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Back in 2005, Hayabusa carried a mini-rover 'MINERVA'. MINERVA was a Solar Powered entity designed to hop around on Itokawa, and capture images. Unfortunately (perhaps due to the time-lag) MINERVA was lost at deployment.

Rosetta bears Philae. Compared to MINERVA's miniscule 591 grams, Philae masses around 100 Kilograms. It is loaded with oodles of instruments - with a mass of over 25 Kilograms.

Yet, like MINERVA, Philae too is apparently to be deployed on a ballistic trajectory by it's parent craft towards the target body; in this case the comet Chury.

  • What lessons were learnt from the loss of Hayabusa's MINERVA?
  • Are any precautions in place to prevent loss of the Rosetta Lander the way Hayabusa's MINERVA was?
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Minerva was fired towards the asteroid by Hayabusa, but it did not have its own propulsion system. The problem was that Minerva was released at the wrong moment:

While awaiting the ground command to deploy MINERVA, the Hayabusa mothership was under autopilot control to maintain a set distance above the surface. As it drifted up or down to a range limit, Hayabusa fired small gas thrusters to stay within the designated interval.

According to Kawaguchi, the deployment command happened to arrive at Itokawa during a period when Hayabusa was drifting away from the surface. Since the escape velocity associated with the asteroid's faint gravitational pull is so small — about one-half of a mile per hour (20 centimeters per second) — MINERVA was sent on a flight path that took it away from the asteroid.

Philae, on the other hand, has its own propulsion system. The Active Descent System is a cold-gas thruster. This can be used during the descent, and it is used after landing to make sure that Philae doesn't bounce off. The lander also has shock absorbers on its landing legs. During the landing, threaded spikes are extended from the landing legs to screw the legs to the surface. After the landing, a harpoon is fired into the ground, again to hold the lander down. Philae contains cameras that will start imaging the landing site before touchdown, so ESA can compare before and after and detect any damage to the landing area.

Video explanation using a Lego model of Philae.

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  • $\begingroup$ Wonder whether that harpoon may have the ability (given that the comet will be out-gassing) to damage the landing area $\endgroup$
    – Everyone
    Dec 25, 2013 at 13:47
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    $\begingroup$ @Everyone: there is going to be an impact (landing legs with spikes, harpoon), so damage is a given. ESA is going to take before and after images so they can detect the damage. $\endgroup$
    – Hobbes
    Jan 20, 2014 at 13:07

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