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Comment by martijn_himself | Hacker News Reader

I associate 'bouncing' with a certain amount of elasticity. If the comet's surface is much harder than expected, I would expect the lander to be smashed to bits.

Where is the flaw in my thinking? Is this to do with it being very small forces exerted over a long time period?

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The gravity and forces we're talking about are tiny. The craft hit the ground moving slower than 1 m/s and then bounced up over 1km before landing again. The forces involved with something moving that slowly and stopping are minuscule.

This is fascinating. I imagine they probably ran simulations with different margins and as a few bounces would have been one of the expected outcomes i.e. it would have been factored into the design of the craft?

The craft had a small thruster on its top side to gently push it into the surface of the comet, but this seems to have been faulty and couldn't be primed prior to Philae's separation. It sounds like the sort of equipment that might have mitigated such a bounce!

In the end, the speed at which Philae left the comet after the initial bounce was not far off reaching the escape velocity of the comet!

The thrusters and harpoons failed, and I don't think the ice screws have really had a chance.

Think about dropping a golf ball onto concrete vs. sand. The harder surface will result in less loss of energy due to deformation. Thus, the ball bounces higher from the hard surface.

You're missing speeds and acceleration. Comet 67P is a very, very tiny world, with barely the slightest hint of gravity compared to Earth's surface. Consider that an ordinary walking speed is about 1 m/s. That's the escape velocity on 67P. A human could literally jump off the surface under their own power, or they could run into a low orbit (low altitude orbit velocities are 0.7x escape velocities, in general).

The speeds and accelerations that we're used to in the context of bouncing from the surface within the context of Earth are about ten thousand times greater than on comet 67P. The amount of force generated by Philae's collision with the comet's surface was very tiny, but it only takes a very tiny force to throw things long distances across the surface of the comet, due to the extremely low gravity (about 1mm/s^2).

Philae hit the comet at around 0.5m/s. 0.5m/s is a very small velocity. The probe is definitely much elastic than the comet, for start, the legs were designed to be elastic.

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