The European Space Agency made history yesterday by successfully landing Philae to Comet 67P. Philae was carried by the Roseta spacecraft that traveled for more than 10 years until it reached its destination. Philae successfully landed on the surface of the comet, but, according to the latest information available, it has not been able to attach itself to the comet and is running a risk of detaching from the comet’s surface. It still transmits truly unique data from the surface of the comet.
The scientific focus of the Mission is on "elemental, isotopic, molecular and mineralogical composition of the cometary material, the characterization of physical properties of the surface and subsurface material, the large-scale structure and the magnetic and plasma environment of the nucleus." (Bibring et al. 2007). This findings should be of tremendous interest to all geoscientists and geotechs!
Philae weighs 100 kgr and is made of carbon fiber. Information about Philae can be found in this factsheet: http://www.dlr.de/dlr/Portaldata/1/Resources/documents/Philae_Lander_FactSheets.pdf Because the cometary gravity is very weak, anchoring is needed so that the lander does not bounce back into space. The anchoring system itself weighs 1.4 kgr. According to the blog of Dr. Böhnhardt, the lander’s lead scientist (https://www.mps.mpg.de/3086295/Philae-Blog ), and a nice summary presented in Wikipedia, Philae would touch down at a velocity of around 1 m/sec (3.6 km/h; 2.2 mph) and the legs are designed to dampen the initial impact and avoid bouncing. The impact energy would drive ice screws in the surface and within seconds Philae would also fire two harpoons into the surface. The harpoons would be ejected at a 70 m/s (250 km/h; 160 mph!) and it was estimated that they would reach a 2.5 m maximum depth. The depth of anchoring is of course site conditions dependent. A downward thruster would keep Philae on the ground to reduce the recoil from harpoon firing.
According to Biele and Ulamec (2013) “The surface strength of comets is still not well constrained but believed to be in the 1 kPa – 100 kPa range. Philae has been designed for compressive strengths between 2 kPa and 2 MPa. For a compressive strength less than 2kPa, Philae’s baseplate would touch the ground (but then effectively stopping further penetration) and the 360° rotation capability of the landing gear would be compromised. Still, all experiments could be performed. Only for compressive strengths < 100Pa (equivalent to tensile strengths of less than 5 . . . 10Pa) the mission objectives would be compromised. For compressive strengths > 2 MPa (solid ice), the harpoons may not anchor safely.”
It is not clear what went wrong with the harpoons. ESA did announce that analysis of telemetry indicated that the landing was softer than expected, but that the harpoons had not fired upon landing. The downward thruster did not either. More information is expected.
This is still a tremendous success and truly valuable data is reportedly being collected. We can’t wait. According to the original plan, the lander will also drill in the comet and collect samples, but, as far as we can tell, the anchoring was necessary for drilling to take place.
Geoengineer.org will stay tuned for more information, and we will keep you posted!
Biele, Jens; Ulamec, Stephan (2013). "Preparing for Landing on a Comet - The Rosetta Lander Philae". 44th Lunar and Planetary Science Conference. 18–22 March 2013. The Woodlands, Texas.
Bibring, J.-P.; Rosenbauer, H.; Boehnhardt, H.; Ulamec, S.; Biele, J. et al. (February 2007). "The Rosetta Lander ("Philae") Investigations". Space Science Reviews 128 (1-4): 205–220. Bibcode:2007SSRv..128..205B. doi:10.1007/s11214-006-9138-2.
Wikipedia listing about Philae lander:http://en.wikipedia.org/wiki/Philae
European Space Agency website: http://www.esa.int
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