I can't for the life of me understand why Rosetta doesn't have equipment to determine Philae's relative position to cm resolution, except perhaps in the case where the comet is between them.
i was thinking, couldn't they afford to put a simple signaling mirror or corner cube reflector on the lander so rosetta can just scan the surface and see any reflection?
I don't think there is such technology available. All the options that come to my mind need aiming and aiming is not an option as far as I can imagine.
It's possible that Rosetta could have carried a detector that could get the time of flight of Philae and so get a rough sphere where it could be. However, over the distances that we're talking about, you'd probably need a powerful laser with a very low divergence in order to get any signal there/back. LIDAR will break a power budget easily.
If you're thinking why couldn't we use some kind of GPS type positioning, remember that you need four satellites minimum for that (plus ground stations to correct things). There was some discussion about using multiple flybys of Rosetta, but it's not clear whether there'll be enough time to do this before the batteries die.
We can locate things on Mars because we have excellent cartography of the planet and the landers were the right way up. We're able to figure out where things are based on the images they send back. Once we know where a lander is the first time, it's easier to figure out where it is afterwards based on this imagery.
In Earth-based GNSS systems, it's for clock synchronization. You've got 4 unknowns (x,y,z,delta t) and 4 signals; if you only had 3 signals, you couldn't solve the system of equations.
Yes it can. You don't need LIDAR when you the thing you are tracking can have a purpose-built transmitter attached to it.
Obviously I'm not talking about GPS, I'm talking about some sort of EM radiation (e.g. the IR on your remote control) being emitted from Philae and being simply seen, as a bright dot in space, by Rosetta. Distance, obviously, you do with timing or with luminosity.
Doubtlessly there is a list of many other things I haven't considered, at the top of which are nonintuitive weight, power, and reliability restriction. But your comment doesn't answer my question at all.
Frankly I find your response rather snarky, remember comments aren't just for your benefit. Hopefully this helps answer your question.
You asked why it doesn't have a positioning system: the simple answer is the battery (Philae has only 130Wh or so), cost and whether it's necessary to do the science: it wasn't.
If you measure distance by timing light, you're using a LIDAR system by definition. A laser isn't explicitly necessary, but because the power received is proportional to R^-4, using an isotropic emitter is wasteful. A pulsed LIDAR system with cm accuracy over 20km is doable, you'd need to time the light to within around 30ns. However we'd struggle to get that kind of accuracy on Earth, let alone orbiting a comet.
Luminosity isn't good enough to measure distance (certainly not cm level), it depends on the efficiency of the emitter and the efficiency of the detector, both of which might have changed during flight. It also depends on the transmission medium, in this case it may well be dusty. It could potentially give you a location, but I'm sceptical Rosetta would see it above the background unless it was an extremely bright source. I think putting the transmitter on the lander would be the backward route.
A corner cube reflector would probably be the most viable solution. We're assuming that Rosetta can overfly Philae and fire its laser in the right place, but I think that would ok. LOLA on the Lunar Recon. Orbiter flew as low as 30km and only got down to around 10cm resolution and required around 15-30W.
My comment regarding GPS was that many people assume that you can get a 3D fix on an object with a single recieve/transmit pair (like in the movies) and it simply doesn't work like that. A directional antenna could work, perhaps.
Thank you for the info. I apologize for the snark. I was responding to what I believe was a reply that sounded confident but in fact didn't answer the question, a pet peeve of mine.
> LOLA on the Lunar Recon. Orbiter flew as low as 30km and only got down to around 10cm resolution and required around 15-30W
Thank you, this was interesting. Note that Rosetta's solar arrays generate 1500 W.
> You asked why it doesn't have a positioning system: the simple answer is the battery (Philae has only 130Wh or so), cost and whether it's necessary to do the science: it wasn't.
I'll claim that a small, pulsed IR LED array would be sufficient to give sufficient directional information on sub-second timescales with negligible power draw.
I think the scramble to try to determine how Philae had bounced and to locate it on the comet surface is good evidence that ESA thought location info had an important impact on the scientific mission.
2) It's a first one. It wasn't really known up to the last detail what to expect.
If there's the next mission, some things will certainly be done differently. Especially if this one results in more willingness to fund the next.
Also note that the precise location on Earth (GPS) needs a lot of satellites in orbit providing only that. There's only one device now orbiting around the comet's core. The position will probably be found based on all we have now (1), but having it very fast is significantly harder.
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?
"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."
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).
"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)
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 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.
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?
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.
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.
"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.
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.
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."
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.
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.
Latest from Ms.Lakdawalla on the location of Philae [1][2][3] :
So after a tip from Gerald at
http://www.unmannedspaceflight.com/index.php?
showtopic=7896&view=findpost&p=215245 … I wonder if
bright spot is Philae and dark spot its shadow?????
So the latest news from Darmstadt is that telemetry is coming in! Philae is alive! :) That's fantastic news, because there was a real concern that it would have died off due to the limited solar irradiance. The most important thing to figure out is whether the drill that was extended 200mm actually grabbed hold of anything. Also, it's possible that Philae got displaced in the meantime.
Getting live updates from mission control at the moment ... exciting! They're in the midst of performing a "lift and turn" maneuver to try to get Philae into a better position.
I believe that regardless of whether or not they have solar power, Philae should run for ~60 hours without power if I remember what I had heard correctly.
edit: there's a chance the shift in position about 20 mins ago might help with solar recharge. Also there is chance of better solar around Christmas when the comet orientation towards the sun is different.
Wait, so Philae could possibly reboot months from now when it gets more sunlight? It sounded like charging could only be initiated if the computer was already operational and the batteries were already heated to 0C.
If it will get enough sun when hibernated it will heat battery up and start charging. When there's enough charge (possibly after many periods of charging and hibernation) it can start working again.
But it's probable that the dust will acumulate on the solar cells before comet is close enough to sun for this to work.
Comet has very low gravity and - think of classic 'comet tails' - stuff gets stripped off by solar winds etc near sun. So yes I think it can be a bit dusty
"Philae's planned mission is expected to come to an end when batteries are exhausted sometime on Saturday; future contacts are possible if the illumination conditions change as the comet orbits closer to the Sun, enabling solar power to flow again."
Harpoons didn't fire. They kept identical harpoons in vacuum here on earth, and after 10 years, they didn't work. I guess that component just didn't survive the journey.
re: batteries: I gather the 1.5hrs a day would never be sufficient to get enough for a wake-up. Some hope that shift in position earlier today may help. Also some hope that comets orientation to sun will change (likely around Christmas)
They mentioned at the google hangout yesterday that it wasn't out of the question they could wake it later again(weeks/months) when the comet is closer to the sun and receives more light and has charged up the lander.
> Also, if the batteries die, can't they just wait a few days until the ~1.5hrs/day of sunlight charges them enough to do something useful again?
The explanation I saw was that you need a set amount of energy to warm up the batteries enough to be able to charge them effectively, and 1.5 hours of weak sunlight won't be enough to hold whatever you can charge up until the next day. You'd never accumulate. (Batteries hold less at lower temperatures.)
This is an instrument that does analysis on whatever samples the drill picks up. We know the drill successfully deployed earlier, so hopefully that means it picked up something and we now have the data from that!
Looks like that's pretty much it for the battery though, unless last minute manoeuvrings result in the potential for a recharge in the future:
Edit: CONSERT data now coming in! CONSERT is an instrument to detect a radar pulse sent out by Rosetta that travels through the centre of the comet to tell us something about its internal structure. Also another ROLIS image (panorama) has been taken, though it's not clear to me if it's been transmitted yet. Very low battery now.
Edit: I missed this before, but it looks like Ptolemy data has been received too! This is another instrument that's supplied with samples from the drill.
When I think about this it gets progressively more amusing. Year 2014 is a time point when Jesus was between 4 and 6. That level of imprecision contrasts starkly with the time stamps being discussed. That Jesus even comes into it is fascinating. Maybe 2014 CE is less controversial, although somewhat dishonest as it uses the same time point as reference.
http://en.m.wikipedia.org/wiki/Anno_Domini
Eh, I think at this point the provenance and significance of the zero date is less important than just having a zero, any zero, that everyone can agree upon and that matches up with the standard used more-or-less globally for the last century at least. I don't think there's anything dishonest about CE using the same reference point as AD; it's just the most convenient one.
The beginning of the Holocene around 11700 BP might serve as a good objective reference point. Nearly every interesting thing that happened in the history of human civilization took place in the Holocene. No more negative years to worry about!
"Present" in "before present" (BP) usually means 1950 AD [1], so this year would be Year 11764 of the Holocene epoch. Brb, gotta find some COBOL programmers to fix that Y10K problem.
But then, the starting date of 11700 BP is also kinda arbitrary. No single short-term event defines the boundary between Pleistocene and Holocene, so we're still left with a sizable margin of error, probably even larger than the error in Jesus' date of birth.
I imagine it's either to make the date internationally unambiguous (re: interpreting yyyy.mm.dd vs yyyy/dd/mm) or to save two bytes per string. I'd bet it's to make better use of bandwidth, since these time stamps are likely sent enough to have some payoff for it.
I'm confused, if after the first contact the lander expected to be stationary but was actually moving 20cm/s away from the comet, how is the lander still on the comet? 20cm/s seems like it would take it off into space pretty quickly. How does a lander "bounce" on a comet without an appreciable gravity, wouldn't it only bounce once?
"But then the lander lifted from the surface again – for 1 hour 50 minutes. During that time, it travelled about 1 km at a speed of 38 cm/s. It then made a smaller second hop, travelling at about 3 cm/s, and landing in its final resting place seven minutes later."
It's not that its direction was directly away from the comet. It's that it's estimated that it slowly moved during almost 2 hours. The sensors detected landing once then almost 2 hours later again. The speed is an estimate, but it's reasonable. Also note the shape of the comet, it isn't spherical.
Aren't these space missions sort of like firing a pebble off a gun and hitting a 1 cm target half way around the earth? It always amazes me how low the failure rate seems to be.
That comparison is a bit disingenuous. There's plenty of opportunity to correct the trajectory as and when you need to, which is something you can't do with a bullet. Still a huge achievement though.
What I find particularly impressive is that they managed to get in a fly-by of two different asteroids as well! So that the orbit was optimised to hit not one but three targets (though presumably they had a lot of asteroids to chose from).
Aren't the adjustments minute, though? Although I guess that even minute adjustments of the angle will result in huge changes in the final trajectory, over thousands of kilometers.
But you're also ignoring the fact that unlike a bullet, there isn't any atmosphere or unpredictible factors in your way. You can do the math and be sure about it. You can also use your math to course-correct after the fact.
That's not at all like pointing your gun and pulling the trigger.
I find it amazing how we've gone from a dim blur on a photographic plate (which was found by accident when trying to photograph a different comet entirely) to what we see now.
I also find it fascinating to think how close we were to landing on an entirely different comet if the original launch window had been hit. There's just so much out there. What was on that other comet (46P/Wirtanen) that we might never get to see?
Also each Philae experiment seems to have a "personal" twitter account set up too, and with a short description, worth a quick reading when they link to them.
Some specially noteworthy recent updates from accounts above:
@elakdawalla:
The sun will rise on Philae around 6:00 am UTC. It is theoretically possible it could wake up then. Next comm pass would be at around 11:00
@chrislintott: Current comms mode is 1 packet every a few minutes - but no science data left on @philae2014 - it all got uploaded! Wonderful! #cometlanding
@FrancescoTop:
Consert PI is confident that using tonight's data w/ previous one we can triangulate and find where #Philae is on the comet.
@elakdawalla:
Even if the solar panel is in a better position, it's quite possible it won't wake up tomorrow, but could recharge over days, wake later
It was immense luck all the time. Since harpoons failed, there was serious chance for lander not to remain on the comet at all, or to get seriously destroyed or disabled earlier than now. Now it seems that most (all?) of the experiments were performed. Wow.
But in order to get lucky, we have to first dare to try and then invest in work. Congratulations!
Yes, the harpoons definitely weren't the only problem in the mission. We can always imagine even better outcomes (like, bouncing but ending on a place with a lot of sunlight) but it appears it was very good even so: until recently we weren't sure if any movement would launch the lander back into space. Or if the battery would last long enough for more experiments to be done. Or the results to be sent.
@FrancescoTop is my boss! :) Lots of ups and downs over the last couple of days, but I think we can say that Philae is an unmitigated success. There's a lot been said about the problems faced, but despite that, there's been some great science data downlinked and hopefully a lot of fantastic discoveries ahead. Philae has helped shed light on a lot of outstanding issues in Solar System formation and planetary science. The hope is that this will help spur on further research and lead to more insights into the ingredients that go into the models that describe where we came from and where we're going.
For mine, scientific endeavors such as these are worth much more than the experiments they entail; they help inspire the next generation of scientists, engineers, mathematicians and so on.
This is has been a great story. Many great space missions have had a major component failure and still provided data that changed our view on things. I find it especially interesting to see if the lander gets 'spit out' when the comet starts outgassing, we might get still more data out of it if it returns to a spot with good solar coverage. Given the lander's mass and the lack of harpoons, if it is in a position to get pushed out I expect there is a very good chance it will.
89 comments
[ 216 ms ] story [ 2819 ms ] threadCouldn't Rosetta have told them that Philae had bounced? I might have the wrong scale in my head of just what Rosetta could see.
In any case this is all still wondrous.
they've had them on the moon for decades.
http://en.wikipedia.org/wiki/Corner_reflector
It's possible that Rosetta could have carried a detector that could get the time of flight of Philae and so get a rough sphere where it could be. However, over the distances that we're talking about, you'd probably need a powerful laser with a very low divergence in order to get any signal there/back. LIDAR will break a power budget easily.
If you're thinking why couldn't we use some kind of GPS type positioning, remember that you need four satellites minimum for that (plus ground stations to correct things). There was some discussion about using multiple flybys of Rosetta, but it's not clear whether there'll be enough time to do this before the batteries die.
We can locate things on Mars because we have excellent cartography of the planet and the landers were the right way up. We're able to figure out where things are based on the images they send back. Once we know where a lander is the first time, it's easier to figure out where it is afterwards based on this imagery.
The second satellite gives you a circle of intersection between the spheres.
The third satellite gives you two points where three spheres intersect. One of these is usually in space so it's discarded.
The fourth one lets you correct for timing errors in the reciever. A microsecond uncertainty corresponds to around 300m (i.e. c * 1µs).
You could get coarse information with three satellites, for instance if you already know your altitude, but four+ is preferable.
Yes it can. You don't need LIDAR when you the thing you are tracking can have a purpose-built transmitter attached to it.
Obviously I'm not talking about GPS, I'm talking about some sort of EM radiation (e.g. the IR on your remote control) being emitted from Philae and being simply seen, as a bright dot in space, by Rosetta. Distance, obviously, you do with timing or with luminosity.
Doubtlessly there is a list of many other things I haven't considered, at the top of which are nonintuitive weight, power, and reliability restriction. But your comment doesn't answer my question at all.
You asked why it doesn't have a positioning system: the simple answer is the battery (Philae has only 130Wh or so), cost and whether it's necessary to do the science: it wasn't.
If you measure distance by timing light, you're using a LIDAR system by definition. A laser isn't explicitly necessary, but because the power received is proportional to R^-4, using an isotropic emitter is wasteful. A pulsed LIDAR system with cm accuracy over 20km is doable, you'd need to time the light to within around 30ns. However we'd struggle to get that kind of accuracy on Earth, let alone orbiting a comet.
Luminosity isn't good enough to measure distance (certainly not cm level), it depends on the efficiency of the emitter and the efficiency of the detector, both of which might have changed during flight. It also depends on the transmission medium, in this case it may well be dusty. It could potentially give you a location, but I'm sceptical Rosetta would see it above the background unless it was an extremely bright source. I think putting the transmitter on the lander would be the backward route.
A corner cube reflector would probably be the most viable solution. We're assuming that Rosetta can overfly Philae and fire its laser in the right place, but I think that would ok. LOLA on the Lunar Recon. Orbiter flew as low as 30km and only got down to around 10cm resolution and required around 15-30W.
My comment regarding GPS was that many people assume that you can get a 3D fix on an object with a single recieve/transmit pair (like in the movies) and it simply doesn't work like that. A directional antenna could work, perhaps.
> LOLA on the Lunar Recon. Orbiter flew as low as 30km and only got down to around 10cm resolution and required around 15-30W
Thank you, this was interesting. Note that Rosetta's solar arrays generate 1500 W.
> You asked why it doesn't have a positioning system: the simple answer is the battery (Philae has only 130Wh or so), cost and whether it's necessary to do the science: it wasn't.
I'll claim that a small, pulsed IR LED array would be sufficient to give sufficient directional information on sub-second timescales with negligible power draw.
I think the scramble to try to determine how Philae had bounced and to locate it on the comet surface is good evidence that ESA thought location info had an important impact on the scientific mission.
2) It's a first one. It wasn't really known up to the last detail what to expect.
If there's the next mission, some things will certainly be done differently. Especially if this one results in more willingness to fund the next.
Also note that the precise location on Earth (GPS) needs a lot of satellites in orbit providing only that. There's only one device now orbiting around the comet's core. The position will probably be found based on all we have now (1), but having it very fast is significantly harder.
1) https://twitter.com/FrancescoTop/status/533417398662336512
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:
http://elakdawalla.tumblr.com/post/102615327170/philae-updat...[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!
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...
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.
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.
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?
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.
"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)."
It seems the only benefit would be speed, and space folks are patient.
Technology to build and use Radioisotope Thermoelectric Generators in Europe has not been developed, largely for political reasons
https://twitter.com/elakdawalla/status/533256200088211456
http://en.wikipedia.org/wiki/SNAP-10A
I think there's been others as well
"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.
[2] http://www.unmannedspaceflight.com/index.php?act=attach&type...
[3] http://www.unmannedspaceflight.com/index.php?s=af65642091d8b...
Getting live updates from mission control at the moment ... exciting! They're in the midst of performing a "lift and turn" maneuver to try to get Philae into a better position.
edit: voltages dropping right now https://twitter.com/FrancescoTop/status/533401941163114496
edit: looks like its going dark right now. https://twitter.com/elakdawalla/status/533403569660379136
edit: link still up but expecting to lose it any time. Getting science data in https://twitter.com/esaoperations/status/533406632278261760
edit: there's a chance the shift in position about 20 mins ago might help with solar recharge. Also there is chance of better solar around Christmas when the comet orientation towards the sun is different.
But it's probable that the dust will acumulate on the solar cells before comet is close enough to sun for this to work.
http://blogs.esa.int/rosetta/2014/11/15/turning-philae/
(Note: that was written before the sucessful reorientation occurred)
I hope the adjustment will mean more power
Also, if the batteries die, can't they just wait a few days until the ~1.5hrs/day of sunlight charges them enough to do something useful again?
re: batteries: I gather the 1.5hrs a day would never be sufficient to get enough for a wake-up. Some hope that shift in position earlier today may help. Also some hope that comets orientation to sun will change (likely around Christmas)
The explanation I saw was that you need a set amount of energy to warm up the batteries enough to be able to charge them effectively, and 1.5 hours of weak sunlight won't be enough to hold whatever you can charge up until the next day. You'd never accumulate. (Batteries hold less at lower temperatures.)
Oh, explained better with a quote a little downthread: https://news.ycombinator.com/item?id=8609965
This is an instrument that does analysis on whatever samples the drill picks up. We know the drill successfully deployed earlier, so hopefully that means it picked up something and we now have the data from that!
Looks like that's pretty much it for the battery though, unless last minute manoeuvrings result in the potential for a recharge in the future:
https://twitter.com/Philae2014/status/533403430489178112
Edit: CONSERT data now coming in! CONSERT is an instrument to detect a radar pulse sent out by Rosetta that travels through the centre of the comet to tell us something about its internal structure. Also another ROLIS image (panorama) has been taken, though it's not clear to me if it's been transmitted yet. Very low battery now.
http://en.wikipedia.org/wiki/CONSERT
Edit: I missed this before, but it looks like Ptolemy data has been received too! This is another instrument that's supplied with samples from the drill.
http://sci.esa.int/rosetta/31445-instruments/?fbodylongid=89...
2014.318.14.09.20.272 - 2014.318.22.31.05.464
It seems to be YYYY.dayOfYear.hh24.mi.ss.miliseconds
http://en.m.wikipedia.org/wiki/Nativity_of_Jesus#Date_of_bir...
"Present" in "before present" (BP) usually means 1950 AD [1], so this year would be Year 11764 of the Holocene epoch. Brb, gotta find some COBOL programmers to fix that Y10K problem.
But then, the starting date of 11700 BP is also kinda arbitrary. No single short-term event defines the boundary between Pleistocene and Holocene, so we're still left with a sizable margin of error, probably even larger than the error in Jesus' date of birth.
[1] https://en.wikipedia.org/wiki/Before_Present
"But then the lander lifted from the surface again – for 1 hour 50 minutes. During that time, it travelled about 1 km at a speed of 38 cm/s. It then made a smaller second hop, travelling at about 3 cm/s, and landing in its final resting place seven minutes later."
It's not that its direction was directly away from the comet. It's that it's estimated that it slowly moved during almost 2 hours. The sensors detected landing once then almost 2 hours later again. The speed is an estimate, but it's reasonable. Also note the shape of the comet, it isn't spherical.
If my math is right[2], I think that puts escape velocity above 20cm/s out to about 33 kilometers.
[1] http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimen...
[2] http://www.wolframalpha.com/input/?i=%282+G+%2810^13+kg%29%2...
What I find particularly impressive is that they managed to get in a fly-by of two different asteroids as well! So that the orbit was optimised to hit not one but three targets (though presumably they had a lot of asteroids to chose from).
But you're also ignoring the fact that unlike a bullet, there isn't any atmosphere or unpredictible factors in your way. You can do the math and be sure about it. You can also use your math to course-correct after the fact.
That's not at all like pointing your gun and pulling the trigger.
https://www.flickr.com/photos/europeanspaceagency/1539815083...
I find it amazing how we've gone from a dim blur on a photographic plate (which was found by accident when trying to photograph a different comet entirely) to what we see now.
I also find it fascinating to think how close we were to landing on an entirely different comet if the original launch window had been hit. There's just so much out there. What was on that other comet (46P/Wirtanen) that we might never get to see?
https://twitter.com/ESA_Rosetta
https://twitter.com/Philae2014
https://twitter.com/elakdawalla
https://twitter.com/chrislintott
https://twitter.com/FrancescoTop
Also each Philae experiment seems to have a "personal" twitter account set up too, and with a short description, worth a quick reading when they link to them.
Some specially noteworthy recent updates from accounts above:
@elakdawalla: The sun will rise on Philae around 6:00 am UTC. It is theoretically possible it could wake up then. Next comm pass would be at around 11:00
@chrislintott: Current comms mode is 1 packet every a few minutes - but no science data left on @philae2014 - it all got uploaded! Wonderful! #cometlanding
@FrancescoTop: Consert PI is confident that using tonight's data w/ previous one we can triangulate and find where #Philae is on the comet.
@elakdawalla: Even if the solar panel is in a better position, it's quite possible it won't wake up tomorrow, but could recharge over days, wake later
Best news there is! :)
But in order to get lucky, we have to first dare to try and then invest in work. Congratulations!