Philae status, a day later
elakdawalla.tumblr.com
elakdawalla.tumblr.com
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?
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.
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!
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
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."
[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
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.
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?????
[2] http://www.unmannedspaceflight.com/index.php?act=attach&type...
[3] http://www.unmannedspaceflight.com/index.php?s=af65642091d8b...
"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.
Couldn'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.
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
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