An ejecta blanket is a generally symmetrical apron of ejecta

Ejecta

Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...

 that surrounds crater

Impact crater

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...

; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge.
After an impact event

Impact event

An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...

, the falling debris forms an ejecta blanket surrounding the crater. Approximately half the volume of ejecta falls within 1 crater radius of the rim, or 2 radii from the center of the crater. The ejecta blanket becomes thinner with distance and increasingly discontinuous. Over 90% of the debris falls within approximately 5 radii of the center of the crater. Ejecta which falls within that area is considered proximal ejecta. Beyond 5 radii, the discontinuous debris is considered distal ejecta.