mass, thermal management, vibration/structural considerations (mostly launch), regulatory, fitting in the launch vehicle.... doing anything with a spacecraft is insanely complicated and you have to carefully engineer, test, and report on the tiniest of details.

It seems the only benefit would be speed, and space folks are patient.

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