
OSIRIS spots Philae drifting across the comet - lentil_soup
http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-Philae-drifting-across-the-comet/
======
grey-area
The original blog post is here:

[http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-
philae-...](http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-
drifting-across-the-comet)

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](http://i.imgur.com/4m4WqAN.png)

[3] [http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-
philae-...](http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-
drifting-across-the-comet/#comment-201918)

------
madaxe_again
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.

~~~
markcerqueira
How long is Rosetta going to orbit 67P? Indefinitely? We need Rosetta to be
able to talk to Philae.

------
martijn_himself
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?

~~~
maaku
Yes, yes it would.

~~~
jessaustin
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.

~~~
narag
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".

Edit:
[http://www.nowykurier.com/toys/gravity/gravity.html](http://www.nowykurier.com/toys/gravity/gravity.html)

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.

~~~
jessaustin
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.

~~~
iliis
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](https://en.wikipedia.org/wiki/Gravity_of_Earth)

~~~
jessaustin
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.

~~~
stromgo
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).

~~~
jessaustin
Oh!

Thanks for the correction; sorry for the noise.

------
brianbreslin
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.

~~~
acqq
If you haven't already, do watch which orbit changes of Rosetta were needed to
deploy the lander:

[https://www.youtube.com/watch?v=4a3eY5siRRk](https://www.youtube.com/watch?v=4a3eY5siRRk)

Must watch. The video was made by ESA.

~~~
kens
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.

~~~
clarkmoody
Milli-Newtons :-)

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.

[1]:
[http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimen...](http://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko)

[2]:
[http://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynam...](http://en.wikipedia.org/wiki/Sphere_of_influence_\(astrodynamics\))

[3]:
[http://en.wikipedia.org/wiki/Orbital_period](http://en.wikipedia.org/wiki/Orbital_period)

~~~
stromgo
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).

~~~
clarkmoody
That's absolutely possible -- I did some in-my-head units conversion.

Maybe another look at the units:

    
    
      G - 6.67384e-11  [N (m/kg)^2] = [m^3 / (kg s^2)]
      M - 1.0e13       [kg]
      a - 30000        [m]
    
      sqrt([m^3] / ([m^3 / (kg s^2)] * [kg]))
      = sqrt([s^2]) = [s]
    
      2*pi*sqrt(30000^3 / (6.67384e-11 * 1.0e13))
      = 1,263,787 s
      = 14.6 days
    

Well, how about that :-)

------
3rd3
It seems to me that this shot is missing:
[http://blogs.esa.int/rosetta/2014/11/16/philae_spotted_after...](http://blogs.esa.int/rosetta/2014/11/16/philae_spotted_after_first_landing/)

However, there seems to be a confusion about the time. If it’s indeed one hour
after touchdown the bright and dark blobs are probably just dust.

~~~
grey-area
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:

[http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-
philae-...](http://blogs.esa.int/rosetta/2014/11/17/osiris-spots-philae-
drifting-across-the-comet/)

~~~
3rd3
It says below the article that they confused the time due to fatigue.

~~~
grey-area
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.

------
seccess
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?

~~~
sp332
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.

~~~
disputin
Can Rosetta not be angled to reflect sunlight to Philae, Iridium style? How
close can Rosetta be placed to the comet?

~~~
arrrg
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.

------
lisper
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.

~~~
woodchuck64
Really too bad the harpoons failed. Why I wonder?

[http://image.slidesharecdn.com/rosettamediabriefing16octbiel...](http://image.slidesharecdn.com/rosettamediabriefing16octbiele-141016091057-conversion-
gate02/95/rosetta-media-briefing-16-october-dr-jens-biele-dlr-philae-lander-
payload-manager-16-638.jpg?cb=1413468719)

~~~
bunderbunder
Apparently the propellant they used was found (after launch, unfortunately) to
be unreliable in a vacuum.

[http://www.wired.com/2013/10/in-space-no-one-can-hear-
your-n...](http://www.wired.com/2013/10/in-space-no-one-can-hear-your-
nitrocellulose-explode/)

~~~
ryandvm
Ouch. This strikes me as a particularly avoidable mistake.

~~~
seanflyon
It's not that they don't work in vacuum, it's that they don't work after a
decade in extremely cold vacuum.

~~~
eps
... unlike a not-so-harsh room-temperature vacuum ;)

~~~
seanflyon
OK, after a decade extremely cold and in a vacuum.

------
ck2
Remember the old lunar lander game?

Someone needs to make a Philae Lander game.

for those that weren't around in 1979:

[https://www.atari.com/arcade/lunarlander/play#!/arcade/lunar...](https://www.atari.com/arcade/lunarlander/play#!/arcade/lunarlander/play)

(js version
[http://www.somethinghitme.com/projects/jslander/](http://www.somethinghitme.com/projects/jslander/)
)

~~~
iliis
You can always reenact it in Kerbal Space Program ;)

~~~
marvin
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.

[http://wiki.kerbalspaceprogram.com/wiki/Gilly](http://wiki.kerbalspaceprogram.com/wiki/Gilly)

~~~
iliis
Indeed, KSP gives a good intuition about orbital mechanics and space travel.
It makes you appreciate the skills of NASA/ESA/... engineers even more!

Also, KSP now has 'real' asteroids!
[http://wiki.kerbalspaceprogram.com/wiki/Asteroid](http://wiki.kerbalspaceprogram.com/wiki/Asteroid)

------
jessmartin
Is it just me, or did Jonathan Amos slip a subtle Star Wars homage into the
article? "It's a trap" is one of the section headings.

~~~
ebbv
That's about as subtle of a Star Wars reference as slicing someone's arm off
with a light saber.

------
Osmium
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).

~~~
codeulike
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

~~~
privong
> 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.

------
unclebucknasty
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?

~~~
Xylakant
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.

~~~
unclebucknasty
Yeah, I think this mission was worthy of a cutting edge imaging solution
though!

As far as transmission windows, well we had download-continuation browser
solutions back then.

Seriously, not to say it's that simple, but this mission solved much harder
problems than imaging would _seem_ to pose.

Really would be interested in knowing how the imgaging solution selection
process went and what the key drivers were.

~~~
sondr3
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.

~~~
rasz_pl
>you can't put a camera, lenses or processors on a device going into outer
space without having done some serious testing

old timers have been saying this for a very long time, and it took years to
convince someone to JUST FUCKING SHOOT A PHONE UP THERE

[http://www.space.com/21036-space-smartphones-photos-nasa-
pho...](http://www.space.com/21036-space-smartphones-photos-nasa-
phonesats.html)

Shaun Meehan (almost 100 private satellites currently in orbit, all more or
less smartphones in space) talks about this in the podcast:
[http://www.theamphour.com/220-an-interview-with-shaun-
meehan...](http://www.theamphour.com/220-an-interview-with-shaun-meehan-
doctiloquent-dove-deployer/)

------
Pxtl
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.

~~~
m-app
Note that the final image in this mosaic (labeled 15:43) is not the final
resting place for Philae. They still do not know exactly where it landed.

------
martijn_himself
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?

~~~
ergzay
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.

~~~
martijn_himself
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?

~~~
estel
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!

------
udev
Philae's landing is commonly represented as a vertical descent. In fact, it
seems there was considerable lateral velocity.

My guess is that Philae landed tumble-weed style, jumping, and rolling until
it hit a wall, in this case, the cliff that shadows it.

~~~
udev
Philae is 1km away from the original landing site. There must've been lateral
velocity.

~~~
theoh
No, the comet is spinning. That is what happened between the first and the
second landing sites.

~~~
ajuc
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).

So I doubt it's so simple.

------
sliverstorm
Is it completely out of the question to reflect light onto Philae using
Rosetta (once they locate the probe of course)?

~~~
anigbrowl
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...

~~~
MichaelGG
Or future missions could just stick an RTG on it, right?

~~~
chx
Please read [http://nucleardiner.com/2014/11/15/wasnt-philae-given-
rtg/](http://nucleardiner.com/2014/11/15/wasnt-philae-given-rtg/)

~~~
MichaelGG
Those sound distinctly non-technical.

------
netcan
This layer of soft dust in low gravity is hard for me to get my head around.

~~~
robin_reala
It‘s just time, and low rather than no gravity. It helps that there’s no air
resistance to blow it off either.

------
modzu
are these photos in B&W or is the comet really that gray?

~~~
matheweis
Probably B&W. Even color images (aka like you see from the MER mission) are
just a B&W imager multiple exposures - each with a different color filter.

~~~
modzu
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?

~~~
dandelany
We will get color photos eventually - they're being taken, they're just
subject to a 6-month embargo.

