Graviton ellipse
A graviton ellipse (designated as spatial anomaly 521 by the Borg) is a stable pocket of normal space moved by a surrounding elliptical gravimetric distortion perimeter measuring thirty million terajoules of subspace energy.

Graviton ellipses travel through subspace, emerging into normal space and extra-dimensional realms only when they are in proximity of objects that emit electromagnetic energy, such as spacecraft and dark matter asteroids. The ellipses themselves generate an electromagnetic radiation field of dangerous levels to humanoids that make the anomaly react like a magnet drawn to another magnet, relative to its target. The Borg had discovered that cutting down  and simultaneously reversing the polarity of deflector shields would result in the anomaly no longer being attracted to the ship. The anomalies remain outside of subspace for some hours only.

In addition to the electromagnetic field, these anomalies emit level 9 gravimetric distortions through subspace to the space around them, creating severe turbulence. As an ellipse emerges from subspace, the initial target it was drawn to is already in range of the distortions and would no longer be capable of forming warp fields to escape. In addition, the target ship would experience a ship-wide power drain. An ellipse could overtake the ship even if it was traveling at maximum impulse.

The stress caused by the gravimetric interference was higher closer to the perimeter and could tear some starships apart. Some ships could, however, survive the ordeal intact. The actual elliptical perimeter of the anomaly is composed of gravimetric currents of increasing gravimetric shear intensity closer to the interior of the phenomenon. As the anomaly changes course or devours objects, it discharges gravimetric surges in the perimeter that travel outward and even affect objects thousands of kilometers away.

The core of a graviton ellipse is totally free of gravimetric distortions. Seven of Nine described Tuvok's reference of the core as "the eye of the storm" as "an apt metaphor". The electromagnetic activity of the anomaly creates a natural luminescence, and the ambient temperature of the interior is −260 degrees. Chemical interactions form a primitive, unbreathable atmosphere. The pocket of normal space remains completely stable and calm regardless of what the speed, location, or movements of the anomaly are. The Borg theorized it to be possible to dissipate the anomaly from within its interior. To that end, they developed shield enhancements to allow ships passage through the gravimetric currents. These enhancements could also be added to shields used by Starfleet craft such as the  or a Class 2 shuttle. Modifications took several hours and required an operator familiar with Borg technology to successfully maintain the protection.

One such graviton ellipse, with a perimeter over a kilometer in diameter, had been in existence from the time of the formation of the Milky Way Galaxy. It was found to have devoured asteroids and spaceships from all of the quadrants of the galaxy and from extra-dimensional realms. An atmosphere had even formed from the interactions of chemicals it had accumulated. In 2032, this anomaly emerged in the Sol system and devoured the Ares IV Mars orbiter. This was mankind's first encounter with a spatial anomaly. The same ellipse attempted to engulf  in the Delta Quadrant in 2376. Commander Chakotay suggested that the anomaly be renamed as the "kitchen sink anomaly" due to the variety of debris contained inside.

Graviton ellipses have been observed only a handful of times, and no known ship had ever survived the encounter or escaped being devoured, before Voyager. ()

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fr:Ellipse de graviton
Subspace phenomena