Rosetta’s Philae Lander Exposed Primitive Water Ice During Its Bouncy Landing

Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; Data: ESA/Rosetta/Philae/ROMAP; Analysis: O’Rourke et al. (2020).

In 2004, the European Space Agency (ESA) launched the Rosetta probe, accompanied by the Philae lander, to comet 67P/Churyumov-Gerasimenko. Almost 11 years after launch from Earth, Philae separated from Rosetta to land on the comet’s surface. However, upon descent, its anchoring harpoon failed to deploy. This resulted in Philae bouncing twice before landing under an overhanging cliff. Surface material properties, such as porosity, thermal inertia, and compressive strength, estimated from the first and third landings have been analyzed previously, providing insights into the thermal and mechanical properties of cometary nuclei. A recent study led by Laurence O’Rourke from ESA’s European Space Astronomy Centre reproduced Philae’s landing trajectory and analyzed the data gathered from the second landing (known as touchdown 2, or TD2), including images and spectra of the site taken by Rosetta. The impact of Philae penetrated and exposed the interior of an icy boulder, perhaps not exposed since the comet’s formation. Spectrophotometric analysis of data from TD2 showed a mixture of approximately 46 percent water ice (by volume), plus dark, organic-rich material. Based on the depth of the impression Philae made in this dust-ice mixture, it was determined that this material has low compressive strength, weaker than freshly fallen light snow. The dust-ice ratio and estimated porosity of 75 percent are also in agreement with radar measurements from Rosetta for the comet nucleus as a whole. Results from this study will lead to a better understanding of the nature of cometary boulders and provide both scientific and strategic information that will aid future landers searching for volatile-rich regions on comets and sample-return opportunities. READ MORE