We all have seen the photos indicating the exact planned landing site, and the (much bigger) candidate area where Philae might have finally touched down, also the pictures that show Philae "in-jump" after first two touchdowns.

I've also learned that Rosetta's orbit propagator used is of a completely new kind, allowing for very slow, controlled and accurate Rosetta flight paths around the comet.

Given these circumstances, wouldn't it be rather easy to visually locate Philae? ESA must have enough images pre- and post-touchdown to compare, and if not, why don't they take highres pictures of the final touchdown candidate rectangle and look carefully for Philae? Maybe even in a crowd project where everybody can look at the pictures?

So why is it so difficult, or at least unsuccessful so far, to figure out where Philae is located, and how it is oriented?

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    $\begingroup$ Knowing where Philae is a bit academic because that knowledge will be of little help; Philae has no effectors. Rosetta still has a science mission to do. Unless locating Philae happens to coincide with that science mission, the science mission takes priority over finding what is now an essentially dead spacecraft. $\endgroup$ Nov 19, 2014 at 20:43
  • $\begingroup$ @DavidHammen: Philae can move its legs fast enough to jump, and still has the flywheel which can very well send it flying (if a little erratically and rather randomly), and knowing its location would allow us to estimate when (if) it would start receiving enough sunlight, plus optimize Rosetta's orbit for maximizing transmission window - plus learn which area of the comet we're studying... $\endgroup$
    – SF.
    Nov 20, 2014 at 13:03
  • $\begingroup$ I can't see how a flywheel could MOVE anything rather than ROTATING it. $\endgroup$
    – jumpjack
    Nov 22, 2014 at 12:22

2 Answers 2


Below is pair of a high resolution images from the OSIRIS camera of where Philae's first touched down, overlaid upon one another. One was pre-touchdown and the other very shortly after touchdown (and bounce):

Image source: http://www.esa.int/spaceinimages/Images/2014/11/Philae_spotted_by_Rosetta_after_first_landing

The Rosetta operators knew to look at exactly that location because that is where Philae was targeted to land. The circled dark spot is where Philae made its first bounce. It disturbed a nice-sized area on the comet's surface. The spots marked "Philae" and "Philae's shadow" are barely visible.

Look at all the sensor noise in that image. Philae is barely more than a pixel across in that image, making it indistinguishable from that sensor noise. The only thing that let the operators identify those objects as Philae and its shadow is that they knew Philae had to be in that image somewhere, and very close to the touch down location. This is a well-lit image taken at a time when Philae's location was fairly well-known. Now the operators have to look for Philae when they don't know where it is. They are looking for a pixel-sized object amongst a haystack-full of artificial pixel-sized objects.

To make matters worse, Philae is in extremely poorly-lit conditions. It is completely in shadow most of the time, partially in shadow almost all of the time.

  • $\begingroup$ You mean the answer is "because the image resolution is too low to easily find the lander, and because Philae might stand in the dark most of the time"? $\endgroup$ Nov 20, 2014 at 9:17
  • $\begingroup$ That's the TL;DR version. $\endgroup$ Nov 20, 2014 at 14:11
  • $\begingroup$ i still think my summary is correct, and that does not mean it's not a good explanation. +1 $\endgroup$ Nov 22, 2014 at 18:35

Since they managed to acquire some more data points they can now estimate at least the axis towards the second landing spot, and the smart Flight Dynamics team should be able to calculate the landing area.

OTOH, ~200m (my calculation; 28cm/px, 730px distance) traveled from the landing site at 15:34 until 15:43 (= 9 minutes) is about 0.37m/s in horizontal direction. The second touchdown was at 17:25, 111 minutes = 6660 seconds later. That would be almost 2.5 kilometers of travel if it was flying over flat terrain. 67P is approximately 4.1 kilometres by 4.3 kilometres* so obviously the trajectory is curved, but that puts the second landing spot roughly half the comet away from the original one, probably somewhere in the middle "saddle" area - the area with deepest gorges and cliffs, most shade, most irregular terrain, and most hidden from all directions. And even if the flight dynamics lets us calculate that second spot, there were another 7 minutes of flight after the second bounce, and we have no clue about its direction (though hopefully the speed was low.)

Adding David's notes to that - Philae being hidden in deep shadow - 1.5 hours of sunlight a day means it's (partially) visible (possible to appear on photos) about 6% of the time. Since it's highly unlikely (...6% chance per photo) that any of prior photos of Chury caught that area when it was lit up by Sun, chance that the 'differential analysis' (like for the first landing spot) will be viable is proportionally diminished. even if we take the photo with Philae in sunlight - simply no photo with visible ground without Philae to compare to!

Probably the best approach to finding Philae would be by finding the moments when radio connectivity is cut off/re-established with Rosetta with it orbiting the comet at various angles, and modelling that data - Rosetta's radio "field of view" edges - onto the comet's 3D model, finding where the edges of "visibility" from moments of entering/exitting the "view" converge. But for that to happen, Philae must talk. As long as it's silent, it's seeking a needle in the haystack... at night.

  • $\begingroup$ Isn't it possible to just triangulate the position using radio emission and rosetta ever-changing position ? $\endgroup$
    – Antzi
    Nov 20, 2014 at 9:52
  • $\begingroup$ I could imagine this is more complicated than expected due to the comet´s irregular shape, and its movement. And maybe Philhae does not have enough priority right now. $\endgroup$ Nov 20, 2014 at 9:58
  • $\begingroup$ >>even if we take the photo with Philae in sunlight - simply no photo with visible ground without Philae to compare to!<< Why? At least most of the planet has been digitized. $\endgroup$ Nov 20, 2014 at 9:59
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    $\begingroup$ @Antzi: Wide-angle antenna; you get signal strength, but no direction. We're only getting the appearance/disappearance moments which give us the 'horizon line' on which Philae is expected. Several such lines from different angles should intersect at the point where Philae is. The problem is Chury is very irregular and the moment (and so the line) is very approximate, so instead of a neat intersection of some neat meridians, you have a totally chaotic (and not even continuous) line. $\endgroup$
    – SF.
    Nov 20, 2014 at 10:04
  • $\begingroup$ @TheBlastOne: About all of the lit area of the comet. The pits of darkness are just approximated. They aren't a major part of the area but unfortunately Philae is in one of them. $\endgroup$
    – SF.
    Nov 20, 2014 at 10:06

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