Gravity Tethers while not directly explained in the Dead Space universe appeared 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 it's smelting bays and guiding ships into it's docking bays. Common procedure also seemed to be outfitting the colony itself with a massive array of gravity tethers along the colony's Megavents which aided in planetary extraction.
In the final chapters when Isaac brought the Marker back down to Aegis 7, the Marker released a shockwave that disabled the gravity tethers holding the massive chunk of planetary rock that the Ishimura extracted causing it to plummet down the planet. It was never stated why the Marker disabled the gravity tethers. The Ishimura's tethers are presumably still active so how they were disabled is unknown.
It was 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 used the Ishimura's gravity tethers to drag the Government Sector into position, lining up the tracks and allowing Ellie to ride the tram over to the Government Sector.
- The Kinesis Module seemed 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 was said that if a Gravity Tether got damaged and did not get fixed, it will create a mini black hole, thus destroying the colony and planet or renedering them unhabitable.