Philae (probe)

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Illustration by Philae
1: 1 model by Philae in the exhibition Outer Space - Fascination of Space at the Bundeskunsthalle
1: 1 model by Philae in the exhibition Outer Space - Fascination of Space at the Bundeskunsthalle
10-minute video report from DLR about the Philae mission (1080p HD )

Philae is the first spacecraft to land softly on a comet . The lander of the German Aerospace Center was on its way to comet 67P / Tschurjumow-Gerassimenko with the Rosetta space probe since March 2, 2004 and landed on it on November 12, 2014. 67P / Tschurjumow-Gerassimenko is the first comet and the seventh celestial body in total that has been or is being explored by a landed space probe.

Naming

Like Rosetta itself, her country is named after a place in Egypt that played an important role in the deciphering of hieroglyphics . Philae was an island on the Nile near Aswan , which is flooded today by the Aswan dam . In their temple in were ancient Egyptian goddess Isis , the Nile and the sun revered as a living donor. An obelisk found there , in which the king names Ptolemy and Cleopatra are chiseled in two languages, helped identify ancient Egyptian characters , similar to the stone from Rosetta , which was also from the Ptolemaic period and was inscribed in three languages .

Mission objectives and organization

Philae was designed to study the material from 67P / Churyumov-Gerasimenko, who at the time of landing was between the orbits of Mars and Jupiter some 510 million kilometers or 3.4 astronomical units from Earth . The comet has a diameter of almost four kilometers. The results should contribute to the understanding of the formation of the solar system . Among other things, the question is whether the earth got its water and first biomolecules from comets.

The MUSC space user center of the German Aerospace Center ( DLR ) in Cologne was responsible for the operational part of the Philae mission . Philae cost about 200 million euros, about one sixth of the cost of the entire Rosetta mission.

The countries

The comet lander Philae measures approximately 1 m × 1 m × 1 m and weighs 100 kg. The landing gear has three legs, each with a pair of feet with an ice screw and seismometer . The body has two anchoring harpoons and a nozzle for pressure thrust for positioning, solar cells, antennas, seven panorama cameras and a floor camera, magnetometer, spikes for analyzing the soil structure, drills for taking samples and an alpha X-ray spectrometer.

Landing gear

The landing gear is tripod and gimbal , so it has two degrees of freedom . A special mechanism between the landing gear and the probe serves as a damping element for the touchdown maneuver during landing. It also allows the position of the lander to be changed by tilting and rotating it and houses the electrical connections between the probe and the sensors in the feet. The cardanic element performs additional damping functions by slowing down the buckling movements in the element. The fixation on the ground should be ensured by three ice screws, one each on a landing gear base, and two harpoons .

Instruments

Philaes instruments

There are ten instruments on board the lander, which make up around 26.7 kg of its 100 kg total mass:

  • APXS (Alpha Proton X-ray Spectrometer). Consists of a movable sensor head with a diameter of 52 mm. APXS analyzes the chemical composition of the comet's surface below the lander. The instrument is an improved version of the Mars Pathfinder APXS .
  • COSAC (COmetary SAmpling and Composition). COSAC will take soil samples and record volatile components.
  • PTOLEMY will use a combination of gas chromatograph and ion trap mass spectrometer to determine the chemical composition of the cometary material, in particular the isotope distribution.
  • CIVA (Comet Nucleus Infrared and Visible Analyzer). CIVA consists of a stereoscopic panorama camera system, a microscope in the visible spectral range and an imaging infrared spectrometer for the soil samples drilled by the Sample Drill and Distribution (SD2).
  • ROLIS (Rosetta Lander Imaging System)
  • CONSERT (COmet Nucleus Sounding Experiment by Radiowave Transmission). With the help of the Rosetta probe, CONSERT will measure electromagnetic wave propagation through the core in order to draw conclusions about the internal structure and composition.
  • MUPUS (MUlti-PUrpose Sensors for Surface and Sub-Surface Science). MUPUS consists of several sensors: a temperature and thermal conductivity sensor, which is driven 30 cm into the comet's floor (PEN), and an infrared sensor (thermal mapper), which measures the temperature of the top crust. In addition, accelerometers for testing the strength of the comet's floor and temperature sensors are built into the two anchor harpoons; When it is shot into the ground, the strength of the soil is determined from the acceleration curve; later the temperature sensor determines the temperature curve in deeper layers. MUPUS will also follow the temperature balance of the coment's nucleus as a long-term experiment.
  • ROMAP (Rosetta Lander Magnetometer and Plasma Monitor). ROMAP consists of a fluxgate magnetometer , an electrostatic analyzer and a Faraday beaker . The fluxgate magnetometer has a measuring range of ± 2000 nT with a resolution of 20 pT and a frequency range from 0 to 32 Hz. The plasma monitor can measure ions with energies of up to 8000 keV and electrons with energies of up to 4200 keV become. An additional Penning vacuum meter and a Pirani vacuum meter can also measure pressures between 10 −8 and 10 mbar. The magnetometer was developed under the aegis of the Institute for Geophysics and Extraterrestrial Physics (IGEP) at the TU Braunschweig in cooperation with the Institute for Space Research of the Austrian Academy of Sciences in Graz and the Max Planck Institute for Extraterrestrial Physics. The KFKI Budapest, the Max Planck Institute for Solar System Research and the IKI Moscow were involved in the plasma monitor.
  • SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment). SESAME consists of three parts: CASSE (Cometary Acoustic Surface Sounding Experiment) determines the structure of the material by analyzing the sound propagation in the cometary floor . DIM (Dust Impact Monitor) measures the flow of dust in the vicinity of the lander in three dimensions. PP (Permittivity Probe) electrically probes the substrate.
  • SD2 (Sampling, drilling and distribution subsystem). SD2 is the drill that provides samples for COSAC, CIVA and PTOLEMY from a depth of up to 20 cm. Strength parameters can be determined from drilling performance and advance speed, while the vibrations caused by drilling (such as hammering in the MUPUS-PEN) represent a sound source for the SESAME-CASSE experiment.

Course of the mission

Rosetta team in the European space control center ESOC in Darmstadt in joy: probe with Philae awakens from "hibernation" of the outward journey (January 20, 2014)

Selection of the landing site

Philaes landing site Agilkia seen by different imagers (NAVCAM, OSIRIS, ROLIS).

Before Philae was dispatched, the surface of the comet was optically mapped and analyzed by Rosetta, as its exact nature was not known prior to the arrival of the probe. After the separation, Philae should approach the comet at a speed of about 1 m / s in free fall and land on the comet with the help of his landing gear, if possible without bouncing off again due to the low gravity . The gravitational acceleration of the comet on its surface was  estimated at 10 −3 m / s².

In the middle of September 2014 the landing site “J” on the comet was chosen, which is located in a varied, not too rugged landscape with good lighting from the sun and hardly any steep slopes. In mid-October 2014, a public competition for the designation of the final landing site was held, for which proposals could be submitted online. As a result of this competition, the landing site was named " Agilkia " on November 4, 2014 , based on the Nile island of the same name, to which the temple complex on Philae Island was relocated before the Nasser Reservoir was created.

landing

Landing of Philae on Churyumov-Gerasimenko (artist 's impression )
Place of first touchdown

The lander was separated on November 12, 2014 at 8:35 a.m. UTC at a distance of 22.5 km from the comet's core. After about seven hours in free fall and a rate of descent of about 1 m / s, the probe touched down on the comet for the first time. The landing was followed by the main control center of ESA in Darmstadt and the lander control center of the German Aerospace Center in Cologne. An initial confirmation of the landing was received at 5:03 p.m. CET, with the signal taking around 28 minutes to cover the distance of around 500 million kilometers from Earth.

Shortly after the landing, however, there were signs that it was not completely smooth: the lander was probably not able to anchor correctly on the comet's surface. Initially it was reported that the mooring harpoons might not have triggered - but it could also mean that he was "stuck in soft material". In addition, there were irregularities in the radio connection. The first time window for possible corrections was tight, however, as after another two hours the lander was in radio shadow with regard to the mother probe until the next morning and therefore no communication with the control center could take place.

On the morning after landing, contact with Philae could be made again between 07:01 and 10:58 CET in the ESA control center . After evaluating the telemetry data, it seems to be certain that the harpoon anchoring did not trigger on the first contact with the ground at 15:34:06 UTC. The ice screws on the feet could not cause a fixation without counterpressure from the defective gas nozzle and because of the floating harpoons in view of the low gravity. Philae bounced back from the comet at a slower speed of about 38 cm / s.

After the first contact with the ground, the gyroscopes used for attitude stabilization were shut down, which caused the lander to rotate on its own during the second jump. The probe only returned to the surface at 17:25 UTC after almost two hours and a jump vertex of almost 1000 meters. After the second contact with the ground, it cushioned again and continued to fly at a remaining speed of around 3 cm / s, reaching a vertex height of around 20 meters, before finally coming to a standstill at 17:32 UTC, around one kilometer from where the first touchdown. The respective flight times of the two jumps could be determined from measurements of the magnetic field.

The exact position of Philae on the comet was not known until September 2016. The lander only touches the ground with two of its three legs and seems to have come to rest in the shadow of a towering wall. At this point, the solar radiation is only 1.5 hours per 13 hours of cometary days, instead of the six hours as planned. This was very disadvantageous for the energy supply of the lander and meant that part of the mission could not be carried out or not fully.

The ricochet made it possible to draw conclusions about the nature of the ground at the first landing site called Agilkia. Agilkia is covered by a 20 centimeter thick layer of dust with the strength of fresh snow. The final landing site was a lot tougher.

power supply

The probe is equipped with a non-rechargeable primary battery and a rechargeable secondary battery powered by solar cells .

The minimum life expectancy of the probe was specified by the ESA in advance as 64 hours; it was hoped, however, that the probe could operate for several weeks. Since the probe apparently remained in the shadow of a rising wall, the probe could only be operated for a few days with the primary battery. To improve the low light yield, the probe was rotated 35 degrees so that a larger solar cell can make better use of the solar radiation.

On November 15, 2014 at around 1:28 a.m. CET, i.e. after 2 days, 7 hours and 56 minutes, the lander switched off all instruments due to insufficient operating voltage and went into standby mode . All instrument data that had been collected during the last data session of the primary mission could still be transferred.

From this point on, the daily energy gain from Philaes solar cells was insufficient to heat and charge the rechargeable secondary battery to operating temperature.

It appeared possible that the lander would load and boot again closer to the sun and / or in a different configuration on the comet's active surface . An attempt by Rosetta to contact you on March 12, 2015 failed.

Measurements

Philaes ice screws (illustration)

After landing on the comet's surface, Philae was supposed to take various physico-chemical measurements. Among other things, the question of the origin of terrestrial water should be examined for its isotopic composition using a mass spectrometer . As regards the question of the origin of life , the comet's ice should be examined for organic compounds such as amino acids . Chiral compounds are separated into their enantiomers so that they can be related to the homochirality of the terrestrial biomolecules.

By November 14, 2014, 80 percent of the scientific goals had already been achieved, with the COSAC experiment, which is still running, 90 percent would be achieved. The measured data was successfully transmitted to earth.

On November 17, 2014, DLR announced the first results of the Philae measurements. The ground beneath the probe is very hard and probably consists of water ice covered by a layer of dust about 10 to 20 cm thick. Philae's hammer couldn't do anything on the surface. The COSAC instrument detected the presence of organic molecules in the comet's atmosphere. What kind of organic molecules these are, or how complex, could not yet be said.

Using the CONSERT (Comet Nucleus Sounding Experiment by Radio wave Transmission) instrument on board the Rosetta probe, the position of Philae could be located approximately on the edge of the Hatmehit crater . “The area where Philae is believed to be 16 meters wide and 160 meters long. In the photos you can see some bright spots in the shadow that were not there when the pictures were taken before landing. However, it is unclear whether it is the robot. "

Radio contact 2015

After almost seven months of standby operation, data from Philae arrived again for the first time on June 13, 2015 at 10:28 p.m. in the ESA control center. The lander sent 300 data packets to Earth via Rosetta for 85 seconds  ; more than 8000 more data packets are still in Philae's mass storage device. The operating temperature was −35 ° C, the electrical output of the solar cells was 24 watts.

After another status report on June 14th and a change in the trajectory of the mother probe, which carries out the data transfer to Earth, 185 packets with current data were transmitted to Earth in a 19-minute connection on June 19. The operating temperature was 0 ° C. The last of eight short radio contacts came about on July 9th. After that, the distance became too great as Rosetta had to retreat from the dust-blowing comet - the closest approach to the sun was in August. Analyzes of the contacts and the geometrically possible, but not materialized contacts indicate that two of the three Philae transmitters are defective and that the third does not work reliably either. The last possibility of contact was in January 2016, then again closer to Rosetta, before it got too cold for Philae. On February 12, 2016, DLR announced that the probability of a radio signal from Philae was approaching zero. The contact attempts were terminated on July 27, 2016.

Rediscovery in 2016

On September 5, 2016, ESA announced that Philae had been found, as one of the previously suspected objects that could have been Philae, a few meters from the location predicted on the basis of the CONSERT data. The prediction had been improved over the previous year along with the model of the comet. Two of the three legs and some of Philae's instruments can be seen in the image, which had already been taken three days earlier with the OSIRIS camera from a distance of only 2.7 km. The robot lies in the shadow of a ledge, which explains the problems with the solar cells. Discovering the exact location of Philae was important in order to be able to classify some of the scientific experiments of the first few days. On September 30, 2016, Rosetta was targeted to the surface of the comet and destroyed in the process, ending the mission. Since then there is no longer any possibility of contacting Philae.

Philae's final location in the shadow of a rock
Philaes landing site Agilkia seen by different imagers (NAVCAM, OSIRIS, ROLIS)

See also

Web links

Commons : Philae  - collection of images, videos and audio files

Videos

Individual evidence

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  24. ↑ Touchdown confirmed for Philae at 17:03 CET. In: ESA . November 12, 2014, accessed on November 12, 2014 (English, ESA landing confirmation for Philae).
  25. ↑ Funk keeps tearing down. In: Focus Online . November 12, 2014, accessed November 12, 2014 .
  26. ↑ Interview - Deutschlandfunk: Information in the evening. Around 6:30 p.m. - (currently not to be found in the local media library).
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  29. ↑ 'I'm feeling a bit tired, I might take a nap': Philae comet lander goes into hibernation mode after batteries fail - but controllers say mission was 'total success' and craft sent back all of its scientific data from surface. In: Daily Mail Online. November 14, 2014, accessed November 21, 2014 .
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  35. ↑ Successful comet landing with obstacles. ( Memento of the original from November 24, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. On: WeltDerPhysik.de. Retrieved November 13, 2014. @1@ 2Template: Webachiv / IABot / www.weltderphysik.de
  36. ^ Turning Philae. On: Blogs.esa. Retrieved November 15, 2014.
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  38. ↑ Rosetta's mating call faded: No news from Philae yet. On: heise.de. March 12, 2015, accessed March 12, 2015.
  39. ↑ Uwe Meierhenrich : Amino acids and the asymmetry of life. Springer-Verlag, 2008. ISBN 978-3-540-76885-2 .
  40. ↑ Philae has fallen silent - All scientific data transferred. NZZ, November 15, 2014 - as well as the print version of the NZZ from November 15, page 26: “Philaes batteries lame” with the 80/90 percent, “regardless of whether the batteries charge or not”.
  41. ↑ Churyumov-Gerasimenko: Hard layers of ice and organic molecules. DLR, November 17, 2014.
  42. ↑ Comet landing: Organic molecules detected by Philae. BBC News, Nov. 18, 2014.
  43. ↑ Alexander Mäder: Philae. The laboratory on the comet is back on track. In: Stuttgarter-Zeitung.de. June 15, 2015, accessed June 30, 2015.
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  46. ^ ESA: Rosetta and Philae: One year since landing on a comet. November 12, 2015.
  47. ^ Comet probe Rosetta: DLR sends wake-up call to the Philae landing robot. heise online, accessed on January 9, 2016 .
  48. ^ Comet probe Rosetta: Lander Philae remains silent. heise online, accessed on January 11, 2016 .
  49. ↑ A farewell in installments: Time to say goodbye, Philae! At: dlr.de.
  50. ↑ Take care Philae: DLR says goodbye to Rosetta's comet lander. heise online, accessed on February 13, 2016 .
  51. ↑ Final radio silence: Bye, "Philae"! Spiegel Online, accessed July 28, 2016 .
  52. ↑ Philae found! In: blogs.esa.int. Retrieved September 5, 2016 .
  53. ↑ Comet Tschuri: Researchers discover "Philae" on photo In: Spiegel Online from September 5, 2016