Rosetta and Philae: Searching for a good signal

After seven months in hibernation on the surface of Comet 67P/Churyumov-Gerasimenko, Rosetta’s lander Philae communicated with Earth via the orbiter on 13 June. Since then, seven periods of connection have been confirmed between the orbiter and lander, but all have been intermittent. One of the key issues being worked on is to adjust Rosetta’s trajectory to see whether a more reliable communications link can be established with Philae. This report describes the status of those efforts as of 26 June, and has been prepared with inputs from ESA’s Rosetta Science Ground Segment team at ESAC and the flight control team at ESOC, along with the Lander Control Centre at DLR.

Credits: ESA-C.Carreau

Artist impression of Rosetta transmitting a signal to Earth. Credits: ESA-C.Carreau

When have contacts been made?
Confirmed contacts between Rosetta and Philae have been made on 13, 14, 19, 20, 21, 23, and 24 June, but were intermittent during those contact periods. For example, the contact on 19 June was stable but split into two short periods of two minutes each. Conversely, the contact on 24 June started at 17:20 UT (on board Rosetta) and ran for 20 minutes, but the quality of the link was very patchy and only about 80 packets of telemetry were received. Prior to this, on Tuesday, 23 June, there was a 20-second contact, but no stable link was established and consequently no telemetry data were received.

How frequently do Rosetta and Philae try to make contact?
Comet 67P/C-G rotates with a 12.4 hour period and thus Philae’s location is not always visible to Rosetta. Roughly speaking, there are two opportunities for contact between the two spacecraft each Earth day, but their duration depends on the orientation of the transmitting antenna on Philae and the location of Rosetta along its trajectory around the comet. Similarly, as the comet rotates, Philae is not always in sunlight and thus not always generating enough power via its solar panels to receive and transmit signals. At the moment, the predicted contact windows vary between a few tens of minutes and up to three hours. During these contact windows, the ideal situation would be that a powered-up Philae hears Rosetta’s calling signal and responds by establishing a link back to the orbiter, then transmitting the data stored on-board via that link.

Why do we care about a stable connection?
Data are stored in two mass memories on-board Philae, and in order to download the data in the most efficient way possible, a stable ‘call’ duration of about 50 minutes is desired. It can take around 20 minutes for the data to be dumped from each one to the Rosetta orbiter, and additional time is needed to confirm that a stable link has been acquired in the first place, and also for uploading new commands.

Artist impression of Philae on the surface of 67P/C-G. Credit: ESA/ATG medialab

Artist impression of Philae on the surface of 67P/C-G. Credit: ESA/ATG medialab

Can the lander still be operated with short communications links?
Yes, but this situation is not ideal because it has an impact on the overall time available to perform scientific operations in the long term. That’s because each time a new science sequence was initiated, it would take longer to get the accumulated science data back and free up on-board storage before new commands could be uploaded and subsequently executed.

What might be affecting the link from the lander’s point of view?
A number of factors regarding the lander’s current status may contribute to the quality of the communications links observed so far. These include:

  • Lander power availability: the orbiter needs to be flying overhead the lander’s position when the lander is ‘awake’, that is, when it is generating enough power to have its receivers and transmitter switched on.
  • Lander location and orientation: the orientation of the lander on the surface of the comet determines how its antenna pattern is projected into space, and the rugged topography immediately surrounding Philae can also distort that antenna pattern.
  • Lander health status: errors in the various on-board units could also affect the chances of making a stable link.

What might be affecting the link from the orbiter’s point of view?
Equivalently, there are a number of parameters related to the orbiter that could be influencing the quality of the communications link observed so far:

  • Distance to the comet: the strength of the signal received by the orbiter diminishes as the square of the distance between the orbiter and the lander, and thus the chances of a stable link are reduced if Rosetta is too far from the comet.
  • Trajectory of the orbiter: to make a link, the antenna pattern of the lander must overlap with that of the orbiter, and given the constraints set by the lander antenna pattern, certain trajectories of the orbiter around the comet will be more effective at seeing a ‘clean’ lander signal than others.
  • Pointing of the orbiter: the exact orientation of the orbiter in space plays a role, because if the dedicated, non-steerable antenna used to communicate with the lander is not pointed directly at the comet, the strength of the signal received from the lander will be reduced. Some science observations being made by Rosetta require the orbiter to be pointed off the nucleus, but steps are being taken to avoid that situation during potential contacts with Philae.

Can any of these factors be changed?
Until a stable link is achieved between the orbiter and the lander and new commands uploaded, it is obviously not possible to try and ‘tune’ the parameters on-board the lander. Thus, present efforts are focused on improving the factors related to the orbiter. However, this is not straightforward, as the spacecraft operations team must keep the safety of the orbiter as their highest priority at a time when the comet is becoming more and more active.

Processed NAVCAM image of Comet 67P/C-G taken on 15 June 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Processed NAVCAM image of Comet 67P/C-G taken on 15 June 2015. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

How close can Rosetta get to the comet and still remain safe?
In order to navigate around the comet, Rosetta uses its star trackers to determine its orientation in space, and thus keep its instruments and high gain antenna pointed in the right directions. However, in the dusty environment of a comet, individual dust particles can mimic stars, making it difficult for the star trackers to operate effectively. If the star trackers are unable to determine the spacecraft’s orientation, it will go into safe mode, as experienced during one of the March close fly-bys of the comet. In the worst case, contact with Earth may be lost, which would lead to the spacecraft entering an autonomous mode that could take days or weeks to recover from.

The increasingly-active environment of Comet 67P/C-G is proving to be dustier than planned for when Rosetta was built and thus since March, the spacecraft has been flying at safer distances of roughly 200 km from the comet to avoid similar issues occurring. It has also been moved into a so-called ‘terminator trajectory’ around the comet, also aimed at reducing the impact of the dusty environment on the star trackers.

The spacecraft operations team are slowly edging Rosetta closer to the comet in this terminator trajectory, closely monitoring the performance of the star trackers in ‘continuous tracking mode’, and planning the trajectory for the days ahead. At the moment, Rosetta is following a trajectory scheme that allows it to come as close as 165 km from the comet, and there are signs that dust interference is becoming an issue again. A manoeuvre planned for Saturday morning will move the spacecraft to 160 km by 30 June and the team will assess the star tracker performance at that time in order to determine if even closer orbits are possible, or if Rosetta needs to be moved further away again.

How is the trajectory being changed?
The current terminator trajectory of Rosetta has it flying over the boundary between comet day and night. The main change that can be made within this scheme is to the latitude of the ground track of the orbit on the surface of the comet. This is currently being stepped down from +55 degrees (on 24 June) to –8 degrees (on 26 June), with a better quality signal between Rosetta and Philae being detected at lower latitudes. For comparison, just after landing in November 2014, Rosetta was flying over latitudes of +15 to +25 degrees. In the coming week, the latitudes will slowly be stepped back up again from –8 to +50 degrees, with a careful assessment being made of signal strength at low latitudes again.

How long is it going to take to resolve the situation?
This is a very dynamic, real-time process, and thus it is hard to predict when a stable link might be made between Rosetta and Philae. The mission teams are working on a short-term trajectory planning schedule, which is updated every Monday and Thursday morning. Changes to Rosetta’s trajectory are made depending on the latest information with regards lander communications and the performance of the orbiter’s star trackers in the days between each decision point. In addition, representatives from ESA’s Rosetta team, the Lander Control Centre at DLR in Cologne, and the Lander Science Operations and Navigation Centre at CNES, Toulouse discuss daily the latest status of any lander communication events.

We will keep you posted as further information becomes available.



  • Guili says:

    Argh !
    Emotional rollercoaster

    • JvZ says:

      J.-P. BIBRING ET AL.
      Communication of Philae uses the Orbiter as a relay for data and command transfer.
      Short range communications between the Orbiter and Philae are realized by means
      of a specific and fully redundant RF subsystem working in S band (Sarthou, 1998).
      The RF subsystem is organized around a high performance and low cost transceiver.
      The equipment has an overall consumption of only 6.5 W (1.5 W for the receiver
      and 5 W for the transmitter with 1 W RF output) and a total mass of 1 kg (transceiver
      +filter). Communications between the Orbiter and Philae can be established in full
      duplex at a 16 kbits/s bit rate, in a distance range from a few hundred meters up to
      150 km; given the system overheads, actual rate for scientific data uplink to the Or-biter have been measured during cruise checkouts at∼10 kbits/s. Lander data trans-mission will be scheduled according to the visibility of the Orbiter from the landing
      site. The RF subsystems are interfaced with the CDMS (Lander side) and the ESS
      (Orbiter side), which manages the protocol and its redundancy (Cluzelet al., 1998).

      • JvZ says:

        Kontakt im kommenden Jahr

        Wahrscheinlich im Frühjahr 2015, so schätzt DLR-Wissenschaftler Dr. Stephan Ulamec, kann das Lander-Kontrollzentrum des DLR wieder mit Philae kommunizieren und eine kurze Rückmeldung erhalten, wie es dem Lander auf Churyumov-Gerasimenko geht. Im Sommer 2015 könnten dann auf dem Kometen Temperaturen herrschen, die es Philae erlauben, seine Batterie aufzuladen. “Der Orbiter wird bei seinen Überflügen auf Empfang sein und hören, sobald Philae wieder aus dem Winterschlaf aufwacht.”

    • JvZ says:

      Saft’s LSH20 primary lithium battery is part of the 100 kg Philae Lander craft, which is a key part of
      the Rosetta mission. The lander will touchdown onto the surface of the comet on November 12
      2014. Its goal is to study the composition of the comet dust and ice structure for around 4 months
      as it travels towards the warmer inner reaches of the solar system. 10 instruments on board Philae
      will obtain the first images from a comet’s surface and make the first in-situ analysis of a comet’s
      composition. As the comet is believed to contain the basic ingredients for life, it may shed light on
      how life started on Earth.
      Saft’s LSH20 are availble by

  • David says:

    Nothing else for it then but a “geo”stationary orbit at around 3km, staying above the lander, ignore the star trackers. May not be easy, but there’d be a nice stable radio link whenever the lander could transmit . .

    • Marco says:

      That’d be a duck-stationary orbit :-). There is really no chance to do this until near the end of the mission. It would risk the safety of the orbiter even with no activity at all.

  • Dave Bennett says:

    Has anything been done to try to adjust Philae’s orientation on the comet? As in, upright the lander?

  • Knipjo says:

    Is there a chance that Rosetta’s position as well as signal strenght and such can help triangulate Philae’s position?

  • Fred J Heller says:

    I am curious what is known or not known about the precise location of Philae now that there are some signals to go on.
    How do the recent signals match up to the earlier ideas about location based on visual surface analysis?

  • Chuck0 says:

    I guess leaving the terminator plane to get a longer communication window is out of the qustion? Also, has philae to be instructed to charge its battery or will it do that itself allowing for contact at comet night?

    Anyways following this mission is as always extremely exciting! Keep up the great work!

  • Casey Schesky says:

    Thanks very much! All of us followers appreciate such a detailed explanation of the ongoing activities of the mission team. We are all holding our breath and sitting on the edge of our seats waiting for news and even the littlest tidbit is great to hear. Good luck to all and thanks for keeping us updated.

  • Margarita says:

    As ever – so grateful that you provide this level of detail and in real time. If hand wringing and/or positive thinking helps – I’m doing both!

  • Jim Robertson says:

    I am curious, but lazy.

    Someone here must have done a quick calculation. How far from the center of mass would Rosetta have to be for a 67P-o-synchronous orbit around it? A hundred meters? A kilometer? Ten?

    And yes, I realize that would be way to close for safe operation, and flight dynamics would never attempt such a thing.

    But it would be a sort of limiting case — Rosetta hovering over the spot where Philae has settled.

    • Gerald says:

      Stationary orbits would be a few kilometers above surface (at most), 3.2 km from the center of mass, if my back-of-an-envelope-calculation is correct, not feasible at the moment due to dusty environment.

    • JvZ says:

      The philae transmitter have a range between a few hundred meters,and up to 150 km.
      Source: J.P. Bibring ET AL.

  • Nick says:

    Thoughts: I hope the team are doing everything they can to try to narrow down the location and orientation of Philae. in this way surely they could aim the orientation of the Rosseta Orbiter in a better way. if as, as I thought, the lander is on its side then the attena might be pointing in a strange direction, or certainly not parallel to the surface.

    It also strikes me that thier seems to be a resistance in releasing data, I am pleased we have this update, however surely if they released every scrap of data and & image that they might have relating to the location of Philae then the power of this huge community might go a long way to tracking down the location of the lander.

    In particular I am thinking or orisris narrow angle photographs and everything that would help with the modelling of 3D rotational graphics

    Ii may be crediting the wrong person however I think it is Thomas that has posted the best 3d models I have seen for far and nothing of the sAme nature or quality from the “official sources”

    I hate being negative but have followed every space mission since Apollo moon landing what I was just old enough to remember to the Viking lander opportunity and spirit rovers and countless other missions but the frustrations of this is to say in a British rather non-descript way rahter than swearing “most-frustrating”

    Hugs to Rosseta Philae and all the teams & supporters across the world all the sAme.

    • Ramcomet says:

      Hi Nick,

      Although I have not seen Thomas’s 3-D images and would love to, you might refer to Mattias Malmer at

      Scroll down and go back a page or two, to older pages for some amazing 3D movies of his own shape models that Mattias has done! 😉

  • Peter says:

    I’m curious, what is the period of Rosetta around the comet?
    From the description, i.e. “can change the latitude”, it must be cometo-synchroneous, and on the equator, and therefore the terminator is the equator of the comet. (However the terminator changes, but the equator of the comet remains stable on the comet’s track).

    • Peter says:

      Just thought, maybe Rosetta flies in parallel to the comet, on an ellipsis around the sun, on its own, and does not rotate around the comet? (gravity related to the sun, not to the comet)

    • Gerald says:

      The terminator is the line between day and night. Usually this isn’t the equator; it can even be perpendicular to the equator. Terminator and equator intersect at two points on the surface of a spherical object at a given instant. The angle between the terminator and the equator depends on the angle between the rotation axis of the comet and the vector to the Sun.

  • Harry Doughty says:

    The images show the discharge of material towards the sun from the hot side of the comet which makes sense. A comet’s tail however points away from the sun so I assume the material progresses some distance towards the sun before being stopped by the solar winds and reversing direction?

  • Peter says:

    Peter is more curious: With the orbiter Rosetta on the terminator, it is sunset for people right below the orbiter, at 18h local comet time, and the solar panels might not get sun. Wouldn’t it be an option to move the orbiter into the sun, perhaps to 15 h local comet time, or even into noon (12h), so that people right below, i.e. lander Philae, get better sun. — Anyway Philae was able to make weak contact at “sunset” with Nadir pointing.

    • Marco says:

      I was wandering about this also. To the point that it would probably work at 400 km distance if the sun side activity was the issue. The best connection would be line of site at day time, thus when Philae is looking into the sun should be also looking at Rosetta. A distant, sun side parking orbit should connect better than a close, terminator parking orbit.

  • Richard Krause says:


    Just want to say that you provide very informative and clear updates.

    You are far superior than any site that deals with the Mars Curiosity. FAR superior!!!!

    Thank you.

    I am TOTALLY amazed at all the people involved on this project. I wish everyone including Rosetta and Philae the very best.


  • Ian says:

    Many thanks for taking to trouble to keep us up to date with such an informative report. I hope the many issues can be resolved or at least improved to the point where Philae can do some more useful work. Good Luck 🙂

  • Kamal says:

    On patchiness: Was the first contact (13th 20:28) at local evening? If so, possible Philae had charged all day before that, and the activity of Hatmehit was quietening down.

  • masanori says:

    Wish any of you in the mission teams are not too disappointed or too frustrated! I hope time & your efforts will give you an answer, hopefully soon! Thank you very much for your hard works for humans!!

    Now I think I might have been wanting to hear new reports on the situation of the radio link too soon & too much. And as I came to think so, I happened to think:

    How about NOT starting to send signal to Philae too soon as the “window” opens?? For example, how about starting to send signal to Philae at one hour after the “window” opens and see what happens?? (Philae gets sunshine for 3 hours per comet day, right?)

  • Pete says:

    Fascinating. Thank you so much for taking the time to explain all that to the ordinary punter.

  • Tony schaefer says:

    A very clear explanation of the issues of communication between Rosetta philae and earth.

  • originalJohn says:

    Tricky stuff Emily. Do the team take into account the possibility of electric or magnetic fields affecting the performance or survival of the lander or do they ignore that and focus only on gas and dust effects.

    • ianw16 says:

      Why would they bother about electric or magnetic fields affecting the lander? It hadn’t measured any before it went into hibernation. And the RPC team have a suite of instruments on Rosetta that can measure these things. Besides, the minimum 9 fold increase in magnitude in the solar wind ion flux, required by EU, would have wiped it out already.
      So not much point, really.

    • Harvey says:

      I would imagine they fully take it into account by ignoring it 🙂

  • Peter says:

    By the comments to and, the cometo-stationary orbit radius is 3280m, at 12.4 earth hours period. Therefore by Keplers Third Law, at 200km the rotation period is 246 earths days. I hope I answered my question correctly, with all uncertainty in the figures, i.e. somewhere between 100 and 900 earth days and a very slow rotation around the comet.

  • Terran says:

    I really like these detailed, also at times technical news we get here. Especially as there’s a general trend in dumbing down tremendously (first-person perspective in Twitter for instance). GJ emily

  • Александр says:

    выключайте мобильную антенну!!
    В прошлом месяце, незадолго до того, как Розетта выпущена филе приземлиться на комету 67р/С-Г, мы выложили статью на этом блоге под названием “поющая комета”. Он представил звуковой дорожки на основе данных, собранных с одним из инструментов от Розеттского плазмы консорциума (НПК) на борту орбитального корабля и .включили мобильную антену!!
    Осенью 2014 года на Солнце произошли несколько вспышек высшего класса мощности за короткий срок. Сильные вспышки были 25 октября 2014 года, на следующий день 26 октября солнечная активность продолжала нарастать, зафиксирована очередная вспышка наивысшего класса.
    Токи Фуко возникают под воздействием переменного электромагнитного поля и по физической природе ничем не отличаются от индукционных токов, возникающих в линейных проводах. Они вихревые, то есть замкнуты в кольце.
    Тепловое действие токов Фуко используется в индукционных печах — в катушку, питаемую высокочастотным генератором большой мощности, помещают проводящее тело, в нем возникают вихревые токи, разогревающие его до плавления.

    С помощью токов Фуко осуществляется прогрев металлических частей вакуумных установок для их дегазации.

    • Peter says:

      What mobile antenna you think of? And why turn it off? What do you want to say, mentioning eddy currents, and in the context of the search for Philae?
      Какую мобильную антенну Вы имеете в виду? Да почему выключить её? Какое значение Вы приложите вихревым токам, и особенно в связи с поиском Филая?
      Спасибо и по возможности на английском– или немецком?

      • Peter says:

        да не приложите, а прикладываете

    • Rocky says:

      Александр are you thinking that Foucault currents will increase, possibly to the point of destroying Philae’s electronic, as we approach perihelion? Wouldn’t the lander design have taken precautions against this? I would like to see more of your thinking on this subject. Thanks for the post, it made me consider something I hadn’t thought about…

  • Thomas Allekotte says:

    Dear Emily,

    thanks again for keeping us up to date with all these complicated matters. Great Job !

    As there are so many comments stored in this blog by now, is there a chance to search not only the blog itself (your blog articles) but the comments as well.
    I tried in vain to do so, am I doing anything wrong?

    The blog is a wonderful source of information right now, it would be even better, if the hole content could be searched by socalled “google-search”.

    In case this should be impossible, is there a chance to list all your blog entries in an Explorer-like view so that lots of topics can be seen in one screen? Would be much easier to scroll through.

    Hope you can help me.

    Thomas Allekotte

  • sandel says:

    this story has been such a joy to follow and keeps getting better. The animated movie coming out is a must once the story is done! That’s a lot of mainstream attention that should not go unexploited, it might even found a new mission with hollywood’s latest earnings.

    …ceres, the god of PS, invented the the levels & curves filter just to get some comparable attention, for a while :))

  • JvZ says:

    When have contacts been made?
    Confirmed contacts between Rosetta and Philae have been made on 13, 14, 19, 20, 21, 23, and 24 June, but were intermittent during those contact periods. For example, the contact on 19 June was stable but split into two short periods of two minutes each. Conversely, the contact on 24 June started at 17:20 UT (on board Rosetta) and ran for 20 minutes, but the quality of the link was very patchy and only about 80 packets of telemetry were received. Prior to this, on Tuesday, 23 June, there was a 20-second contact, but no stable link was established and consequently no telemetry data were received.

  • Ralph Calvert says:

    Why are the updates not as good as NASA? What happened about the trajectory changes last weekend?

  • JvZ says:

    signal quality
    signal duration
    Have a nice weekend.

  • George Nixon says:

    Apart from being able to sample sub-surface, the sniffing and surface sampling has provide a lot of information, and there may not be much more surface information available to be gleaned.

    If it is not possible to fasten the lander to allow drilling, then I say congratulation to the Team in their determination to not unnecessarily endanger Rosetta. After all, there are many more months of exploration ahead of Rosetta as comet 67P travels outbound from the Sun. Patience is a necessary virtue as displayed to-date by the members of the Team.

Comments are closed.