How has the CONSERT instrument measured the distance of Philae and its rotation during the descent?

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During the descent of Philae on the comet, the CONSERT instrument has measured the distance between the orbiter and Philae by measuring the signal exchanged between the antennas located on both the orbiter and the lander (see the animation below). The maximum amplitude of the signal, as may be seen below, periodically varied while decreasing with time.

Maximum signal amplitude measured by CONSERT during the descent.
©ESA/ROSETTA/Philae/Consert

The overall decrease in signal intensity is due to the distance from Philae as he progresses towards the Chury. The periodic variations are due to the rotation of Philae on itself. Indeed, the gain of the antennas, the tuning between them, depends on their orientation.
Why was Philae rotating? In order to descend in a stabilized manner, the Philae landing gear is equipped with what is called an inertial wheel located inside the gear. When it is in operation, this wheel turns and by reaction the aircraft rotates in the opposite direction. This continuous rotation of the inertial wheel stabilized the aircraft and maintained the Philae’s attitude throughout the descent.
Philae was rotating at a slower and slower rate. This can be seen in the diagram above: the signal period increased slightly during the descent, which means that the Philae was turning slower and slower. This was not supposed to happen. So scientists are now trying to understand what could have caused this slowdown. If it was internal or related to the environment of Philae that may have changed during the descent.

Additional Resources

The CONSERT instrument: Comet Nucleus Sounding Experiment by Radiowave Transmission . The experiment consists in propagating a radio signal (90 MHz) from the lander landed on the comet, through the cometary nucleus and to receive it on the orbiting probe. Like an X-ray, the propagated signal contains information about the environment it has passed through and will provide knowledge about the physical and electrical properties of the comet’s nucleus, a first and unique experiment on Rosetta. With several observation orbits, it will be possible to image the internal structure as a whole. The detailed analysis of the radio signal that has passed through the comet nucleus will give strong constraints on materials, inhomogeneities and will allow to identify blocks, gaps or voids. With this information we will try to answer some questions about the constitution of comets. The answers to these questions should help to better understand the essential problem of comet formation. Did they form from unprocessed interstellar grains or from condensed grains in the presolar nebula? How did accretion take place? By first forming comets and then by collisions forming kilometric bodies?.

Principal Investigator: Wlodek Kofman, CNRS researcher at the Institute of Planetology and Astrophysics of Grenoble (CNRS/Université Joseph Fournier) .

  • CNRS laboratories involved :
  • IPAG (CNRS/Université Joseph Fourier),
  • LATMOS (CNRS/Université Versailles Saint-Quentin-en-Yvelines/ Université Pierre et Marie Curie),
  • IRAP (CNRS/Université Paul Sabatier).

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