At the end of the film,
Iron Man grabs and steers the nuclear missile through the open portal. At the time just before the suit malfunctions due to atmosphere/cold, his thrusters are firing him towards the Chitauri ship.
Objects in space continue moving in the same direction unless new force is applied. Wouldn't he have just kept traveling towards the Chitauri ship, or does Earth's gravity somehow exert itself through the portal?