The graviton ellipse was a rare subspace anomaly that travelled the galaxy consuming objects in its path, including the Mars mission command module Ares IV in 2032.
The ellipse is the centre of the sixth-season episode One Small Step. In October 2032 it emerges near Mars and engulfs the orbiting Ares IV with Lieutenant John Kelly aboard, an event that nearly ended early human spaceflight.
In 2376 Voyager meets the same phenomenon in the Delta Quadrant. Chakotay, a keen student of the period, takes the Delta Flyer inside with Paris and Seven of Nine and finds the module intact in the calm core, alongside debris from many civilisations.
His insistence on towing it out leaves the Flyer crippled when a dark-matter body strikes the ellipse. Seven beams aboard Ares IV to salvage a part and plays Kelly's last logs, which show that he kept gathering data until he died. Kelly is given a Starfleet burial.