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After Four Days, Philae Team Gets to Rest
The Associated Press reports on one happy consequence of the inevitable shutdown of the Philae lander, after its incredible landing on Rosetta: the team that was in control of the lander here on earth finally gets to take a well-deserved break, after four nearly sleepless days and nights. It seems unlikely -- though it's not impossible -- that Philae will get enough solar energy to briefly wake up again; its bouncy landing and harpoon malfunction mean that the craft is in shadow rather than the sunlight that it was hoped to bask in. From CNN: Originally, it was supposed to have seven hours of light per comet day -- which lasts just 12.4 hours. Now it is exposed only 1.5 hours a day. That's likely not enough to juice up Philae's rechargeable secondary battery, ESA said.
There is one last hope.
"Mission controllers sent commands to rotate the lander's main body, to which the solar panels are fixed," ESA says in on its blog. "This may have exposed more panel area to sunlight."
First things first.
We've been calling this comet "Rosetta" thanks to the media, but it's actual name is 67P/Churyumov–Gerasimenko. Rosetta is the name of a probe that photographed it. But it does have a certain ring to it that may better stick in the minds of your average news reader.
The comet has a rotation period of about twelve and a half hours. Its orbit lasts 2,398 days. We may be calling that a "Rosetta year" soon. To actually calculate whether the comet's orientation will allow sunlight to strike the main panels for longer stretches, we'd need to know more about the cliff it's under. The ESA is no doubt crunching those numbers now, but it's possible that if this situation will resolve itself then it will take years.
A smaller panel got sunlight when the drill was used to rotate the probe. So, if it is powered down and we wait, it should eventually charge back up. Each time that happens, the ESA can work at getting it into a better position, little by little.
It's not dead; it's just napping. It will eventually be back online. The big question is, when? If the ESA knew for sure, they'd probable tell us. So, we wait.
As I understand it, the problem is that the batteries must be at 0 degrees C to accept charge. The limited sun it's getting now isn't enough to heat the batteries (surface temperatures are about -70C IIRC).
It took ten years to get the Rosetta mission to the comet. By then a RTG would be fairly depleted too.
That isn't a legitimate reason to not use RTG for Philae. The lander only uses 32 watts of power. The MMRTG used in Curiosity provides 125 watts of power initially, and 100 watts after 14 years. The mass of that specific RTG (the MMRTG) would be too great for use in Philae, but then it also produces 3 times more energy than needed (even after 14 years). RTGs have been made in many sizes for many different applications, so it would simply have been a matter of designing an RTG that produces 40-45 watts of power after 10 years.
However, one of the main uses of the 32 watts of power required by Philae is just to keep the batteries warm so they don't fail. RTGs produce more "waste" heat than they do electricity. For example, the MMRTG used in the Curiosity rover produces 2 kW of heat, of which 125 W is converted to electricity. The extra heat is used to keep the various temperature-sensitive parts of the rover nice and warm so they don't fail. With Philae, a good portion of the 32 watts of the solar power it requires is just to keep the battery warm. So if an RTG were used, it wouldn't even need to produce 32 watts of electricity since it can keep the lander warm directly.
Looking at the mass and wattage produced, the RTGs ("SNAP-19") in the Pioneer probes would have been just about perfect for Philae. They produce 40 watts of power and weigh 13.6 kg. Philae's current electrical system weighs 12.2 kg, so that's at least in the ballpark. The RTGs on the surface of the moon, as manually placed by Apollo astronauts's would have been a bit heavy at 20 kg. One of those RTGs was still producing 90% of its power after 10 years.
Regardless, the fact that the Philae mission would last more than 10 years is not a reason to not have used RTG. Other issues (obtaining the radioactive material, environmentalists throwing a fit, inexperience of the ESA with that kind of power source, delays in production, etc) certainly dictated that an RTG wasn't used, but it was most certainly not due to any technical limitation.
Better known as 318230.