Science with the lander – what to expect when Philae meets 67P

When Rosetta’s Philae lander touches down on comet 67P/Churyumov-Gerasimenko in two months time, a new chapter will begin for the mission. At the European Planetary Science Congress this week, participants heard from the lander team about the science plans.

Rosetta’s orbiter instruments are producing their first preliminary scientific results and many of these were presented at the European Planetary Science Congress (EPSC) that took place this week in Lisbon, Portugal. While scientists are eagerly examining the data and making the first steps towards characterising comet 67P/C-G, there is also something else on their mind: the selection of the primary landing site and the prospect of the science that can be done when Philae is safely settled on the comet.

The approximate locations of the five candidate landing sites are marked on these OSIRIS narrow-angle camera images taken on 16 August from a distance of about 100 km. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The approximate locations of the five candidate landing sites are marked on these OSIRIS narrow-angle camera images taken on 16 August from a distance of about 100 km. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

During the Rosetta Special Session at EPSC, the lander science team explained the landing site selection process that began almost as soon as Rosetta arrived at the comet in August, and that will conclude in mid-October with the formal Go for landing from ESA. They emphasised the important role played by the orbiter instruments, in particular ALICE, MIRO, OSIRIS, ROSINA and VIRTIS, in identifying suitable candidate landing sites. Philae will get a chance to repay the compliment when on the comet surface, by providing ground-truth measurements for the orbiter instruments.

This weekend, the primary landing site will be selected from the shortlist of five candidates. The team explained that the landing will be a passive one, meaning that the exact location of landing will be determined by the relative position of Rosetta and the comet at the time of Philae’s deployment, and the speed and direction of the deployment: there is no active steering down onto the surface.

As all of these parameters have uncertainties associated with them, the Rosetta and Philae operations teams can only predict the landing point in advance to within an ellipse typically 1 kilometre long on the surface of 67P/C-G. This is larger than any of the apparently smooth terrains on the reachable parts of the surface of the nucleus, adding to the challenge of selecting the best possible site.

As soon as Philae is released from the orbiter, the lander’s first science will begin. This is called the separation, descent, and landing (SDL) phase and it will last about 5 to 10 hours – the duration will depend on which landing site is selected and what trajectory needs to be flown to deliver the lander.

During the SDL phase, many of the lander instruments  will be active. During the separation and descent of Philae:

  • CIVA will make a ‘Farewell’ image of the orbiter;
  • ROLIS will take images during the descent;
  • COSAC and PTOLEMY will sample the ‘atmosphere’ of the comet as the lander approaches the surface;
  • ROMAP will measure the interaction between the solar wind and the cometary plasma;
  • SESAME/DIM and SESAME/PP will measure the dust and the plasma environment, respectively;
  • CONSERT, along with other experiments on the orbiter and the lander, will measure the rate of descent and, at the same time, will sense the uppermost surface layers of the comet nucleus.

Immediately upon landing:

  • CIVA will make a panoramic image of the landing site; this will be used together with other information from the lander to determine where and how Philae has landed.
  • MUPUS will measure the deceleration of the harpoons as they are fired to anchor Philae to the surface;
  • SESAME/CASSE will measure the elastic properties of the surface.

With Philae safely on the surface, another series of measurements will begin, marking the start of the so-called first science sequence (FSS). This phase will last for a maximum of 54 hours, and the main goal of this phase is to secure a set of the most important scientific measurements at the surface of the comet. The FSS is split into several blocks with distinct science goals.

For the first several hours, a pre-programmed automated sequence of measurements is made. At this stage:

  • ROLIS will take images of the surface with micrometre resolution;
  • ROMAP will measure the magnetic and plasma properties of the surface environment;
  • MUPUS will measure the surface and subsurface temperature at the landing site;
  • CONSERT will begin operations to probe the comet interior through one complete revolution of the nucleus.

While this automated sequence is running, the lander operators will determine how Philae is oriented by examining the telemetry data for the solar panels - the power distribution of the panels will let them work out the position of the Sun with respect to Philae and thus Philae’s orientation. They can also compare the panoramic image of the horizon taken by CIVA-P with images of the surface mapped onto digital terrain models. By the time the automated sequence is completed, the lander operators will know how Philae is oriented and can command the lander to rotate to put it in the best position to illuminate the solar panels – this is crucial to ensure that the batteries can be charged as efficiently as possible.

Philae's instruments. Credit: ESA/ATG medialab

Philae's instruments. Credit: ESA/ATG medialab

The next sequence of measurements during the FSS is mainly dedicated to investigating the composition of the subsurface, thus the most pristine constituents. During this period, SD2 will drill into the comet surface to take a sample of a few cubic millimetres of material from beneath the surface. While SD2 drills, COSAC and PTOLEMY will sense the release of gases in the surroundings. SD2 will drill twice during this block, with each sample being further heated in an oven to release chemical species that do not otherwise sublimate from solid state. The first sample goes to PTOLEMY to measure how much carbon, hydrogen, oxygen and nitrogen there is, and to identify their isotopic composition, while the second sample goes to COSAC to identify and characterise the heavier molecular compounds. Measurements of the dust in the environment will be made with SESAME.

This is followed by some experiments to study the surface properties. The MUPUS hammer is released and embeds itself into the ground so that it can measure the temperature at various depths in the subsurface. The acoustic signals of the vibrations of the hammer action will be detected by acoustic sensors in the feet of SESAME/CASSE and will be used to measure the mechanical properties of the nucleus. APXS will be deployed to measure the elemental composition of the surface material. SESAME/DIM will investigate dust impacts further, and the dielectric properties of the ground will be measured by SESAME/PP, which can provide some indications about the presence of water ice beneath the surface.

In the next block of measurements, another drill sample will be taken by SD2 and delivered to an oven in which CIVA-M will acquire microscopic images of the samples, both in the visible and in the infrared, to infer its composition. The sample will then be analysed with COSAC at lower oven temperatures than before.

Power and battery recharging permitting, there is the prospect of continuing science operations on the comet surface for the long-term science (LTS) phase, which could run from November until March 2015. The emphasis during this period will be on studying how the conditions and environment at the landing site change as the comet gets closer to the Sun, and to make some additional studies that are among the more challenging of Philae’s science goals. These include searching for comet quakes with SESAME, using COSAC to look for evidence of amino acids in a drilled sample, and making tomographic measurements with CONSERT, by transmitting radio signals between Philae and Rosetta through different parts of the interior of the comet to look for heterogeneity on smaller scales.

However, it is expected that by March 2015, the temperatures of the compartments on the lander will have reached levels that are too high for Philae to continue operating, and the lander’s science mission will come to an end.

 

Comments

34 Comments

  • Gianfranco D'Elia says:

    Superb Philae, Superb Mission!

  • Dertutenaix says:

    There are quite a few systems in the lander to assure that it stays put ones it makes contact to the surface. What is a bit worrying is that if the surface is very dusty and soft the feet seam to be very small to prevent is from sinking deep. It lands with a bit under a meter per second but the mass is 100 kg and the inertia is always the same no matter what the gravity is. Why no downward oriented thrusters to make it slow down? Afraid of getting covered in a dustcloud?

    • emily says:

      See the animation at 0:41 for the downward thruster!

      • Dertutenaix says:

        Dear Emily in the animation i see the comment thruster pushing down, upward pointing. What i meant is downward pointing thruster breaking the speed of impact. I see none of those. The only break i know of is the electromechanical suspension and the flexibility of the legs. If 100 kg falls with 0.8 m/s on earth or the comet the impact energy is the same, i know this is tested prior to launch on a bed if sand on earth but the risk that it will sink in a bit more on the comet is major as its surface density is at least 10 times less then sand. This is only a guess but so fa that is all we can do, make qualified guessing.

  • Hannes says:

    This last sentence is so sad, I have tears in my eyes.... "However, it is expected that by March 2015, the temperatures of the compartments on the lander will have reached levels that are too high for Philae to continue operating, and the lander’s science mission will come to an end."

    • Alter schwede says:

      If the lander gets buried deep enough its nice and cool down there for the next million years

  • logan says:

    To all the followers. Take the last Osiris:

    http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/09/comet_on_5_september_2014/14804076-1-eng-GB/Comet_on_5_September_2014.jpg

    And rotate 90º counterclockwise. I can't stop staring at it.

  • logan says:

    Another crystallization on the shadow, near the top of the front lobe. Ice? Marvels of micro-gravity.

  • Denis says:

    Sounds like a very busy and ambitious plan!!
    I really hope the landing goes well, good luck to everybody involved!

  • Errol Coder says:

    The APXS really been a common tool sent out on the landers/rovers. Sojourner had one in 97, the same version left with MER Rovers in 2003, then Rosetta left with a 2004 version. I wonder how different the MER and Rosetta version are. Then MSL left with improved one one in 2011. I haven't heard any talk about it though. Did it actually make it on the lander?

  • logan says:

    Structures are fractal nature to the infinitum 🙂

  • frankebe says:

    Brilliantly skillfully planned out. A wonder of modern engineering.

  • logan says:

    This is as important as pulling the Earth out of the center.

    Where is the world?

  • morganism says:

    Another great article, thanks for turning a checklist into a story !

    Am also a bit concerned about sinking into the dust, but the brazil nut effect may end up being helpful, as the places with more small boulders on the surface are probably not as deep as the smooth crater fills...

    Now i wonder if after the extended mission is done, and things start to heat up, while still harpooned in, could you fire the engines in-situ , and cover the lander with the graphite dust ? Can you refold the solar panels, to keep em clean ?
    If it works as insulation for the comet, it may help the lander make it thru perihelion...

    • Brian Deschene says:

      @morgansim: There are no engines pointing downward that you could fire to accomplish this; Philae falls to the surface without decelerating. Indeed, there is only one small engine and it is pointing upwards on top of Philae to drive it into the surface (watch video). As for the solar panels, they are mounted on various vertical surfaces of Philae and do not unfold or deploy as is customary with spacecraft.

  • Bill says:

    My major concern from seeing these first images is that the "smooth, sandy areas" are composed of larger, heavier particles that have escaped from the jet gas stream and fallen back to the surface. I have no idea of the particle size distribution in the dust jets, but I'd expect sub-micron size particles to remain entrained in the gas jet and sub-millimeter size particles to fall back to the comet body. Think of the surface as being like a deep fluffy light snowfall composed of fine silicates and tholins.

    But there may be many clues to the nature of the surface as we get closer, such as talus ("debris") piles at the bottom of the slopes, which can tell us a lot of the physical nature of the surface, as well as observing any active jets.

    --Bill

    • logan says:

      Hi Bill. The aim of 1km longitudinal is not going to give such 'finesse'. Some cross fingers needed.

    • logan says:

      Can Rosetta 'throw' something in order to test 'fluffy-ness'? ;).

      Can CONSERT make radio wave reflection? OSIRIS and VIRTIS will help on this.

  • Eric Guicherit says:

    I can't even start to imagine the science and math needed for the delivery of all this. I sit back and watch in awe.. Congratulations to the Team!

  • Dertutenaix says:

    Another qualified guessing, once the surface heats up i am sure the amount of stuff around the comet will take away some of the heat due to its filtering effect. There is also a night side to cool it down. I guess there will be a life after the 15/03-2015 and quite some time beyond. The osiris situation is more critical due to the comets growing tail.

  • DexGypMom says:

    As a representative of 'John/Jane Q-Public', I am absolutely fascinated with this entire project and visit the blog everyday monitoring the progress of Rosetta. The closer to the deployment of Philae, the more anxious I become praying for every stage of this mission to reach fruition. This is science at its best!!

    Since I am not a scientist by any means, please forgive my ignorance of deep space mechanics with my following questions. I have perused the DLR Lander site and the ESA site for this information before coming here to ask.

    How deep will the drills of the Philae lander legs screw into the comet's surface securing it in place?

    As stated in earlier blog entries, the comet will shed more and more material as it passes near our sun. Is there a possibility that enough of the comet's surface erodes or changes physical states such that Philae may be ejected from the comet?

  • Alter schwede says:

    On earth mars and moon the gravity is convenient for our machines and persons visiting it in the past and present. No dangerous fluffy dust so far, on this comet the situation is totally different as its gravity is faint. I think NOBODY has a clue what it is supposed to be like. There is nothing but guesses from the audience available. I wonder if some simulations in labs are going on playing around with starch powder and other things making a mess when copies of the Philae hits the dirt. Its a bitt worryingtc.

  • logan says:

    Massive crystallizations on old duct ceilings.

  • logan says:

    Hardly a level 7 environment.

    But I'm going to say nothing 🙂

  • THOMAS says:

    I too am truly and sincerely amazed by the amount and the quality of the planning and engineering work which has gone into the Rosetta mission, including what is hopefully still ahead for the Philae lander.

    Sorry, however, to strike a slightly dissonant note in saying that I believe what the images are telling my eyes (rather than what the old “dirty snowball” theory is still seeking to feed to our mindsets), which is that this comet is just a rather large chunk of extremely hard, misshapen rock which is undergoing electrical discharge phenomena (particularly from the “neck” region). If this is indeed the case, then Philae’s harpoons will, in the best-case scenario, just ping back off the surface and Philae itself will presumably simply drift off into the dark. In the worst-case scenario, Philae’s electric circuits will be shorted out during its descent towards the comet (whatever the chosen landing-site) and we will never have any hands-on information about the precise nature of the comet’s surface or sub-surface. But we’ll at least be even surer about the electric nature of comets and of the “jets” which produce their comas…

    We’re really getting near money-time for paradigm shifts!

    • Alter schwede says:

      If extreme hard rock with a density a third of water exists it is some really high tech material like silicon-carbide or boor-nitride foam. Philaes electro mechanical brake will absorb the major part of the energy of impact if the surface takes away nothing of it. The bounce-back will be the minor problem, staying head up is the main issue and some systems are taking care of this. The major worry is to sink too deep as there is nothing available to prevent this from happening but the hope that fluffy 2 meter deep dust does not exist in this place. I have been skiing in fluffy snow ploughing over one meter down through it and this on planet earth. Gravity has nothing to do with inertia.

  • Teddy says:

    'COSAC and PTOLEMY will sample the ‘atmosphere’ of the comet as the lander approaches the surface;'

    Neither COSAC nor PTOLEMY will be active during descent.

  • Vard says:

    Where were the foot screws tested and on what surfaces?

  • Yves says:

    Having seen the press conference video about the choice of the landing site and how the Philae will be gently falling on the comet surface, I started to wonder, beside the trajectory of its mass center, how it is controlled to avoid rotate and to land say upside down. I'll be very pleased to get explanations (gyroscopes, active feedback...?). THX. Yves

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