"They don't know where it came from" - Elizabeth Darko
The Jet Engine is an Artifact that comes out of nowhere
When a Tangent Universe occurs, those living nearest ot the vortex will find themselves at the epicenter of a new world. An Artifact provides the first sign that a Tangent Universe has occured.
Origins
If an Artifact occurs, the Living Receiver will retrive it with great interest and curiosity. Artifacts are formed from Metal, such as an Arrowhead from an ancient Mayan Civilization, or a Meta Sword from Medival Europe. Divine Intervention is deemed the only logical conclusion for the appearance of the Artifact.
Donnie Darko
In Donnie Darko, the artifact takes the appearance of a Jet Engine that crashes on Donnie's bedroom. At the end of the film, the engine is pulled down by Donnie and enters a wormhole 28 days into the past along with Donnie as the Jet Engine crashes onto Donnie's Bedroom this time killing him.
S. Darko
In S. Darko, the Artifact takes the appearance of a Meteorite that crashes on a windmill where Justin Sparrow was. At the end, Justin time travels the Meteorite 4 days into the past as the meteorite crashes onto the Windmill, killing him.