comet scientists

We are comet scientists and engineers working on Philae and Rosetta. We just triple-landed a robot lab on a comet. Ask us Anything!

We are comet scientists and engineers working on the Philae robotic lander and the Rosetta mission at the German Aerospace Center DLR. Philae landed on the comet 67P/Churyumov-Gerasimenko on November 12, 2014. Rosetta continues to orbit the comet and will escort it as it nears the Sun for at least one more year.

The Rosetta mission is the first in the history of space flight to:

  • completely map the surface of a comet,
  • follow a comet’s trajectory and record its activity as it approaches the Sun,
  • land a robotic probe on a comet and conduct experiments on its surface.


  • Michael F. A’Hearn – Astronomy Professor (emeritus) and Principal Investigator of the Deep Impact mission(ma)
  • Claudia Faber – Rosetta SESAME Team, DLR-PF/Berlin (cf)
  • Stubbe Hviid – Co-Investigator of the OSIRIS camera on Rosetta at DLR-PF/Berlin (sh)
  • Horst Uwe Keller – Comet Scientist (emeritus), DLR-PF/Berlin and IGEP TU Braunschweig (uk)
  • Martin Knapmeyer – Co-Investigator of the SESAME Experiment at DLR-PF Berlin (mk)
  • Ekkehard Kührt – Science Manager for Rosetta at DLR-PF/Berlin (ek)
  • Michael Maibaum – Philae System Engineer and Deputy Operations Manager at DLR/Cologne (mm)
  • Ivanka Pelivan – MUPUS Co-Investigator and ROLIS team member (operations) at DLR-PF/Berlin (ip)
  • Stephan Ulamec – Manager of the Philae Lander project at DLR/Cologne (su)

Follow us live on Wednesday, 26 November from:
| 17:00 CET | 16:00 GMT | 11:00 EST | 8:00 PST |

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With the mission being launched ~10 years ago, and conceived ~25 years ago, what difficulties did you come across using (now) decade old technology? Were there any times you became frustrated because “modern” technology would have made something significantly easier?

[ek] The 20 years old technology worked nearly perfectly. The biggest drawback for Philae was the limited computer power and mass memory. What we could install there (800 MHz CPU and some MB memory) seems to be from another world today

[mk] Faster computers always help… and more memory. In the SESAME Experiment, we record time series of vibrations of the soil (CASSE Instrument). This is a kind of seismology. What a seismologist usually wants is: more data. Having just a continuous record over our few days on the comet would make a big difference. But I do not want to complain: we have a lot of data, more than I expected after the first bounce, and I am optimistic that we can make science of it. With the bouncing, it even got more exciting, since we now have data from two points on the comet.

So theoretically you could control it from your smartphone?

[??] On principle your smartphone has more computational power but it wouldn’t survive in space.

How confident are you that you will be able to recharge the batteries and continue the mission? And how long will it be until that is achieved?

[ma] Well, it isn’t really the batteries’ fault! The batteries were meant to be recharged by solar panels but because Philae hopped a couple of times after the first touchdown, it ended up in shadow for nearly all the time (roughly one hour of sunlight every 12-hour “day” on the comet). It is like trying to power your house with solar panels when you live in Alaska just below the arctic circle during the winter. We are not sure yet where Philae is, but if it is at what some people think is the most likely place, the seasonal change toward spring in Philae’s hemisphere should bring Philae into much more sunlight on a time scale of months. That, coupled with the fact that the comet is getting closer to the sun, should warm up the batteries enough to take a charge and then keep them charged. I.e, don’t blame the batteries, blame Philae for wanting to go into a winter den for hibernation.

[mm] We expect to have enough energy to boot around March next year. Then Philae needs to be heated until we can think of starting to charge the battery. So enough power to run the system, heat it and do charging or other operations we can expect early summer. Once charging can be started, it might take some comet days to charge the battery completely.

How long is a comet day?

[ip] The rotation period is about 12.4 hours. So the CG comet day is roughly one half Earth day.

What effects does the success of this mission have on the future of space exploration? Will we see more comet landings in the future?

[uk] I guess that the rendezvous mission is an important step and will encourage the agencies to think about the next step, a comet nucleus sample return. There are already several proposals on the table

[??] I certainly hope so! On the US side, there are several groups proposing cometary missions to the next round of NASA’s Discovery Program (these are small, at least small by NASA standards, missions led by a scientific PI). Those proposals are due early next year and by next summer NASA will select a few, probably 3, of them for more detailed studies, with the final selection in 2016. Whether a cometary mission succeeds is hard to predict, but in the last round a cometary mission was one of the three selected for detailed studies. There are also at least two teams working longer term on proposals for a sample return mission from the surface of a comet. Those will be submitted to NASA’s New Frontiers program (larger, i.e., more expensive missions than the Discovery program). However, the deadline for those proposals has not been announced and it is likely to be in 2016.

What was your most significant discovery so far?

[ma] We will probably disagree on which discovery was most significant. For me the most significant discovery from the Rosetta orbiter thus far is that the outgassing at these large distances from the sun is not driven the way we expected (super-volatiles rather than water). And again just for me, the most significant discovery from Philae was the existence of a hard surface underneath the soft material at the first touchdown point.

[mk] One of the most significant discoveries to me was simply the shape: how likely is it that we chose the most bizarre object in the solar system as our target? And second: yes, the hard surface. I expected the comet to be more fluffy.

What was the biggest surprise you encountered during the Philae mission?

[ma] The biggest surprise to me was the path of Philae after the first touchdown. That was completely unexpected.

[ek] There have been many surprises during the mission: the strange shape of the comet that made the landing not easier, the hardness of its surface but also the precise landing within some seconds and some dozens of meters of the pre-calculated touchdown scenario

How did you get to be involved in such an awesome project (career wise)?

[uk] I was already involved in the ESA Giotto mission to comet Halley being in charge of the Halley Multicolour Camera. Right after this mission we started to work on the Rosetta mission and here again I was responsible (PI) of the scientific camera system OSIRIS. Most of my research has been connected to cometary physics.

[ek] At the beginning it was more or less accidently. I studied physics and started my career in space science. But meanwhile I’ve been working in this mission for 20 years, nearly half of my life

With the knowledge you have now, Would you have done the landing of Philae differently? If so, what kind of adjustments would you have made?

[mm] I think the landing could again be done as it was planned. If the harpoons and ADS would have worked, we would have had a perfect mission on a very good landing site with lot of power for an extensive long term science mission.

When you think of the future of space exploration (next 20 -30 years or so) what prospective missions excite you?

[uk] I think one of the key missions is a comet nucleus sample return. Bringing back a sample of the most pristine material we can imaging will provide a major step in the understanding of the physical and chemical conditions of the early solar system (nebula) out of which our planetary system formed. Investigations in laboratories on earth provide much more capabilities than even the most sophisticated instruments on spacecraft. See the example of Stardust mission.

Although you all have an intimate understanding of the entire project, do you ever have moments of thinking “are we really controlling a couple of man-made machines as they interact with a comet hundreds of millions of miles away”?

[sh] Sometimes 🙂
It takes a lot of work to keep a missing like Rosetta running. So on a daily basis it is easy to get lost in the details. But it is actually quite incredible that you can talk to a spacecraft 510 million kilometers away through 34 to 70 meter big radio telescopes located all over the world. The transmission takes place through a microwave signal which is comparable to a flashlight in energy.

[ma] We all have those moments the first time we are directly involved with these communications during critical events, but some of us adapt our thinking fairly quickly so that it no longer seems “insane”, while others still get that feeling even after doing it many times. For me personally, I no longer think of it as “insane”, but neither have I reached the point that I am blasé about it.

With your new knowledge, what type of anchoring system do you think would be best in the future?

[su] We are currently analyzing the exact reason for the failure. But in principle, I believe the design is very good (and would have worked).

How are we doing on narrowing down Philae’s position and when will Rosetta be in a position to get a better high resolution image of the landing site?

[uk] Currently the suspected landing area is imaged by OSIRIS from a distance of about 30 km, just barely close enough to resolve Philae. End of next week a closer orbit is planned.

[ma] OSIRIS has taken more pictures from the orbiter to search for the lander but those images are still on the spacecraft, waiting for their turn to be put into the downlink queue. There have been several suggested identifications of the final landing place in OSIRIS images, but none of these are good enough to be sure, hence the additional images. Tying the images with the information from the CONSERT experiment (talking to Philae through the nucleus can tell them where Philae is by looking at the variations in what is between the orbiter and Philae.

How do you go about determining the gravitational pull of a comet?

[sh] You measure the doppler shift (frequency shift) of the X-band radio signal to find the acceleration of the spacecraft caused by the comet. Rosetta is equipped with an ultra stable oscillator which makes this possible. This directly gives the mass of the comet.

[su] Ideally by measuring the trajectory of an orbiting spacecraft (in this case Rosetta)

What more would you have been able to do if Philea’s solar panels were well positioned and it still had power? Why not use a plutonium battery on Philea (like Voyager 1 and 2 and Cassini)?

[mm] In our planned scenario our mission was expected to be finished when reaching about 2 AU due to a possible overheating of the internal compartment. Now the mission profile changed completely and we expect to restart at 2 AU or less due to the limitation on power. We could not use radioactive sources as this was not possible in Europe. Therefore Rosetta is designed as “green spacecraft”.

Will you make the data collected in the mission open the others to study as well in the general community?

[uk] All data will be archived and are then public. They will be archived in an understandable well calibrated format. This will take some time (1/2 to 1 year) and is additional work for the instrument teams.

[??] All the data will be publicly available in 6 to 12 months in the ESA’s Planetary Science Archive.

Can you make software updates on Philae’s Control System? For example, in case you guys notice a bug on the Operating System or any other part, how do you proceed?

[mm] Generally yes, but we have to get enough power to boot, even more power to communicate and then we can think of uploading updates.

[su] Yes we can. The S/W has been updated during cruise.

Would you consider this mission a complete success? If so, why? If not, what do you wish you had achieved that you didn’t?

[ek] Thank you! I think Rosetta is a huge success! We already have a lot of very interesting data from all 21 instruments. Rosetta is the first mission that orbits a comet, that lands on it and follows it on its way to the Sun. I had wished that anchoring of Philae would have worked but nothing is perfect, particularly at 500 Mill. km away from home

[uk] The mission is a tremendous success but this will only be complete when Rosetta operates through the perihelion of comet C-G at the end of next year. We want to observe the increase in activity to more than an order of magnitude beyond what it is now

Can the instruments/equipment on Philae survive such a long time in the cold?

[mk] I’m not an engineer and should not speculate… but we’ve been in space for ten years, and part of the trajectory brought us much farther out than we are now. This could mean there is a chance to survive a few months more.

[sh] When you design space mission you always test the equipment in a certain temperature range (called the qualification range). You then try to ensure that the spacecraft never leaves this range. Philae will get colder than the qualification range which means that we are in terra incognitae. This means that the engineers will not guarantee that the lander will work.
It has on the other hand been seen before that hardware works also after a deep freeze. The Soho spacecraft for example survived a multi year freeze when contact was lost. It has now been working flawlessly for years afterwards.

What are your favourite pictures taken by Rosetta/Philae from the mission to date?

[mk] Images from planet (or comet) surfaces are always exciting, but what I liked most here is the OSIRIS picture that shows Philae drifting away from the Orbiter – blurred details in black sky, and all legs, antennas, booms perfectly deployed. Tough little ship!

[uk] One is the picture of Philae above the surface of C-G just before the first landing. More physics oriented I find the large “wall” of the small lobe at the neck awesome particularly I did not expect the surface to look like this.

What is the latency from when you push a button on earth to when philae reacts?

[mk] the travel time of a radio signal is currently close to half an hour. Some additional delays may add since we did not communicate with Philae directly, but relayed through the Deep Space Network on Earth, and through the Rosetta Orbiter.

[su] Signal travel time at the Moment is 28min

What kind of emotions did you perceive during the last minutes before the lander’s touchdown? How did your attitude evolved during the project’s life?

[ip] Those whose prime goal was to land surely experienced their happiest moments when the lander was brought down successfully. The instrument teams waiting for operations to start experienced further up and downs emotionally, especially towards the end when it was unclear how long the battery would still be alive.

[??] Although I was in Göttingen and not in Cologne or Darmstadt the feelings were strange. To stand in front of several hundred people together with a few people to comment the life stream. I thought of the PIs sitting in the small room in Cologne waiting for their instrument data after successful landing.

Do we know how high the cliffs are that are near Site B, which was not selected?

[??] The cliffs at site B are about 40 m high.

Apart from lifting and rorating, did Philae’s main body move physically by any physical movement of his instruments/equipments after final touchdown?

[su] By analyzing the illumination at the solar Panels, apparently, Philae did not move.

Where do you see the knowledge of asteroids and comets going in the next 10-20 years? When do you think humans will be able to land on asteroids, and when do you think it will be economically feasible to commercially mine such objects?

[mk] Predicting the evolution of knowledge is beyond my skills… economically feasible: I personally do not belive in this. It is simply to expensive to bring stuff out there. returing makes it even more difficult. One example: to mine the Moon for an amount of He3 sufficient for the current power consumption of the US would require a heavy industry at a scale you do not even find in coal mining on earth. The same electrical power could be generated by simply covering all our roofs with solar panels.

Do you think there is a possibilty of single-celled life on the comet?

[mk] organic molecules just means molecules with a few carbon atoms in them. This does not necessarily involve life. I don’t think there could be life, except when brought there from elsewhere (e.g. by a landed spacecraft 🙂

[ek] No, I wouldn’t expect that. We know that there are organic molecules but no liquid water and not very stable temperature conditions for the development of life

What exactly is a triple land?

[ip] This refers to the fact that we landed at the foreseen landing spot, bounced off, hit the surface again for a second time and shortly after reached a final landing position which is not yet known.

How long until commercial mining missions are common?

[mk] My personal opinion is: very long. There is currently nothing that is expensive enough even to account for the costs of mining on the moon (including the famous He3). Asteroids and comets will be even more expensive.

[??] This is very hard to predict. I am old enough that I doubt it will be in my lifetime, but for anyone much younger than I, you should live to see it.

Is there any other mission planned in the future to further explore comets, or does this depend on the result that Philae delivers? Also, what would you love to find in the data that Philae already gathered?

[sh] There are several missions being proposed but unfortunately there are currently no selected mission. I sure hope that the success of Rosetta may change this.
What I would like to see as a result of Philae/Rosetta?… There are of course many questions to be answered but If we understand the way cometary activity works at the end of the mission I will be very happy. Currently the mission is creating more questions than answers. But this is the way a successful mission always work.

[ma] Well, it depends on what you mean by planned! We scientists (collectively) have envisioned a variety of future missions to comets (as mentioned in a previous post higher up the page). Some would go to the surface, some would orbit, some might fly past. We have developed plans for how to do the missions, what they would cost, what equipment would be needed, what kind of specialists would be needed, and so on.
On the other hand, neither NASA nor ESA nor JAXA nor IKI nor ISRO has planned to spend money specifically on a cometary mission. The planning process for funding varies dramatically from one space agency to another, but there are certainly plenty of scientists pushing in one way or another, depending on the space agency with which they deal, to get the funding committed for a cometary mission. When that happens, the planning becomes much more intense and much more detailed.

[ma] Now on to the other question – what would I like to learn from Philae. Given the way Philae behaved, I am very anxious to see the synthesis of all the data, both engineering data and scientific data, that relate to the strength of the cometary material. It seems that at the first touchdown point the lander went into soft material and then suddenly “bottomed out” by hitting a relatively hard surface, from which it bounced. The details of those data will be important in evaluating the details of the materials. We hope to get more images from OSIRIS of that first touchdown spot and analysis of the “crater” that Philae made, and how it evolves over time, will be valuable for understanding how comets work. From the instruments on Philae, I am particularly interested in learning what molecules were identified by the COSAC instrument – even if they were only ambient cometary gases rather than samples brought up from below the surface, the differences between these gases and those seen by the ROSINA mass spectrometer on the orbiter will tell us about the relationship between what is in the nucleus and what we see further out in the coma.

[uk] In the US several mission proposals are being studied to bring back material from a comet in order to investigate it in the laboratory. These studies will go ahead independent of what Philae delivers but may be influenced by the Rosetta results.
Clear information about the physical and chemical (organic material?) properties of the surface

What are the chances that Philea could be reactivated for some hours/days? And when could we hope for that to happen?

[su] We hope, when the comet (and Philae) get closer to the sun, in spring/summer 2015 we will be able to re-activate the Lander.
First, only during illumination periods (~1:20h), later, when the batteries can be re-charged charged, also longer (some hours per sequence).

[ek] We have some hope. After some months there should be enough solar energy available to reactivate Philae (when approaching the Sun). The open question is if the lander electronics will survive the low temperatures on the comet