Humanity's first soft landing on a comet bounced back into space
In November 2014, the European Space Agency's Rosetta probe released the Philae lander toward comet 67P/Churyumov–Gerasimenko for history's first cometary touchdown. Because the comet's gravity is microscopic, Philae relied on thrusters and ice harpoons to secure its footing. Both failed upon impact, sending the lander rebounding roughly a kilometer back into space. Philae drifted for nearly two hours before settling unevenly in the shadowed crevice of an icy cliff.
The Challenge of Landing in Microgravity
When the European Space Agency launched the Rosetta mission in 2004, its ultimate objective was something no space agency had ever attempted: to enter orbit around a comet and deliver a robotic lander directly onto its surface. Comets are primitive remnants from the formation of the Solar System, composed of volatile ice, rock, and dust. Because they preserve primordial chemical compounds relatively undisturbed by billions of years of solar heating, studying their surfaces up close offers unprecedented insight into the early building blocks of planets and the origin of water on Earth.
Delivering a spacecraft onto a comet presents an entirely different physics problem than landing on the Moon or Mars. Comet 67P/Churyumov–Gerasimenko is a small, irregular body whose gravity is negligible compared to Earth. With an escape velocity of roughly one meter per second—a brisk walking pace—any standard impact would cause a lander to bounce straight off the surface and float back out into interplanetary space. A traditional touchdown was out of the question; the landing was fundamentally an exercise in docking with an uncharted mountain tumbling through the vacuum.
The Triple-Lock Anchoring System
To keep the small, washing-machine-sized Philae lander from rebounding into the void, European engineers designed an elaborate multi-stage anchoring system. The three-legged landing gear was built with internal dampers designed to absorb the mechanical energy of the initial impact. In addition, footpads were equipped with rotating ice screws meant to bite into the surface material as the legs settled.