I am not convinced that when a particle enters the gravity field of a massive body (which may extend for millions of miles), that massive body is affected by the gravity field of that particle (which may extend less than millimeter).

Well, I would say, get used to it. It is **Newton’s third law**:

The mutual forces of action and reaction between two bodies are equal, opposite and collinear.

Let’s do the calculation. You throw a weight of 1 kg with a velocity of 12 km/s away (that is you throw it away that hard that it leaves the earth’s gravitational field ).

That means you gave it a momentum of **p** = m.**v** = 1 kg x 12 km/s = 12 km.kg/s

The earth’s mass is 6×10^24 kg (a 6 with 24 zeroes).

Now the earth will get a velocity of your throw of **v** = **p**/m = 12/(6 x 10^24) = 2 x 10^-24 km/s in the opposite direction.

That is 3600 s/h x 2 x 10^-24 km/s = 7.2 x 10^-21 km/h: 0.000000000000000000006 km/h. Do you think somebody notices?

Of course, a 1 kg meteorite flying by at millions of kilometers away will have less impact, but it will cause a tiny change. A photon even less. But if you want, you can try to calculate now for yourself.

Edit: typo