They feel themselves immobile, but they think that's impossible, they must have some speed, and they want to know it. One day, they pass by the earth, and they decide to send a shuttle that will stay near the earth during 2 years, and will thereafter resume the journey at the same speed as the vessel. One of the systems will age less than the other, and by exchanging signals, they hope to find out which one, and so determine their speed. But they fail. Whe have the earth E, the vessel V and the shuttle S. 1.
E is motionless.
V sends S toward E with speed 0. S's time flows normaly, V's is still slowed. V has aged 1 year, S 2 years. Its time is slowed again. At that moment V is 1 year old.