What changes and challenges does an astronaut's body face when he/she comes back to Earth after a long mission at the ISS (International Space Station)?

There are two phases of concern.  The first is the immediate response to entry (EMS) and the second is the prolonged readaptation to the Earth normal environment (ERS).  The intensity of the effects is usually proportional to the amount of time spent in microgravity, but vary from astronaut to astronaut. 

Is the mass of the earth constant? If no, then what about momentum? Why there is no change in earth velocity?

The measured yearly increase in earth's radius together with the gigantism of past life suggest that earth continually gains mass.Very long baseline interferometry (VLBI) gives us a method of measuring any increase in earth's radius.  It involves detecting an astronomical signal at multiple stations around the globe and looking for changes in the delay between one station

Is the weight of Earth decreasing?

Short Answer: NoLong Answer: KindaThe Earth's mass is not decreasing, or at least in any substantial amount.The Earth can be considered a closed system (although imperfect). In nonrelativistic classical mechanics, a closed system is a physical system that doesn't exchange any matter with

What would happen to the Earth if it had a bigger Moon?

The Moon:First off, how was it formed? (how is any moon formed, for that matter)When planets were first created from the left out debris of the star of our solar system, they were hundreds in number. They collided, shattered one another, and the