Since its landing on Comet 67P/Churyumov-Gerasimenko on November 12 (2014), Philae, which was able to perform a series of meaurements before exhausting its batteries, has not yet revealed its precise position. Where’s the research? Will the probe, which will soon make a close flyby, be able to locate it? Will Philae wake up? What happens to the measurements it has undertaken before falling asleep? Where is Philae and will we able to locate it precisely?
Since Philae’s landing on November 12, 2014, which is thought to have bounced three times, one of the objectives is to be able to locate Philae in images and thus determine its final location. The CONSERT (1) instrument has been of great help in this respect, enabling the search area to be reduced to an area of 200 by 20 metres located on the small lobe of the comet. A specific search around this area, based on images from the OSIRIS (2) instrument, has not yet made it possible to determine the exact position of Philae.
The descent as well as the first bounce on the Agilkia site were well followed by the OSIRIS small field camera (the NAC2, for Narrow angle camera)( see here). The team also identified what they believe to be the lander in a wide-angle photograph (with the WAC – Wilde angle camera) taken 2 hours later over a large depression in the small lobe of the comet called Hatmehit. The image was used to guide efforts to locate the lander and provided a basis for reconstructing its trajectory. According to data from Philae’s ROMAP instrument (see this ESA article), a first bounce occurred at 16:20 UTC.
In total, therefore, the instruments on board Philae recorded 4 successive contacts, including the final contact at 17h32 UTC. The site of the final landing was named Abydos since Agilkia remains the name of the site initially planned for the landing (that of the first contact). Although the images sent by CIVA (3) (see the example below) give information about the nature of the surrounding ground, it is not possible to derive indications about the precise area where the lander is located, a visual confirmation is still necessary.
The research campaigns conducted by OSIRIS in November and December 2014 from a distance varying between 18 and 28 kilometers from the cometary ground did not locate Philae, even though these campaigns targeted, for their passage over the area, the hours during which Philae is illuminated by the sun. Note that the trajectory initially planned for the orbiter immediately after Philae’s landing could have allowed good visibility conditions if the orbiter had docked properly on the Agilkia area.
The image above is an example of an image used in the search for the lander. For the series taken at an altitude of 18 kilometres, the images were taken in pairs: one image with an orange filter and the other with a blue filter to take advantage of the reflection of Philae’s solar panels, which differs from that of its surroundings. Nevertheless this is not enough, especially since at this distance Philae can only represent 3 pixels on the images. And yet, engineers and researchers have searched by eye for a 3-pixel task, but unfortunately this kind of task is abundant on the images that the comet sends back to OSIRIS notably because of the numerous blocks that cover it.
It should be noted, however, that the probe will fly over at a low altitude of 6 kilometres on 14 February. This flight will first provide images and spectra with a resolution never before achieved during the mission, as well as samples of the inner region of the coma, which will allow scientists to better understand its characteristics and how it develops. Scheduled several months in advance, like all the scientific operations of the mission, this overflight is not dedicated to locating Philae, but will be conducted over the largest lobe of the coma. While we probably won’t get a picture of Philae, this flyby promises some nice perspective images. After this low pass, the probe will again be far away from the comet and will not be able to attempt a new search for the Lander until much later in the mission, perhaps even next year. In any case, knowing the position of the Lander is not required to command it* and therefore to make the desired measurements. On the other hand, knowledge of the site would allow us to plan future experiments.
*Let us note that the CONSERT instrument performs a kind of radiography by analysing the shape of the signal received after it has propagated through the nucleus of the comet, from Philae to the probe and vice versa; as such it is useful to know as much as possible about the position of Philae in order to make a more reliable analysis of the measured signals. But this information can be included a posteriori from the data acquisition.
When will Philae wake up?
Those who had followed Rosetta’s awakening knew that it was not just a matter of pressing a switch so that science and observations would immediately start again. The same goes for Philae.
At the originally planned site (Agilkia), Philae was to receive 6.5 hours of sunshine per cometary day, with temperatures becoming too high for it to function properly by the end of March 2015. But, last November, on the Abydos site where it is finally located, the sunshine is more restricted and the batteries were charging only during 1h20 per cometary day (i.e. 1h20 every 12 hours). Indeed, the inclination of the aircraft and thus of the solar panels, as well as the shadows that are cast on it are not the most favourable.
It is thus finally a little the opposite which is expected, since it will probably be only from the end of March that Philae will receive enough light to allow it to restart. Provided, however, that the systems inside Philae have survived the low temperatures it is currently experiencing; which is quite likely, given that they can withstand temperatures as low as -80°C in principle. It will be able to reboot, but it is unlikely that it will have enough power to transmit with Rosetta yet. This will have to wait until May – or June – 2015 (the minimum power required is 17 watts). It will still need some more time for its batteries to be fully charged and to allow full resumption of scientific activities, but this will give it a chance to finally have a front row seat for perihelion – the closest point to the Sun – when the comet’s activity will be at its peak!
The teams are already preparing the programme of instrumental activities that will follow the awakening.
Even if Philae did not wake up, he was able to perform his first sequence of measurements, and even better, collect information at different points on the comet, thanks to his bounces.
We are therefore eagerly awaiting the images that OSIRIS will take during the February 14 flyby, but also the scientific data already collected by Philae, whose analysis will be delivered soon.
Rosetta is an ESA mission involving several national space agencies, including CNES for France. Philae is the result of a European consortium under the leadership of the German space agency (DLR). The CNRS and several French universities are involved in various capacities and on numerous instruments (4), notably through the participation of their laboratories (5).
(1) 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) .
(2) The OSIRIS instrument: Optical, Spectroscopic, and Infrared Remote Imaging System: is a dual-camera imaging system operating in the spectral range of visible light, near infrared, and near ultraviolet. OSIRIS consists of 2 independent cameras sharing common electronics. The small-angle camera is designed to produce high-resolution images of the comet nucleus. The wide angle camera will produce images of the dust and gases emitted directly from the surface of the nucleus.
The OSIRIS-NAC camera, a high spatial resolution imaging instrument designed and developed by the Marseille Astrophysics Laboratory (CNRS / Aix-Marseille University) in partnership with the ASTRIUM company and several European laboratories.
Comet nucleus Infrared and Visible Analyzer. The CIVA-P instrument is a set of 7 cameras designed to realize the stereoscopic panorama of the landing site, with a millimetric resolution near Philae, and metric on the horizon. CIVA-M is itself a set of two microscopes.
(3) CIVA-M/V is an optical microscope: samples taken by drilling will be successively illuminated in 3 colors from green to red, and the images will allow to identify details of a few micrometers. CIVA-M/I is a hyperspectral infrared microscope, which will image the samples in more than 500 colours scanning the near infrared range: the aim is to characterize the composition of ice, grains and organic molecules present in the cometary material. These analyses are non-destructive, so the samples can then be subjected to further measurements, in particular by mass spectrometry (Ptolemy and COSAC instruments). In total, CIVA should make it possible to highlight the active processes at the surface of the cometary nucleus, and to characterize its major and minor constituents. All its systems have been developed under the responsibility of the IAS, as well as their central electronic control unit.
Jean-Pierre Bibring, professor-researcher at the Institut d’Astrophysique Spatiale (CNRS/Université Paris Sud). CNRS laboratories involved :IAS, LAM The CIVA instrument is composed of three subsystems.
(4) Experiments to which the CNRS laboratories contribute :
- Orbiter (9 instruments out of 11): ALICE, CONSERT, COSIMA, MIDAS, MIRO , OSIRIS , ROSINA , RPC, VIRTIS.
- Lander (5 instruments out of 10): APXS, CIVA, CONSERT, COSAC and SESAME.
(5) CNRS laboratories involved in Rosetta-Philae :
- CRPG, Centre de recherches pétrographiques et géochimiques (CNRS/University of Lorraine)
- CSNSM, Centre de sciences nucléaires et de sciences de la matière (CNRS/Université Paris-Sud)
- GET, Environmental Geosciences Toulouse (CNRS/CNES/IRD/Université Paul Sabatier)
- IAS, Institute of Space Astrophysics (CNRS/Université Paris-Sud)
- ICN, Nice Institute of Chemistry (CNRS/UNS)
- IPAG, Institute of Planetology and Astrophysics of Grenoble (CNRS/University Joseph Fourier)
- IRAP, Institute for Research in Astrophysics and Planetology (CNRS/Université Paul Sabatier)
- LAAS, Laboratory of Systems Analysis and Architecture (CNRS)
- LAM, Marseille Astrophysics Laboratory (CNRS/University of Aix-Marseille)
- LATMOS, Laboratory of Atmospheres, Environments and Space Observations (CNRS/Université Versailles Saint-Quentin-en-Yvelines/ Université Pierre et Marie Curie)
- LERMA, Laboratoire d’étude du rayonnement et de la matière en astrophysique (CNRS/Observatoire de Paris/Université de Cergy-Pontoise/Université Pierre et Marie Curie/ENS)
- LESIA, Laboratory of Space Studies and Instrumentation in Astrophysics (CNRS/Observatoire de Paris/ Université Pierre et Marie Curie/Université Paris Diderot)
- LISA, Inter-University Laboratory of Atmospheric Systems (CNRS/Université Paris-Est Créteil/Université Paris Diderot)
- LPC2E, Laboratory of Physics and Chemistry of the Environment and Space (CNRS/University of Orléans)
- LPP, Plasma Physics Laboratory (CNRS/École Polytechnique/Université Pierre et Marie Curie/Université Paris-Sud)
- UTINAM, Universe, transport, interfaces, nanostructures, atmosphere and environment, molecules (CNRS/Université de Franche-Comté)