11.   Astronomers theorize that these isolated stars were displaced from their home galaxies as a result of galactic mergers or tidal forces from nearby galaxies.

12.   Besides cameras, the spacecraft would carry a laser altimeter to measure the changing shape of Europa as its surface bulges and subsides in response to powerful tidal forces.

13.   However, the ice is riddled with cracks caused by tidal forces, he says.

14.   Nor is it subject to the same flexing tidal forces that heat up the closer moons.

15.   Other planetary scientists agreed, but noted that the relative absence of cratering on Europa indicated that the strong ocean tidal forces must have existed in the recent past.

16.   Scientists reasoned that the strong tidal forces of Jupiter caused regular expansions and contractions of Europa.

17.   That comet was torn apart by tidal forces after coming too close to Jupiter on a prior orbit.

18.   The difference in force is called the tidal force and is like that in the ocean, except in more extreme form, Begelman said.

19.   The heat responsible for the eruptions could be the result of tidal forces, the gravitational tug-of-war that goes on between giant Jupiter and its four major moons.

20.   The outermost Callisto, almost as large as Ganymede, appears to exist in a deep freeze away from the gravitational tidal forces.