There's a nice image[2] which shows this in context linked to in the comments [3]. ESA seem to think the lander ended up somewhere over in the dark cliffs of the large crater filling the right hand side of this image.
If there's one plus in all of this (notwithstanding the data already returned!) it's that if there wasn't a thick enough coating of dust to absorb the shock of a light impact in the absence of the harpoons or retro-thrusters, and the surface was hard enough to break the thermometer... there may also not be enough dust to cover the panels as 67P nears the sun, which means we may not have heard the last from Philae.
Does anyone know if the highly irregular shape of the comet would cause a wild variation of the strength of the gravitational field near the surface of the comet? (as opposed to a 'perfect' spherical planet).
I'd imagine that would cause quite interesting dynamics and make it potentially quite difficult to calculate where it will end up?
You move from calculations using little-g to calculations using big-G (i.e. the acceleration due to gravity varies significantly since the distance between centers of mass is varying significantly), but this is still a straight-forward problem of one body moving in a gravitational field.
Original question said: when moving near the comet. So ISTM that it's not so straightforward. Some months ago someone posted a gravity simulator here and it was very instructive playing with it and seeing how unstable a small object's orbit is around two bigger objects rotating around their center of gravity, situation that seems similar to the "duck".
Try creating two big objects in a circular orbit and experiment launching smaller objects with the same direction and speed. Some of them will crash, some others will escape.
My impression is that although it is not spherical, the core of the comet is a single object. It may therefore be treated as a point mass for these purposes.
You can treat anything as a point mass as long as you're far away enough ;) And even with point masses only, orbital mechanics can get quite complex.
When you are close enough that a significant portion of the comet's mass is "besides" yourself (somewhere off to the side) it will pull you to the side.
This even holds true for spherical objects (say, you standing on earth), where you get gravitational pull not only towards the center but also towards the sides. For a perfectly symmetrical sphere, these sideway forces cancel each other out tough.
But earth isn't a perfect sphere, so even on our own planet you get (very small) variations in gravitational pull (stronger pull at the poles or near mountains (where the crust is thicker)): https://en.wikipedia.org/wiki/Gravity_of_Earth
I encourage you to think a bit more about what is meant by center of mass. By definition, it takes into account all the mass of the body. If the force of gravity is pulling you "somewhere off to the side", then the center of mass is also "somewhere off to the side". Think about the extreme case of two identical-size, uniform-density spheres attached at a single point. That point will be the center of mass, despite the fact that little of the mass of the entire object is located near there.
The center of mass is the point where the force of gravity would pull you if the force increased proportionally with distance. In reality the force decreases as 1/r^2, which changes things. Think about the more extreme case of a barbell -- a single object. If you're 2/3 of the way on the bar, you'll be attracted towards the closest endpoint, away from the center of mass (midpoint).
It amazes me that we take for granted the sheer complexity of hitting a moving target like a comet at such an incredible distance in space. Navigation in space has to be so incredibly complex because you are moving in so many planes.
It's like firing a gun and hitting a quarter that was thrown miles away.
The video is very cool. But I find it hard to imagine how to orbit something that has hardly any gravity - the orbiter must be getting just milligrams of force.[1] The scale isn't clear from the video: are they orbiting really slowly? And it seems like it would be very hard to enter orbit around something that has hardly any pull without flying right past it. On the positive side, the orbit changes in the video would require almost no propulsion, right?
[1] milligrams isn't a unit of force, but hopefully you know what I mean; milli-ounces sounds too weird.
The comet[1] that Rosetta is orbiting has a mass of 10^13 kg and an orbit semi-major axis of 518,060,000 km. This gives the comet a Sphere of Influence[2] of roughly 62 km.
What this means is that within 62 km or so, the comet is the dominant gravitational body, so orbits of 16, 20, and 30 km (in the video) are perfectly reasonable.
The orbital period[3] of the larger 30 km orbit is around 40 ksec or 11.1 hours.
Your orbital period of 11.1 hours at 30 km looks wrong. The video shows about 7/8 of a 30-km orbit over 14 days (19 Nov - 3 Dec). The orbital period formula gives 14.6 days (2 * 3.14159265 * sqrt(30000^3/(6.67384e-11 * 10^13)) / 3600 / 24).
What does a "relay phase manoeuvre" mean, and why are they performed? Those are the weirdest ones, looking very much like "heh, let's go in some random direction that we didn't try yet!"...
These maneuvers look strange because Rosetta is too far from the comet at those points for a real orbit. The probe can change course freely without the gravity of the comet affecting it much.
The reason for the maneuvers is to maintain line of sight with the Philae lander so that communications between Earth and Philae can be relayed, hence "relay phase". Then the probe inserts itself back into an orbit.
I don't think "we" do take it for granted. It's just that those of us who don't can't think of much more to say than "Woah! on a comet?! wow." This is incomprehensibly amazing to me.
Thinking about navigating in zero gravity became a lot easier for me when I realised that you can arbitrarily redefine “down” as whatever direction is convenient. Gravity assists and orbits, for example, are simply falling around a bend.
The pic you linked to was taken with a different camera (NAVCAM), the pictures in this article (OSIRIS) are much better, and clearly show the lander before and after touchdown along with marks where it first landed/bounced. What they don't show unfortunately is the final resting place, but that's a bit tricky because of comet rotation, comet shape (not a flat surface), and the parabolic trajectory of the lander.
What's the confusion about the time? Touchdown was at 15:34 UTC (NB UTC), I doubt anyone at ESA is confused about the time, though some of the commenters seem pretty confused on the ESA blog post this BBC article is based on:
That's an older article about old pics though isn't it, not these new pics? It was just a label on the website blog which the web team got wrong and was corrected soon after posting. The pics in the article you link are 4 mins before and 1 minute after landing, not 1 hour after, and they are corroborated by these new images, so no real confusion over timings.
I haven't seen any discussion of the possibility of Philae receiving light to its solar panels as the comet tumbles through space. Is there no chance of the comet turning to a point where the sun can reach the lander?
Philae is getting 1 hour of sunlight about every 12 hours. Right now, it's so cold that it has to warm up the batteries before charging them, and it's getting so little light that it can't even do that. It's possible that the rotation might change, but because the probe is surrounded on 3 sides, that's not likely to help. So we'll have to wait for the temperature to get higher and/or the sunlight to get a lot stronger.
Even if they could (it’s unlikely) … would they? Philea was a pretty risky and relatively small add-on to the main Rosetta mission. Rosetta does important stuff all on its own, it’s not just a vehicle to get Philae there and relay data.
Rosetta will stay with the comet as it gets closer to the sun and observe it.
Even under perfect conditions, this would provide a tiny bit of extra light for a few seconds per orbit, not even close to enough to make a difference unfortunately.
This is probably not feasible, but in order for reflection to help much Rosetta would have to be much farther from the comet, so that its orbit would be comet-synchronous. Actually I doubt Rosetta has any surface reflective enough to help with this anyway. If it were used this way it would probably not be pointing its transmitter toward Earth.
I wonder if there's a reason they didn't design the lander with some sort of energy-absorbing material (something analogous to the crush zone in a car) on the bottom of the landing pads to prevent bounces. I thought I'd "wonder out loud" here because someone on HN might actually know the answer.
Its landing was supposed to be softened by a thruster, which didn't work, and it was supposed to be held fast upon landing by harpoons, but they also failed. They presumably did use some shock absorbtion, but it was probably engineered under the assumption that at least some of the rest of the landing system would have worked.
The thruster, if it had fired, would actually have propelled the lander towards the comet. The idea is that it would hold the lander in place against the ground, not slow the lander's descent.
in the initial race to the moon it was widely speculated that the moon was covered in a deep layer of ultrafine dust, and any spacecraft landing on it would be liable to sink. Arthur C. Clarke's novel "A Fall of Moondust" was based on this thinking.
It seems astronomers have a bias towards assuming dust will cover stuff. Perhaps this says something about their office space.
I thought they calculated the amount of dust that based on the current rate dust accumulation and extrapolating based on the believed age of the universe. It's a little surprising we have not corrected that thought since the moon landing.
Who's to know why they were wrong — it's just amusing to see the same error come up again. I prefer the theory that they look around their offices and extrapolate ;-)
They weren't wrong, there's plenty of dust on the comet. The first photo of the initial landing site is visible precisely because of the shadow cast by a giant cloud of dust kicked up by the lander.
They were wrong about the amount of dust, not the presence of dust -- as was the case with the moon landing. There's plenty of dust on the moon, too, just not oceans of it.
Landing on Gilly, the asteroid orbiting Eve in KSP, will give an idea of the challenges of the low-gravity environment Rosetta/Philae are operating in. The Rosetta comet has lower gravity, though, by two orders of magnitude.
Any news on if the drill managed to pick up anything useful? Curious to find out if the Ptolemy data that came back was of anything useful (as I understand it, Ptolemy's task was to measure isotope ratios so that we could see if the water composition on the comet was at all similar to that on Earth).
They might have had some results from particles/gases in the atmosphere, not sure, nothing announced yet that I know of but they did say they were taking measurements. It might be weeks/months before they announce results.
I hear some of the instruments were owned by private research institutes rather than European taxpayers (who funded the vehicle) so for some areas may take a while to publish. Might have to wait for proper papers etc
> I hear some of the instruments were owned by private research institutes rather than European taxpayers (who funded the vehicle) so for some areas may take a while to publish. Might have to wait for proper papers etc
It is not immediately obvious to me why who owns the instrumentation would be a driving factor in the time to see results. Both private research institutes and public institutes would want to get results out as quickly as possible, to capitalize on the PR effect. But that is moderated by the need to perform sufficienly accurate and detailed analysis, which takes time. Those pressures are the same for both public and private research institutes. If anything, the private institues would feel more pressure to publish results quickly, to improve their chances of obtaining future funding.
I cannot tell if you are comparing the quickness of this press release with the lack of results from the scientific packages on Phile, but I will address the (potential) comparison, just in case... Part of reason this press release (with the image of Philae bouncing) was out so quickly is that it does not require a very detailed and specific analysis (requiring roughly the equivalenth of image stictching in photoshop), and the results are fairly straightforward. Additionally, it is not really providing much in the way of new scientific information and so does not have as high a burden of proof. Additionally, the OSIRIS imager has been used to collect data for a while now, so presumably the OSIRIS team has a decent understanding of the real-world operation of the equipment, enabling them to quickly release images in which they were confident. In contrast, the Philae lander's instruments had not provided actual data before the landing, so the instrument teams still need to do the careful calibrations and analysis to understand the systematics.
But your point stands; I know a lot's under embargo right now, I was more wondering if we knew if the drill had successfully retrieved a bit of comet or not rather than specific experimental results which will take some time!
I have to say this. I have been as amazed as anyone by the feat of engineering that this entire mission represents. It is absolutely mind-blowing and I have been sharing it with others, including my kids.
But, something puzzles me. That is, why didn't they use better camera technology? That's a lot of miles traveled and a lot of effort. Seems the mission would have been much better served by better imaging, both from a scientific perspective and from a public interest perspective (which can help engender support for future exploration).
Frankly, the fact that it even happened amazes me. But, the images disappoint me.
I know it sounds like a nit, given the overall accomplishment, but that's all the more reason to wonder why not ensure that something presumably as simple as the images we capture be as stunning as possible?
The wifi reception is really shitty up there. Jest aside: More pixels, more bandwidth needed and if you think about how much bandwidth wifi systems were capable of achieving 10 years ago, they're probably at the edge of what they could use without resorting to cutting edge tech. You also have to transmit the data from the lander to the orbiter in a reasonably short timeframe since the lander is not always visible from the orbiter.
Maybe they have higher res pictures still on the orbiter that will download later when there's bandwith left.
Not only 10 years ago, but also "known reliable equipment" from 10 years ago. Cutting-edge technical equipment usually isn't sent up on these things due to the propensity for unknown bugs.
Yes, known reliable is what I meant. Or, more accurately, best reliable. Seems like we certainly had better 10 years ago.
Just really odd that this tremendous feat couldn't be paired with much better imaging tech. I mean, the imaging is strikingly poor and unimpressive compared to the rest of the mission.
Found the answer. Those who mentioned that it was state of the art for the time were correct.
The main camera, OSIRIS, is only 4 megapixels. Also, the camera cost $100MM, which is a significant percentage of the overall mission. So, it's not as if they were skimping.
None of the probes has ever used what could be called cutting edge equipment, the science behind it all is cutting edge but you can't put a camera, lenses or processors on a device going into outer space without having done some serious testing. It's just easier to do it with devices and such that have a long track record of working with radiation, incredible cold and so on.
I totally know where you're coming from though, having Curiosity or Rosetta be equipped with some serious photographical capabilities would've been awesome. But I doubt any of the firms are going to put any kind of cutting edge imaging solution on them any time soon.
I hear you, but it's hard to imagine we didn't have better, reliable tech at the time, including hardening for the environment. And, at a minimum, seems they could've developed the tech in concert, and still retained the lower quality camera as a backup.
Sure, they certainly have download-continuation deployed, but they also have a lot of data and a very narrow channel and time window - the lander had only battery for 68 hours most of the time the lander is not visible to the orbiter. The problem is not imaging. The lens is certainly magnitudes better than you can see on the pictures, but transmitting all those pixels back to earth is the real problem. TCP kinda gets hard if RTT is approximately one hour.
68 hours was the lander's initial battery life. The solar panels were to recharge the batteries, and the expected operation was up to 3 months. The expected hard stop is March 2015, when the comet's orbit will take it close enough to the sun that the tech is not expected to survive the heat.
If you're sending millions of dollars worth of technology into space for 10 years, you want to be more than reasonably sure it will still work when it gets where it's going.
That is simply not something you can be reasonably sure of with "cutting edge" technology.
Well, I didn't literally mean cutting edge as in first one ever made. I just meant best reliable technology of the time, which 10 years ago was certainly much better than what we are seeing.
best reliable technology capable of withstanding 10 years in ~ absolute zero while being bombarded with radiation? I'm pretty sure they would still need technology that was "best reliable tech" many years before the launch (and had been tested in space).
I think you may be overestimating the complexity of hardening. It's something we do very well. Consider Hubble, manned missions, and also the probes that have journeyed much farther out than Earth's orbit. Hard to imagine that simply upping the image resolution would pose a major hurdle in that arena.
BTW, the solar panels upon which Rosetta relied to make the journey were pretty cutting edge. And Rosetta is the first mission to go beyond the asteroid belt relying only on solar power.
Had they failed, the entire mission would have failed.
It's because you need to send something that needs to work on very tight energy budgets, after 10 years of radiation exposure and extremely cold temperatures, and on tight bandwidth budgets. They're lifespans and conditions that are rarely considered when designing cutting edge tech.
If you only get one shot at setting up for a mission that won't start gathering data until 10 years later, you probably don't want to take a state of the art camera up there, you want to take a decent camera that you're pretty sure will work well after 10 years.
Hindsight is 20/20, but it really looks like the lander descended too quickly, at least at the final stage. I mean, any smaller bounce would have prevented the shadow landing. But then, there are probably risks associated with a last-second deceleration, and the odds of both the top-thruster and the clamps failing were probably small.
The only way to make it hit at a lower speed would be to have some sort of thruster on the bottom, which could have failed just as the one on the top did. Additional redundancy may have helped, but that would have reduced useful payload for other things, and since the top thruster is more versatile then you may as well go for redundant top thrusters rather than top and bottom.
I associate 'bouncing' with a certain amount of elasticity. If the comet's surface is much harder than expected, I would expect the lander to be smashed to bits.
Where is the flaw in my thinking? Is this to do with it being very small forces exerted over a long time period?
The gravity and forces we're talking about are tiny. The craft hit the ground moving slower than 1 m/s and then bounced up over 1km before landing again. The forces involved with something moving that slowly and stopping are minuscule.
This is fascinating. I imagine they probably ran simulations with different margins and as a few bounces would have been one of the expected outcomes i.e. it would have been factored into the design of the craft?
The craft had a small thruster on its top side to gently push it into the surface of the comet, but this seems to have been faulty and couldn't be primed prior to Philae's separation. It sounds like the sort of equipment that might have mitigated such a bounce!
Think about dropping a golf ball onto concrete vs. sand. The harder surface will result in less loss of energy due to deformation. Thus, the ball bounces higher from the hard surface.
You're missing speeds and acceleration. Comet 67P is a very, very tiny world, with barely the slightest hint of gravity compared to Earth's surface. Consider that an ordinary walking speed is about 1 m/s. That's the escape velocity on 67P. A human could literally jump off the surface under their own power, or they could run into a low orbit (low altitude orbit velocities are 0.7x escape velocities, in general).
The speeds and accelerations that we're used to in the context of bouncing from the surface within the context of Earth are about ten thousand times greater than on comet 67P. The amount of force generated by Philae's collision with the comet's surface was very tiny, but it only takes a very tiny force to throw things long distances across the surface of the comet, due to the extremely low gravity (about 1mm/s^2).
Philae hit the comet at around 0.5m/s. 0.5m/s is a very small velocity. The probe is definitely much elastic than the comet, for start, the legs were designed to be elastic.
This is my understanding from reading the web site and some of the proposal papers associated with the mission.
The plan was that Philae would come in at net 0,0 velocity with respect to the normal of the landing site. That involved putting it into an orbit which intercepted the landing site. This was made a bit more challenging as the top thruster on Philae failed to fire, which meant it was unable to circularize its interception orbit. And that resulted in some horizontal motion at the time of landing.
That said, the motion seems to have been well within the safely margin of the harpoon system from what I can read about that subsystem.
Since we aren't on the comet we can only guess what happened but one speculation I've read that seems plausible is that the harpoons "fired" but did not "release", which is to say they tried to move and did not. Imparting a delta-v that was vertical with respect to the surface normal of the landing spot. That delta-v, combined with the landers imperfect alignment, contributed to the sideways journey.
It did confirm that you really do want something like harpoons to hold you to the comet (and potentially to asteroids if you're doing missions to them).
I thought that too, but then they wouldn't be able to target the first spot so perfectly if they haven't considered that. And assuming 12h day the rotation should be around 2 pi R / 12 h ~= 2 km /h assuming R=4 km.
That would result in much bigger velocity than 0.5 km/h when bouncing, and would exceed escape velocity 2 times, just from horizontal part of velocity).
That sounds too complicated for the amount of light you'd get, although I get your point - I'm sometimes surprised by the light/heat that reaches my greenhouse from a reflection on a window as the sun is setting.
On a related note, though, perhaps future missions will transmit power via laser rather than relying on solar panels. Of course solar panels have also had 10 years to improve since the mission was launched...
I agree that it sounds like a math/astrophysics/trajectory nightmare, but I remind myself that is the kind of thing these agencies are really good at. Remember, they just shot a comet some 400 million miles away.
"Some light contrast enhancements have been made to emphasise certain features and to bring out features in the shadowed areas. In reality, the comet is extremely dark – blacker than coal. The images, taken in black-and-white, are grey-scaled according to the relative brightness of the features observed, which depends on local illumination conditions, surface characteristics and composition of the given area."
we're running spectrographic analysis etc but we don't know the color of the surface? it strikes me as strange we don't have color photos. is it to save bandwidth?
The imaging sensors themselves will be monochromatic (B&W), so there are no color-detecting pixels. Some space imagers slide multiple colored filters in front of a single B&W camera to get colored frames. Those frames are later composited into color photos.
http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-...
There's a nice image[2] which shows this in context linked to in the comments [3]. ESA seem to think the lander ended up somewhere over in the dark cliffs of the large crater filling the right hand side of this image.
[2] http://i.imgur.com/4m4WqAN.png
[3] http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-...