4 ms·

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

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.

jessriedel 12y ago
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.
- micampe 12y ago
  Looking at pictures, I believe Philae solar panels are on the external shell, they don't need to be extended.
- 3am 12y ago
  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?
  - maaku 12y ago
    What shielding? There's nothing live on it.
    - mikeash 12y ago
      Electronics aren't particularly fond of radiation either.
      - beefman 12y ago
        All space-faring electronics need to be rad-hardened, and not because of an RTG source that emits only alpha particles.
joshvm 12y ago
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.

jessriedel 12y ago

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.

micampe 12y ago
Looking at pictures, I believe Philae solar panels are on the external shell, they don't need to be extended.
3am 12y ago
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?
- maaku 12y ago
  What shielding? There's nothing live on it.
  - mikeash 12y ago
    Electronics aren't particularly fond of radiation either.
    - beefman 12y ago
      All space-faring electronics need to be rad-hardened, and not because of an RTG source that emits only alpha particles.

joshvm 12y ago

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.