Hi.

Part of an article I read about Rosetta space probe states :

 

"...The probe and its lander will keep sending back data until their batteries die or the debris streaming off the comet irreparably damages their sensitive instruments.

The mission is different from NASA's Deep Impact probe that fired a projectile into a comet in 2005 so scientists could study the resulting plume of matter. NASA also managed to land a probe on an asteroid in 2001, but comets are much more volatile places because they constantly release dust and gas that can harm a spacecraft..."

 

What forces push debris off the comet ? What forces push dust towards the landed spacecraft ?

Comets made of frozen water, ice, is constantly heated by photons from the Sun.

The closer to Sun, the more energy comet is absorbing.

When it's far far away from the center of solar system it has no tail.

Tail only appears when it's close to the Sun, and it's always pointing in opposite direction to Sun.

Water on the Earth is also absorbing energy from the Sun, from photons, and suddenly is vaporizing and forming clouds.

But our planet has enough gravitation to keep atmosphere. unlike comet.

When relatively near the Sun, comets have 2 tails. The tail illustrated above -always directed away from the Sun- is the ion tail and is composed mostly of gasses. The other tail which is not illustrated is a dust particle tail and trails more or less behind (points away from) the comet and not directly away from the Sun..

 

Besides dust stirred onto the lander by solar forces, I suspect it might accumulate dust through electrostatic forces.

We see shooting stars linked with comets, so they also release some coarse dust, which Rosetta's operators suspect can harm a probe. After some time, dust's speed must differ from the main body, due to Sunlight pressure, hence have a speed relative to the probe.

 

Ice sublimation by Solar heat releases vapour, and lost ice releases dust... Maybe UV light can evaporate water molecules directly, without heating ice first.

 

All these objects must be rather loose aggregates, especially the smaller ones. They're supposed to re-arrange from time to time, and this must release more gas and dust. Holmes 17P may be one such case.