Developed and planned over many years, the pioneering Rosetta mission is one of the most challenging ever undertaken in the history of space exploration. In May 2003, however, engineers were presented with a new challenge when ESA's Science Programme Committee announced that comet 67P/Churyumov-Gerasimenko would replace comet 46P/Wirtanen as Rosetta's objective.

Not only were the two comets following different orbits and timetables, but the team from ESA, industry and academia would have to prepare the Rosetta lander for a hazardous descent onto a much larger ice world than originally anticipated.

Rising to the challenge, the team began to study the implications of exploring Churyumov-Gerasimenko and the modifications that might be required to the fragile lander. After months of intensive studies and simulations, engineers are now confident that everything possible has been done to ensure that the spacecraft will successfully complete history's first soft touchdown on a cosmic iceberg.

"Comet Churyumov-Gerasimenko is a much bigger comet than Wirtanen," said Philippe Kletzkine, ESA manager for the Rosetta lander. "It is about four times the diameter and its gravity could be at least 30 times greater. This means that the landing speed will increase from 0.2 - 0.5 metres per second to 0.7 - 1.5 metres per second.

"In the case of Wirtanen, our biggest problem was avoiding a rebound - the spacecraft only had to bounce slightly and the momentum would overcome the weak gravitational hold of the comet.

"Now, we also have to worry about absorbing the shock from a faster landing and the stability of the lander upon touchdown. In the worst case scenario of a 'hard' comet surface, rough terrain and relatively high gravity, it was possible that the lander could topple over. In order to prevent this we decided to modify the landing gear."

The design team wanted to avoid removing the landing gear or the entire lander from the Rosetta orbiter, which is currently at the launch centre in Kourou, French Guiana. They also wanted something, small, light and easy to fit. The answer was a small bracket, known as a tilt limiter, that could be attached to the bottom of the lander.

"By restricting the angle at which the landing gear can flex on touchdown to only 3

- 5 degrees, we improve the damping effect on touchdown and reduce the possibility of a rebound," explained Jean-Christophe Salvignol, Rosetta lander mechanical engineer.

"The limiter was designed by Astrium GmbH in collaboration with ourselves and the Max-Planck-Institute in Lindau. During pendulum tests with a model of the landing gear, we simulated landing on a wall at different angles of approach, and verified that the spacecraft could successfully touch down at speeds of up to 1.5 metres per second on a 10 degree slope, or up to 1.2 metres per second on a 30 degree slope.

"In parallel, computerised simulations of landings were run by the Max-Planck-Institute to better determine the landing performances for various surface characteristics, impact velocities and lander attitudes."

The tilt limiter was finally delivered to Kourou and mounted on the spacecraft landing gear on 30 September.

"This excellent collaboration between ESA, industry and MPAe has enabled us to adapt to the new mission very quickly and efficiently," said Salvignol.

No major changes are envisaged for the lander's descent profile. However, under the new mission scenario, there will be more time available for the orbiter's instruments to map the nucleus in detail and find a safe haven for the 100 kg lander.

The historic touchdown on the pristine surface of comet Churyumov-Gerasimenko is expected to take place in November 2014. "We anticipate a landing on the 'summer' side of the nucleus, where there is maximum illumination," said Philippe Kletzkine.

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