This invisible substance apparently makes up five-sixths of the universe's matter, and its gravitationalattraction is what's thought to hold galaxies together.
Tide: periodic rising and falling of large bodies of water resulting from the gravitationalattraction of the moon and sun acting on the rotating earth.
The idea is to send a satellite to hover alongside a threatening asteroid and, using the gravitationalattraction between the two objects, to pull the asteroid gradually off course.
The coil is suspended in an external magnetic field and the current running through it adjusted so that the attraction between the two magnets balances the gravitational force on the suspended mass.