Gravity Tethers, while not directly explained in the Dead Space universe, appear to be coherent beams of energy which can lock onto objects and hold them into place, or bring them closer to the ship that is emitting them. They are similar in concept to a tractor beam.
The USG Ishimura was equipped with several gravity tether emitters, both for bringing ore from planet cracks into its smelting bays, and guiding ships into its docking bays. Common procedure also seems to be outfitting the colony itself with a massive array of gravity tethers along the colony's Megavents which aid in planetary extraction.
In the final chapters when Isaac brings the Marker back down to Aegis 7, the Marker releases a shockwave that disables the gravity tethers holding the massive chunk of planetary rock that the Ishimura had extracted; causing it to plummet down the planet. It was never stated why the Marker disables the gravity tethers. The Ishimura's tethers were presumably still active, so how they were disabled is unknown.
It is clear that the massive chunk of earth would have produced cataclysmic effects on the planet thereby killing any remaining life on the surface.
Dead Space 2Edit
Isaac uses the Ishimura's gravity tethers to drag Gov Sec into position, lining up the tram track and allowing Ellie to ride the tram over to Gov Sec.
- The Kinesis Module seems to use a similar technology to the gravity tether, but in a smaller, portable form.
- There are four gravity tethers holding the asteroid that was used to send the SOS. There are two on the inside and two on the outside.
- In Dead Space: Extraction it is said that if a Gravity Tether gets damaged and doesn't get fixed it will create a mini black hole, thus destroying the colony and planet or renedering them unhabitable.