Rosetta’s descent towards region of active pits

Squeezing out unique scientific observations until the very end, Rosetta’s thrilling mission will culminate with a descent on 30 September towards a region of active pits on the comet’s ‘head’.

Rosetta’s last week at the comet. Click for full caption.

Rosetta’s last week at the comet. Click for full caption.

The region, known as Ma’at, lies on the smaller of the two lobes of Comet 67P/Churyumov–Gerasimenko. It is home to several active pits more than 100 m in diameter and 50–60 m in depth – where a number of the comet’s dust jets originate.

The walls of the pits also exhibit intriguing metre-sized lumpy structures called ‘goosebumps’, which scientists believe could be the signatures of early ‘cometesimals’ that assembled to create the comet in the early phases of Solar System formation.

Rosetta will get its closest look yet at these fascinating structures on 30 September: the spacecraft will target a point adjacent to a 130 m-wide, well-defined pit that the mission team has informally named Deir el-Medina, after a structure with a similar appearance in an ancient Egyptian town of the same name.

Rosetta's planned impact site, within a ~700 x 500 m ellipse. Click for high res and full caption and credit info.

Rosetta’s planned impact site, within a ~700 x 500 m ellipse. Click for high res and full caption and credit info.

Like the archaeological artefacts found inside the Egyptian pit that tell historians about life in that town, the comet’s pit contains clues to the geological history of the region.

Rosetta will target a point very close to Deir el-Medina, within an ellipse about 700 x 500 m.

Since 9 August, Rosetta has been flying elliptical orbits that bring it progressively closer to the comet – on its closest flyby, it may come within 1 km of the surface, closer than ever before.

“Although we’ve been flying Rosetta around the comet for two years now, keeping it operating safely for the final weeks of the mission in the unpredictable environment of this comet and so far from the Sun and Earth, will be our biggest challenge yet,” says Sylvain Lodiot, ESA’s spacecraft operations manager.

“We are already feeling the difference in gravitational pull of the comet as we fly closer and closer: it is increasing the spacecraft’s orbital period, which has to be corrected by small manoeuvres. But this is why we have these flyovers, stepping down in small increments to be robust against these issues when we make the final approach.”

The final flyover will be complete on 24 September. Then a short series of manoeuvres needed to line Rosetta up with the target impact site will be executed over the following days as it transfers from flying elliptical orbits around the comet onto a trajectory that will eventually take it to the comet’s surface on 30 September.

The collision manoeuvre will take place in the evening of 29 September, initiating the descent from an altitude of about 20 km. Rosetta will essentially free-fall slowly towards the comet in order to maximise the number of scientific measurements that can be collected and returned to Earth before its impact.

A number of Rosetta’s scientific instruments will collect data during the descent, providing unique images and other data on the gas, dust and plasma very close to the comet. The exact complement of instruments and their operational timeline remains to be fixed, because it depends on constraints of the final planned trajectory and the data rate available on the day.

The impact is predicted to occur within 20 minutes of 10:40 GMT, with uncertainties linked to the exact trajectory of Rosetta on the day, and the influence of gravity close to the comet. Taking into account the additional 40 minute signal travel time between Rosetta and Earth on 30 September, this means that the confirmation of impact is expected at ESA’s mission control in Darmstadt, Germany, within 20 minutes of 11:20 GMT (13:20 CEST). The times will be updated as the trajectory is refined.

Click for full caption.

Click for full caption

Mirroring Rosetta’s wake-up from deep space hibernation in January 2014, where a rising peak at the right frequency confirmed that the spacecraft was alive and transmitting its carrier signal, mission controllers will see that peak disappear for a final time once Rosetta impacts. It will not be possible to retrieve any data after this time.

“Last month we celebrated two thrilling years since arriving at the comet, and also a year since the comet’s closest approach to the Sun along its orbit,” says Matt Taylor, ESA’s Rosetta project scientist.

“It’s hard to believe that Rosetta’s incredible 12.5 year odyssey is almost over, and we’re planning the final set of science operations, but we are certainly looking forward to focusing on analysing the reams of data for many decades to come.”

“This pioneering mission may be coming to an end, but it has certainly left its mark in the technical, scientific and public spheres as being one of outstanding success, with incredible achievements contributing to the current and future understanding of our Solar System,” adds Patrick Martin, ESA’s Rosetta mission manager.

More information

This article is mirrored from the main ESA Portal.

All times and details regarding the end of mission are preliminary and subject to change as Rosetta’s final trajectory is refined. Even on the day, timings will have uncertainties owing to circumstances at the comet beyond the control of the mission team.

For further information, you can consult the end-of-mission FAQs here.

Application for accredited media and social media representatives to attend the event in Darmstadt on 30 September is possible via the Call for Media. Science journalists will also be eligible to attend a briefing on 29 September focusing on the scientific results of the mission. Livestream details will also be provided nearer the time.

An ESA Hangout on Air is planned for Monday 19 September 12:00 GMT (1400 CEST) to present the latest information regarding the details of the last week’s operations and the story of the search behind finding Philae. More details will be provided soon.



  • Gadi says:

    Are extra DSN resources allocated for communicating with Rosetta for these final days in order to maximize the data collected?

    • emily says:

      Yes. DSN 70m is prime, we have a lot of 70m as of 28/09 and double station coverage with ESTRACK.
      (Answer from Sylvain Lodiot, ESA’s Rosetta spacecraft operations manager)

      • Marco Parigi says:

        Hi Emily,

        Thank you for your answers and allowing the conversations on the Rosetta Finale to continue. I have moved on to acceptance , and as I have put in a lower comment post, look forward to the finale and leave behind my bitterness and angst.

        The fact that I feel grief at the mission ending is testament to how well the outreach team has connected with the public.

        Once again thanks!

        • emily says:

          Thank you for writing this note and the one below, Marco. We are all feeling sad that the mission is ending, but rather than let those feelings overwhelm the conversation here, let’s use it as a time to celebrate together Rosetta’s fantastic achievements! And even though the operational aspect will soon be ending, there will be plenty more science reporting to come; it is not over yet 🙂

  • A. Cooper says:

    There had been a plan to do a retro burn at 1.7km altitude and come in at 50 centimetres per second to land. That’s in this NASA document:

    It says JPL are helping ESOC quantify the gravity field so it’s an authoritative document. 90 centimetres per second is indeed the freefall speed from 20km (I made it 99cm/sec dropped from infinity to 1400m radius including surface rotation vectors) and so it’s clear the 1.7km altitude burn resulting in the 50cm/sec approach speed is not going to happen.

    This means the close approach will be nearly twice as fast and be over in nearly half the time as for 50cm/sec. That in turn means half the data return. It’s also 7 times less than the data return from a very close flyby of Apis at 25cm/sec.

    With real time transmission instead of saving data for later transmission, the data return is reduced still further, perhaps by half again. I think one could safely say that a flyby of Apis and later transmission would have yielded ten times more data. In that scenario, if escape wasn’t executed, Rosetta would be locked in the gravity well anyway and probably crash eventually. A small retro burn of no more than 20cm/sec after data transmission would ensure a crash if there were any lingering worries.

    • Harvey says:

      I can see a number of issues with such a late, low burn. The spacecraft would have to be re-oriented twice, taking time, and a large fraction of the ‘sky’ will be filled with comet potentially blocking the star tracker view.
      There is also an issue of error budgets.
      I imagine a lot of simulations have been run.

      • Matt Taylor says:

        Exactly Harvey, LOTS of analysis on this, to get the best end of mission. The plan avoids any communication outages with Earth (which the braking manoeuvre did have) during the final descent, which is NOT good for nerves :). It gives us a VERY nice profile to the comet, Rosina data and other insitu data is going to be unique. remote sensing will be awesome of course, and we wait for that last OSIRIS image with likely many tears in our eyes. We are truly getting the most out of the mission, before power and data rate drop almost completely.

      • Marco Parigi says:

        Hi Matt Taylor,
        And thanks for dropping by. My main worry with abandoning a craft on a body that has been identified as one with a lot of complex organics is a contamination issue.

        The range of possible plans that finish the mission with the probe landing is a set mutually exclusive to a plan that ends in a non impacting heliocentric orbit. There are several examples of both kinds of robotic mission endings and different advantages to either. The cost differential between “abandoned assumed lost” in a heliocentric orbit and “impacted and turned off” should be assumed to be zero, so cost should not be an advantage to either ending.

        Then there is the trade off between a contamination potential versus having something interesting for future 67P probes to look at.

        The other trade off is the potential, however unlikely for future amateurs to try to wake up/reacquire the probe – There are plenty of probes, well past their use by date, giving back rudimentary, but useful long term data.

        This versus the potential and even a teaser that encourages future probes to 67P to see the probe again.

        One such detailed equal cost plan ruled out by impacting is as follows.

        The detail and calculations are extensive and cover all possibilities.

        I, for one, am quite sad that after all the anthropomorphism related to Rosetta and Philae, we are essentially going the “euthanasia” direction with the spacecraft with most of its faculties intact until death.

        For my part, I feel a loss of opportunity of putting Rosetta to sleep, ie. In a hibernated state and ending the ESA’s commission that way. ie. No financial responsibility to be reacquired, but the opportunity and possibility, however remote it ends up being.

        I will indeed cry a lot.

        • Booth says:

          Mr. Parigi!

          Enough! The contamination argument is unjustified! Recall that Philae contacted the surface four times during landing, and the “corpse” has been left behind.

          In other words, 67P has already been contaminated. Landing the Rosetta orbiter will not alter that fact.

          More to follow on the whole hibernation fallacy.

        • Marco Parigi says:

          Hi Booth,

          I’ve also replied to your other comment regarding hibernation.

          There is a great scale of unknown with what we consider “contamination” I am sure when humans used to dump things in the deepest oceans or in deserts or elsewhere remote, there was no thought that it could be harmful to future generations or that there was anything important there to contaminate.

          The difference with Rosetta over Philae is the likelihood of break-up and spread of a wider range of stuff. due to the compactness and design of Philae to be operational and self-contained after impact.

          If we are abandoning the mission anyway, why take the risk of spoiling something we will only realise with more information is important not to spoil.

          • Gerald says:

            Hi Marco,
            comets are mission category II in terms of planetary protection:
            “Bodies of “significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could compromise future investigations.”

            Planetary protection is extensively considered for each mission and according decisions.

          • Booth says:


            Just saw your post!

            I have recently submitted something that has a comparable quote. It should show up soonish. 😀

            Thanks be to ye! Great minds and all ….

            Quick question – I’ve been having certificate and configuration problems with a couple NASA websites (e.g., the one you link above) and the COSPAR HQ website. Have you experienced any issues?

          • Marco Parigi says:

            Hi Gerald,

            The whole point of missions is to work out where life might reside. The categories are quite presumptious at this very early stage of close comet study.

            Of course, even when we do discover life on comets, we could always go to a different comet with more sterilisation protocols…..

  • logan says:

    .These are already the closest, bolder, dangerous flyovers in Space Exploration History:


    On committing to that ‘lake fishing’ 2km flyovers rewards|risks are at a level not seen since Philae working to the last battery drop.

    • logan says:

      Dominant layering 120º. Around 11m from the perspective. Very dry taste. Orange filter, then speculating ambiance light mainly particle-o-sphere.

      [Seeing the same frequency layering (at bottom right) here than at Philae Finding Shot. Consequently instrument related].

  • Kai Roesner says:

    Where (in terms of geocentric coordinates) will 67P/C-G be when Rosetta impacts on the comet?

  • Jochen Alberty says:

    As I understand the FAQ, Rosetta can switch into save mode during the final descent. Why is that feature still turned on at this stage? What situation do you want to a avoid at this stage? Or is the save mode feature something that simply cannot be turned off?

    • emily says:

      Turning off safe mode triggering capability doesn’t avoid safe modes, it just stops the spacecraft from dealing with them. Ultimately in the final trajectory the spacecraft will be in a state of no return, some aspects of the safety triggers will be turned off that could perturb the final descent unnecessarily. But some are left active, so that the spacecraft is passivated upon impact.
      (Answer via Sylvain & Matt)

  • Daniel Pakk says:

    Isn’t Rosetta’s orbital period actually decreasing as the spacecraft gets closer to the comet?

    “We are already feeling the difference in gravitational pull of the comet as we fly closer and closer: it is increasing the spacecraft’s orbital period (…)”

    • emily says:

      The gravity was increasing it, but as stated, small manoeuvres correct this.
      In the overall picture, the apocenter of the orbit is being increased, to keep a 3 day orbit and a fixed pointing plan and timeline.

  • Gerald says:

    A question the EOM FAQ doesn’t target explicitely: Will Rosetta delay or abort the EOM collision in case there occur DSN issues, as there had been some this year at Goldstone?

  • emily says:

    Thanks for the questions! Have passed them on to the operations team and will get back to you soon!

  • Mike says:


    Back in the 60s Ranger 7 transmitted 1,000s of photographs during the final 17 minutes of its descent to impact the moons surface.

    It seems such a shame to end the Rosetta mission with such a one way descent to the surface of 67P. Unlike Ranger 7 would it not be possible during this last decent to bring Rosetta to a stop/hover at about 100m from the surface, then gentle gain altitude to say 500m and letting 67P rotate under it. This could be repeated several times. Thus instead of getting close data on one area several areas could be examined. After each successful “HOP” the stop/hover altitude could be lowered until Rosetta was just above the surface.

    • logan says:

      Hi Mike. This is Rosetta’s dirty ‘environment’ at 5km over 67P.

      Just extrapolate it to 500, or 100m. We don’t even know it She is going to make it to surface [while keeping a strongly aimed transmission link], those huge solar panel ‘wings’ become a liability. Neither know if She’s surviving dangerous fly-over phase.

      This is not an 8 sec. comm. delay, but full 40 min., one way. Same way as your WiFi quickly deteriorates and downgrades on the distance, Rosetta’s link also.

    • Gerald says:

      Hi Mike,
      this appears to be too difficult for the flight dynamics, implying the risk to lose more science value than to gain.
      The close flybys before the final touchdown seems to be the best balance between getting close to the comet, and avoiding a too high collision risk.
      Hovering over the comet would rapidly increase the trajectory uncertainty, and interrupt communication with Earth. Turnaround times for recalculating the necessary correction maneuvers are too long to gain anything compared to better-determined flybys.

  • Kamal says:

    Emily: What would be the local time on the comet (height of the Sun above the horizon) at the site, during the descent and at time of impact?

  • Booth says:

    Mr. Parigi!

    Regarding your numerous hibernation comments scattered over several threads, over several weeks …

    Your desire for a second period of “survivable” hibernation is unjustified, especially when there is still good science to be conducted right now!

    Recall the following facts …

    1) Rosetta was originally designed for a mission to 46P/Wirtanen (aphelion = 5.13 AU).
    2) Launch vehicle problems forced planners to shift target to 67P (aphelion = 5.68 AU).
    3) Design specifications require Rosetta to operate at a max distance of 5.25 AU.
    4) During hibernation, Rosetta flew outside the max design distance for 6 months!
    5) During hibernation, Rosetta reached an aphelion distance of 5.29 AU on 20121003.

    The spacecraft is aging. Some systems no longer operate within specifications …

    1) Rosetta has been in space for more than 12.5 years (exceeding specifications by half a year). During that time, 31 months were spent in hibernation (i.e., systems could not be monitored or managed).
    2) 20060810 : RCS anomaly – A slow leak on the low pressure side of the helium pressurization plumbing was identified – unable to repair / thruster efficiencies degraded.
    3) 20100800 : AOCMS operational degradation – Around the time of the 21 Lutetia flyby, two of four reaction wheels started showing increased noise due to friction (torque) – RW degradation could be managed, but components might fail at any time.
    4) 20110118 : Pre-hibernation rendezvous maneuver #1 anomaly – A pressure instability caused loss of power in thruster #9; other thrusters attempted to compensate; S/C lost attitude control due to C/M asymmetry resulting in safe mode – a side effect of the earlier Propulsion anomaly.
    5) 20140120 : Software/hardware anomaly – Rosetta’s onboard computer rebooted during the wake-up and reactivation sequence – problem investigated / no resolution required at that time.
    6) Two years of operations in the inner coma of an active comet have degraded the performance of numerous systems through dust impact/ablation (e.g., solar panels and camera optics).
    7) Fuel usage continues! Consumption dictates that one day the Rosetta mission will end ….

    Aside – The first hibernation phase was executed before rendezvous! This fact is very important!

    A second hibernation phase requires the following actions / considerations …

    1) To enter hibernation, all AOCMS and RCS functions must be disabled! This means the spacecraft will need to be spin stabilized with the optimal solar pointing direction being set for aphelion. To restate, over the course of more than 50 months, Rosetta’s solar panels will only be pointing directly at the sun for a few short months near aphelion. The rest of the time, they will be offset from optimal, especially at the beginning and end of hibernation.
    2) To enter hibernation, all science instruments and most of Rosetta’s systems will need to be powered down. Given the dramatic reduction in power during this extended hibernation phase, even some of the heaters will be inoperable. So, what components and subsystems do we potentially sacrifice for this hibernation? The onboard computer and wake-up clock are mission critical. That leaves either the science or propulsion! As we will see below, we need to sacrifice the science!
    3) The first hibernation phase was not affected by near-nucleus forcings! In other words, there were no significant external forces that could alter the inertial behaviour of the spacecraft – this led to a recoverable state upon wake-up. In this second hibernation cycle, however, we will be flying blind for over four years near a comet and must account for the comet’s gravity field and potential outgassing! Question – Where does Rosetta need to be at the start of hibernation to “ensure” stable inertial flight? During the rendezvous phase, the Flight Dynamics team started seeing gravitational effects at ~1800 km from the nucleus. Given that kind of information, the engineer in me would argue for a minimum 5x fudge factor in the separation distance. Thus, Rosetta should start hibernation at least 10000 km from 67P, and that is not a typo. Next, given that outgassing events are insolation driven and unpredictable (in magnitude, extent, and solar distance), it would be prudent to position Rosetta at a point directly below the nucleus on the terminator plane to minimize impacts from outgassing. Recall that the southern “hemisphere” is in permanent shadow during this phase of 67P’s orbit. Finally, there is one last significant orbital issue that must be recognized …
    4) Debris! We know that many annual meteor showers are caused by comet debris liberated via sublimating volatiles. These tiny dust particles, once ejected, slowly disperse through space following the comet’s previous orbits. You will note that I have pluralized “orbits” … which, for spacecraft safety warrants an additional standoff distance to the original 10000 km. At a minimum we should double our hibernation starting point to 20000 km. Of course, this distance does not guarantee safety and survival at the end of 50 months, but it helps.
    5) Next question – When should Rosetta be put into storage? Given that 67P will experience a solar conjunction in October, it would have been necessary to start the hibernation process months ago. The window of opportunity is driven by the length of time it takes to move Rosetta to a safe distance of 20000 km. Simple calculations indicate that a thirty day move would require a relative velocity of ~7.7 m/s! That kind of delta-v consumes a lot of fuel! Both for departure and zeroing out the S/C’s motion with respect to the nucleus once the parking position is reached. Returning to 67P at the end of hibernation entails additional fuel, though the approach could be at a less frantic 1 m/s (which, assuming no drift further from the nucleus, would involve a 230 day trip home). Furthermore, the 30 day outbound move should have started mid-August to ensure the spacecraft had reached an appropriate distance before shutting down … to die! That represents a loss of six weeks of real science on the off-chance that Rosetta MIGHT survive a 50 month deep-freeze! Of course, all this assumes there is sufficient fuel to move the spacecraft that far away in such a short period of time. If we wanted to use less fuel, we should have started the departure sequence say … in mid-January! More lost science! Including the February outburst and the northern hemisphere remapping effort completed in June! This hibernation idea is a lose-lose-lose-lose proposition every way you look at it, especially given the loss of genuine science … and for what? A maybe?


    0) Available hibernation power is reduced well below design specifications by flying 0.43 AU beyond the 5.25 AU limit.
    1) Available hibernation power is further reduced below specifications by sub-optimal pointing of Rosetta’s solar panels throughout most of the 50 month aphelion phase.
    2) Available hibernation power will be insufficient to ensure all heaters can be used. Only the mission critical clock and reboot computer get heat, as does the remaining fuel vapours. At what point do we lose power to the fuel heaters?
    3) Comet outgassing and gravity forcings push Rosetta to a hibernation point 10000 km from the nucleus to ensure the spacecraft’s inertia is undisturbed for ~50 months.
    4) The potential for the spacecraft to encounter debris from a previous orbit demands that the standoff distance be boosted to at least 20000 km! Please note, this distance value is non-negotiable!
    5) The opportunity to put Rosetta into hibernation closed some time ago! The loss of tangible science with an aging spacecraft could not be passed up for a future maybe!
    6) Rosetta is not a person! Rosetta is a robot. A robot that has far exceeded its design specifications, yielding the most amazing science!

    Aside – Nothing significant is likely to happen during the four year, “quiescent” aphelion phase of 67P’s orbit. Thus, there will be nothing new to see upon exit from hibernation … if Rosetta survives the cold!

    Personally, I am very pleased with what the mission planners, scientists, and managers have put together! Planning the last intense science cycles can be incredibly stressful! So proud!

    This mission has exceeded all expectations! The scientists, engineers, and technicians have gone above and beyond in terms of design, construction, and operations! I would like to further offer a tip-o-me-cap to the Flight Dynamics team for the most extraordinary bit of “by-the-seat-of-your-pants” flying ever attempted! A thousand thank yous to all involved!

    Post Script – Please! Enough with the hibernation talk and Apis flyby / fly away dreams. Rosetta will complete its mission on 20160930 when it impacts the nucleus of 67P! This EoM scenario has already been initiated!

    • Booth says:

      Oops! Tis funny the points you forget and then remember after the fact.

      First, the design specifications state that the nominal lifetime of the spacecraft is 11 years, not the 12 cited above. Like Rosetta, I too, am getting old.

      Second, if Rosetta were to survive a second hibernation, even more fuel would be required to return, as the spacecraft would need to reenter 67P’s gravity field from the sunward direction, as was the case during the initial rendezvous. This trajectory is required to map out any new large-scale debris that may now surround the nucleus. It should always come back to “safety first” …!

    • Marco Parigi says:

      Hi Booth,

      Thank you for your time and detailed response, but it is obvious to me that you neither understood my post nor read the link which factors in every detail you mention.

      My calculus is the following option 1 versus option 2 done after all the science you want to do up to the 29th of September, with just one day dedicated to saying goodbye and dumping the spacecraft.

      Option 1 – fatally dumping the craft onto the comet.

      Option 2 – probably fatally dumping the craft in a hibernated programmed state.

      With this in mind, you could probably realise that none of the preparatory excercise you talked about designed to ensure or optimise wake up are relevant to this calculus between option 1 and option 2.

      • Gerald says:

        Hi Marco,
        Option 2 lacks the close-up inspection and analysis of the nucleus. Hence option 1 is of higher science value. An easy decision.
        Hibernation would most likely be indefinite, and destroy instruments and electronics, since you get too far outside the designed temperature range, leading to microcracks by differing thermal contraction between materials.

        You would be right, if there wouldn’t be essential science during descent.

      • Marco Parigi says:

        Hi Gerald,
        Thank you also for your time, but you obviously haven’t read the alternative plan either. The alternative plans allows for a complete data set science return from a closer distance with more data. This is because a daring close flyby at about the same speed of 1 m/s duck centric velocity (less than the free fall surface relative velocity of the landing plan) can be hyperbolic in terms of the nucleus, and there is far more time for data transmission, even with interruption/error rate which is unknown and cannot retrieve once impacted.
        There is far more science data with secure redundant transmission than in *REAL TIME*. Real time because if there is a reboot or something, there is no opportunity to recover the lost data.
        This would even be true if the whole plan was executed as for an impact with a last minute burn into a hyperbolic trajectory to give time for a complete data set to be transmitted redundantly.

  • Mike says:


    What is the predicted or target impact speed of Rosetta when it hits the surface of 67P?

    Will the pictures taken during the decent that make it back to Earth be published right away (live )or is the going to be a 6 month delay?

    Is the or do you plan to make a web viewer similar to Google earth or even let Google do the job with the picture data that lets you move around 67P and zoom in to the surface with ever greater detail?

    Are there before and after sets of photos showing the Sun weathering of the surface of 67P?

    Last but not least, a BIG THANKYOU to all the people involved in the Rosetta mission.

    Thank You, Mike

  • Booth says:

    Mr. Parigi!

    It’s interesting the things you find on the interwebs when you go looking for them …

    Following your concerns about additional contamination of 67P by the pending impact of the Rosetta orbiter, I did some digging.

    According to the COSPAR Planetary Protection Policy (2002/10/20; Amended 2005/03/24), Rosetta (and by extension, Philae) is classified as a Category II mission, Under these UN guidelines, there are no requirements for sterilization … None! I find that revelation fascinating! Now, to put things in perspective, Venkateswaran et al (2001) used a traditional swab-and-culture technique to examine unsterilized “spacecraft hardware” and found that an initial bioload of ~10^6 spores/m^2 was typical. It should also be obvious that this technique will not detect unculturable organisms, or the organic residues left behind by dead cells. Hence, bioloads on unsterilized spacecraft hardware is going to be significantly higher than the spore count reported above.

    From the COSPAR PPP – “Category II missions comprise all types of missions to those target bodies where there is significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration.

    Conclusions –
    – Comets (i.e., Category II destinations) are deemed to be biologically inert. Properties like extremely cold temperatures, UV radiation, and cosmic rays have a lethal effect on life.
    – An unsterilized Philae lander has already left considerable terrestrial contamination scattered across 67P’s nucleus.
    – Additional contamination caused by retiring Rosetta to the comet’s surface is not a valid argument against ending the mission as planned/presented.

    Query —
    1) Rosetta, being a Category II mission, did not require any form of sterilization.
    2) While escorting 67P, Rosetta is considered a Mars and Jupiter crossing satellite.
    3) Comet 67P is in it’s current orbit following several encounters with Jupiter’s gravity.
    4) Mars and Europa are destinations with special “protected” status according to the COSPAR PPP.
    5) To prevent possible contamination of the Europan oceans, the Galileo spacecraft (at the end of it’s mission) was commanded to enter the Jovian atmosphere to be destroyed.
    6) If Rosetta is lost in space (due to a failed hibernation cycle for example), it is probable that, at some point, the spacecraft will be perturbed by Jupiter’s gravity into an unpredictable trajectory that could jeopardize either the Martian or Europan environments.

    Given these observations, what is the “safest” or most prudent end to the Rosetta mission?
    An uncontrolled hibernation that is likely to be unsurvivable, or a focused scientific descent to a known final resting place on the comet’s surface?

    Of course, there is a comparable probability that 67P could later become the same terrestrial contamination hazard given that both the orbiter and lander are now a part it’s mass. But then, one could argue, that’s nature!

    At the end of the day, there will be no hibernation. Rosetta will make one last remarkable and targeted scientific descent to the surface. Regardless of your views or wishes, the mission end has already been scripted! I hope you can graciously accept that fact.

    • Marco Parigi says:

      Hi Booth,

      With regards to the sterilisation protocols of Rosetta/Philae, I was quite aghast that they were not sterilised – I think I had researched this back in 2012,

      At any rate, I am glad you dug them out because they are poorly publicised and require review if evidence reaches a critical mass to change the perceived probability of life. The panspermia group isn’t exactly a tiny group, and they would beg to differ in regards to that probability.

      Talking of probability, the probability that a free roaming Rosetta would contaminate Mars or Europa within a million years is negligible, and the same as for 67P itself impacting those bodies, so Rosetta landing there doesn’t really materially change the risk for those bodies, but if you are counting things other than biological contamination (eg fuel contamination, heavy metals) that may contaminate future samples or poison the life that is there, you are comparing very small numbers, but it should not be a net reason for landing.

      Either way, I think I need to move on from my stages of grief (Denial, anger,bargaining,depression) and move on to acceptance.

      I accept that the plan in place has been meticulously scrutinised, and is absolutely the best possible one of the ones that end in impacting – The decision to impact was made well in advance for the very reason that it gives enough time to analyse all the alternative impacting plans. Once the decision is made, the chances become tiny for reversal, and reduce from there.

      I then have to look at my perception of the advantages of definitively ending a mission over leaving it to the gods. Certainly the mission scientists are more motivated to give 100% to the mission right to the last. More calculated risks would be taken, even if technically it wouldn’t make a difference – psychologically, all the things on Rosetta’s bucket list that can possibly be done are squeezed into a small time frame with the full team of highly motivated staff. If there was even an inkling to save something for later, that may reduce the daringness of this month’s fly byes.

      So at this stage, I am not happy with the decision made many months ago, but I want to move on and enjoy the science, observations, and even the last minute drama that impacting entertains. This along with everyone else – I do not want to spoil the fantastic show that it will be with bitterness and angst.

      I sincerely thank the moderators of this blog to allow such conversations to take place also.

  • Kamal says:

    The discussion section of the paper “Pits formation from volatile outgassing on 67p” by Olivier Mousis and others (a draft version is at suggests that observations of changes in the Ma’at pits, which remain illuminated during perihelion, between when Rosetta arrived and now, can potentially differentiate between whether the pits formed over the last 57 years from when the comet moved into a closer orbit after bring nudged by Jupiter in 1959, or whether the pits formed several millenia ago.

    • A. Cooper says:

      Thanks, Kamal

      I took a look. Interesting.

      • Marco Parigi says:

        Hi Emily,

        The study of pits highlights an issue that is yet to be resolved regarding the thermo-physical models of comet nuclei.

        The issue is described in section 7 of the following Rosetta science summary paper (paper summarising and contrasting conclusions from hundreds of recent papers) which describes the issue in section 7:

        Quoting from there:
        “The 67P surface has a very low thermal inertia, close to 20 J K−1 m−2 s−0.5 (Schloerb et al. 2015)”

        And of course it can explain the observed surface, but once we use this information to constrain how dust, pebbles and larger chunks can be dislodged from the surface, calculations based on this thermal inertial parameter fail to allow for over pressure that is thought to lift larger grains from the nucleus.

        It then says later:

        “A second consequence is that local thermodynamical equilibrium (LTE) cannot be applied in models of water and dust ejection from comets. The fact that all thermo-physical models of comets assume LTE (Prialnik et al. 2004) may explain why they fail most predictions.”

        Which makes sense to me – Even from pre-Rosetta missions, comet nuclei have not been physically and thermodynamically predicted to behave in the way that they are observed to. I am not suggesting that the laws of thermodynamics are being broken, but that the axiom that LTE is applicable to comet nuclei is wrong. Ie. That there is something other than a local thermodynamic equilibrium on the comet. My guess would be internal energy sources other than the sun directly, or some sort of Carnot cycle that makes the calculation of thermal inertia unreliable.

        There has been reasonable consensus on a dust mantle:
        “Prialnik et al. (2004) write that thermo-physical models of comets predict the formation of ‘a dust mantle which inhibits gas sublimation when most of the surface is covered by dust (Prialnik & Bar-nun 1988), a result confirmed by the KOSI experiment (Grün et al. 1993)’. VIRTIS observes this mantle: the surface of 67P is covered by an organic-rich layer (Capaccioni et al. 2015), where the few wettest spots contain at most 4 per cent water (Filacchione et al. 2016), but 67P is still very alive. This confirms that some results of the KOSI experiment, based on a dust-to-water ratio lower than one, are misleading.”

        However the contradiction with expelled dust grains is “worrisome” for the models.

        “A dust layer much thinner than the size of the ejected dust has little physical sense, but is still a common assumption of models of water ejection (Keller et al. 2015b).”

    • logan says:

      .And, you bet which one, Kamal? I bet for depletion across millenia, and collapse quite contemporaneous.

    • Kamal says:

      Not a betting man, Logan. But I can tell you what I would like: finding the kind of changes that can take place on a comet over 50-odd years would be fascinating.

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