Tracking Philae’s descent with magnetic data

ROMAP co-principal investigator Hans-Ulrich Auster from the Technische Universität Braunschweig, Germany, explains how the instrument will be used with RPC (on the Rosetta orbiter) to monitor Philae as it descends to the surface of comet 67P/Churyumov-Gerasimenko on 12 November – as well as looking out for any evidence of a ‘fossil’ magnetic field linked to the comet itself.

On 12 November, the magnetic field instruments on-board the Rosetta orbiter and lander will be assigned the task of monitoring Philae as it descends to the surface.

That’s possible because both the lander and the orbiter generate small magnetic fields of their own, due to the electronic circuits inside the spacecraft. These magnetic fields create perturbations in the data that the scientists normally remove in order to analyse the purely natural magnetic fields from the comet and the solar wind. However, on 12 November, these perturbations can be analysed to tell what is happening to the lander as it slowly drops towards the surface of 67P/C-G.

Once the lander has separated, the magnetic perturbation registered near the orbiter will decrease as Philae moves away. Later, when the lander deploys a boom arm for its ROMAP instrument, it will cause a brief variation in the perturbation. A similar variation may also be registered when the three landing legs spring into place.

The perturbations may prove too subtle for the RPC instruments, operating on Rosetta at some distance from Philae, but the ROMAP instrument on the lander will register the changes loud and clear.

We will be able tell what is happening on the lander by the changes in its magnetic field,” says ROMAP co-principal investigator Hans-Ulrich Auster.

These measurements will add to the overall picture of Philae’s progress to the surface of the comet.

Of course, the main focus of these instruments is on science. The comet should retain a memory of any magnetic field that was present in its environs 4.6 billion years ago when the Earth and the other planets were forming. For example, some theories of star and planet formation require a magnetic field to accelerate the growth of our Solar System, while others do not, and thus by measuring the ‘fossil’ field, Rosetta can hope to shed light on this epoch.

In the final few hundred metres of Philae’s descent, ROMAP will detect this magnetic fossil if it exists.

It’s a simple question, is it there: yes or no. We are just a few days away from knowing the answer to this,” says Auster.

After the landing, ROMAP will again play an operational role, by helping scientists determine the angle at which the lander has come to rest. This will be essential to help orient the lander so that as much light as possible falls onto its solar panels.

Although there are other ways in which this information will be monitored and returned to Earth, the contribution from RPC and ROMAP should prove very valuable.

We will do everything we can to help understand the overall state of Philae after landing,” says RPC principal investigator Karl-Heinz Glaßmeier, also from the Technische Universität Braunschweig, Germany.

After all, the more that can be done to keep the lander functioning, the more science will flow from the mission. And the more we will learn of our cosmic origins.


The contributing institutions to ROMAP are: Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Germany; Hungarian Academy of Sciences Centre for Energy Research, Hungary; and Space Research Institute Graz, Austria



  • Mariusz says:

    I am just thrilled and goose-fleshed awaiting for tomorrow. All the details you provide on how things work there in space and at ESOC are very valuable to understand the mission “behind the scenes”. This is a fantastic thing that you share so much details and that we can learn it. Good luck ESA! I keep my fingers crossed!!!

  • Pat says:

    this is excellent information and i am very excited for tomorrows events. It is fantastic that you can share this with us. Good Luck to everyone involved for a successful day!!
    mian is fearr le haghaidh tuirlingt rathúil

  • Sajid Rabbani says:

    What does it mean when we say that Rosetta’s Constellation is Sagittarius. By chance this is period of Sagittarians going on 🙂

    I wish safe and smooth landing of Philae.

  • nico says:

    How do you know the comet is 4.6 billion years old? I don’t believe that is measured fact. I think it is because some theory says so. What if the comet is let’s say about 10000 years old debris from some cosmic accident. Can you do measurements that gives some estimation about comet’s real age?

    • THOMAS says:

      Nico, there are no possible measurements which can give the age of anything independently of the premises of a particular theory, whether up in space or even down here on Earth, for that matter. You’re absolutely right, the “4.6 billion years” figure is purely an article of faith which is being fiercely defended as an essential pillar of the standard Big Bang theory.

      If, on the other hand, correct scientific method is applied to the findings pouring in from the Rosetta mission and if those findings prove that 67P is NOT the “dirty snowball” vestige from the origins of the Solar System which it is simply theorized to be but, more mundanely (but more spectacularly), a largish lump of stratified rock which is being subjected to increasingly powerful electric discharge events as its elliptical orbit increases its charge imbalance with respect to the Sun, then its age as a “comet” could, as predicted by the competing EU model, be as little as a few tens of thousands (or even thousands) of years.

      In both cases, the “age” is estimated within the framework of a theoretical model. It is simply now a case of determining which model is best supported by Rosetta’s evidence.

  • This mission is amazing. The whole family is waiting on tenterhooks for tomorrow – This is one of the moments that shows humanity at its best, exploring our origins and the origins of our universe. Good luck! Bonne Chance! Viel Glück! And congratulations to everyone at ESA.

  • Brian says:

    Absolutely astonishing that ROMAP can sample and “see” the lander from so far away.
    Great tribute to the many Engineers involved over many years.
    Well done and best wishes/ fingers and toes ever so crossed!!

  • Mike says:

    Am I the only one this reminds of Eurovision?

  • jk says:

    Congratulations on you work!

    A request:
    Most of your text is extremely pale. Tiring to decipher, even this email.
    PLEASE change to a dark colour – good old black would be nice.

    Many thanks.

  • THOMAS says:

    During the break between two live CometLanding webcasts, I’d like to record my personal doubts, during this money-time descent of Philae towards the comet, about its chances of landing safely on the surface and sending back the information it was designed to acquire.

    I was literally moved to tears, as many others on this blog presumably also were, when we got confirmation of the successful separation and the start of Philae’s descent this morning. The engineering achievement of all those who have been involved in the Rosetta/Philae mission over the years is quite simply phenomenal.

    I believe, however, that Philae has just been sent into the jaws of a lion whose existence had not been suspected (or sufficiently accounted for by the theorists) during the planning stages around 15 years ago. Even if Philae’s circuits are not quite simply shorted out as it approaches the electromagnetically active landing-zone, I fear at least that its communications with the Rosetta probe will be severely disturbed (and perhaps lastingly curtailed) by the same electromagnetic phenomena. We know that this sort of thing happens down here on Earth when radio signals are disrupted each time the Sun sends a coronal mass ejection in the direction of the Earth. The risk would seem to be far greater for Philae.

    But I sincerely hope I’m wrong because I’m as keen as anyone for Philae to provide the observations and data which should settle the conflict between competing cosmic models once and for all…

  • Dr. V. Laxmanan says:

    I would be interested in getting a simple table of distance from comet surface (d) versus time (t) since the lander separated from the orbiter. This would permit an estimate of the acceleration g due to gravity near the comet’s surface. Together with data on the orbit periods for Rosetta, we could actually redetermine the product GM and also decompose this product, with M being the mass of the comet and G the universal gravitational constant introduced by Newton (and retained by Einstein).

    • Dr. V. Laxmanan says:


      Here’s how I arrived at this numerical estimate. It is now posted on my Facebook page and is written for the layman and/or elementary physics students. .


      Hey! For the science enthusiasts here. I was able to make a preliminary estimate of the gravitational acceleration near the surface of the comet 67P, where ESA has managed to soft land a probe named Philae.

      For starters, the gravitational acceleration near the surface of the earth is 9.8 meters per second squared or 32.2 feet per second squared. This means that the velocity (or speed, for simplicity) of an object falling freely will increase by 32.2 feet per second, or 9.8 meters per second, in each second of the fall.

      Based on data given in the Wikipedia article on Rosetta spacecraft, it settled into an orbit at a height of about 30 km (30,000 meters) from the surface of the comet. The lander proble Philae seems to have hit the comet surface with a velocity of about 1 meter per second. I don’t expect to find more detailed velocity versus distance data, since it would be quite difficult to measure accurately. (Hopefully, they did have sensors to measure the distance from the comet surface … if so we would have more data.) Even so, an estimate of the gravitational acceleration g is now possible based on just the above information.

      As we learned in our elementary physics classes the formula that gives the gravitational acceleration g is the following, relating the final velocity v = 1 m/s to the distance s = 30,000 m of the free fall.

      v*v = 2 g s since the initial velocity at start of fall is zero, in the direction of the acceleration. The Philae lander has thus experienced an acceleration of g = 3.33 E-05 meters per second squared which is nearly 3.4 million times lower the acceleration experienced by a body falling freely near the earth surface. (3.33E-05 /9.8 = 3.4E-06)

      Bingo! Now if we can refine this estimate with more distance-time or distance-speed data during descent, we would know a lot more

      With the knowledge of g = GM/R^2 we can know go a step further since we know the times taken for Rosetta to orbit the comet. I have not yet been able to get this info but should be available. Here M is the unknown mass of the comet and G is Newton’s Universal Gravitational Constant (also used by Einstein in his theory) and R is the “average” radius of the comet. As we have seen, the comet has a very head shape and looks like two “blobs” stuck together – much like a snowman made by kids – of one big blob and a smaller blob which makes the head of the snowman. The landing site was chosen to be the “head” of the comet in such a configuration. The formula for the period T of the orbit also includes the product GM. Hence, we can determine both G and M independently but we are faced with the challenge of what to use for the “average” radius R – the distance to the surface where Philae landed.

      However, scientific progress is made by first making reasonable approximations and then refining them.

      Now, I turn this over to the ESA team and will post it on their blog. May be they can do the refinements or share the data they have to arrive at a better understanding of both G and M and also g near the comet’s surface.

  • Dr. V. Laxmanan says:

    It looks like the total time that would be taken for Philae’s descent from Rosetta to the comet surface was estimated quite accurately by ESA. – the seven hours of “terror”. Hence, I am curious how the estimate was arrived at. The acceleration due to gravity calculation based on distance traveled (30 km) and velocity at impact (1m/s), which is just an estimate, leads to a huge discrepancy in the time estimate.(a factor of 1.8). Something intriguing going on and would be interesting to gain some more insights.

  • Dr. V. Laxmanan says:

    Thanks for accepting my comments. As noted, I have tried to create interest in this descent problem on my Facebook page. Hopefully, someone at ESA will pay attention and provide more details about the descent as a function of time, along with the information about the time it takes Rosetta to orbit the comet 67P. I have NOT been able to obtain any information about the time it takes to complete one orbit around the comet. Now that the lander is on the surface, it provides a nice reference point and we should be able to determine the orbit times accurately and thus also decompose the product GM from these two types of measurements (orbit and the descent). I know this was NOT the original objective of the mission but would lead to a greater understanding of the basic physics of orbital motion and gravity in a totally “alien” environment, like never before. 🙂

  • Dr. V. Laxmanan says:


    I did NOT follow this mission until a couple of days before the scheduled landing of Philae. So, I have been reading up on several blogs by ESA. Here’s some of the relevant data used to refine the earlier estimate of gravitational acceleration near the comet’s surface. .

    Philae separated from the orbiting Rosetta at a height of 22.5 km at 9:03GMT (time when signal was received on earth).

    The descent was entirely due to gravity, without any propulsion or guidance of any kind.

    Touchdown on the comet surface was confirmed at 16:03 GMT (time when signal was received on earth).

    The velocity (or speed) of Philae was estimated at 1 m/s upon impact with the comet’s surface.

    Hence, we can now refine the estimate of the local value of the gravitational acceleration near the surface of the comet. The relevant equations of motion were given in earlier posts.

    s = (1/2) gt^2 where s is the distance travelled and t is time and t^2 is square of t.

    v = gt where v is the velocity at time t.

    Hence, we can eliminate g and get the following relation for s, v, and t and the gravitational acceleration g.

    2s/v = t = s/(v/2)

    Also, v^2 = 2gs or g = 2s/(v^2)

    The revised value for g = 4.44E-05 meters per second squared (as opposed to 3.33E-05 based on 30 km descent and 5.00E-05 based on 20 km descent reported earlier. Here I have only used the s and v values to determine g. The value of g is 4.56 million times weaker than the value of g = 9.81 meters per second near the the surface of the earth.

    If we use the time of descent to determine g from s = (1/2)gt^2, we get a higher value of g = 7.09E-05 meters per second squared. This is a significant discrepancy and might be due to the “estimate” of v = 1 m/s for the velocity upon impact. Based on the higher value of g from the s-t data, the estimated velocity upon impact with the comet surface is 1.79 m/s instead of the reported 1 m/s.

    Finally, it appears that my earlier comments about determining the individual values of G and M and decomposing the product GM were premature. From the ESA blogs that I have since looked at, it appears that Rosetta is NOT entirely executing orbital motion under the influence of a central gravitational pull. It appears to have thrusters and is being “guided”. The gravitational acceleration g = GM/(R^2) where R is the “average” radius of the comet, M is the mass of the comet and G is the universal constant introduced by Newton (and also retained by Einstein) when he developed the theory of gravitation. The same produce GM also appears in the expression for the period T of an orbiting body. Hence, the two INDEPENDENT measurements of g and T will permit a determination of the separate values of G and M. However, this assumes that Rosetta is executing orbital motion due to a gravitational force. If Rosetta is being “guided” due to other mission considerations, we will not be able to decompose the produce GM, as suggested.

    At this point there are still some questions/concerns about the exact location of Philae since the harpoons that were supposed to anchor Philae to the comet’s surface did NOT deploy during soft landing Nonetheless, the mere landing of Philae is a remarkable achievement. This may be further appreciated by the fact that, like the earth, the comet is also rotating about its own axis. The earth takes 24 hours and this comet takes 12.4 hours (a little over one-half the time for the earth). Hence, during the 7 hour descent, the comet itself was rotating slowly and the chosen landing site was moving away after the probe was released by Rosetta. Considering the very uneven and rough terrain below, any errors would have meant Philae missing the chose site, a relatively flat area, named Agilka, based on earlier images of the comet surface taken from Rosetta.

    Rosetta will continue to orbit for more than a year around the comet, regardless of the fate of Philae. Hence, I do hope we will be able to develop good data for the orbital period T. But, let’s hope the ESA team figures out a way to make the best planned use of Philae in spite of the landing missives.

Comments are closed.