A scale for estimating the potential danger of an Earth-approaching asteroid. The levels of danger are rated from 0–10, with corresponding colour coding: 0, coloured white, means the asteroid presents no hazard; 1 (green zone) means the asteroid will pass near the Earth but poses no unusual level of danger; 2–4 (yellow zone) mean the discovery merits attention by astronomers; 5–7 (orange zone) mean the asteroid is rated as threatening; 8–10 (red zone) refer to a certain collision, the exact number assigned indicating the anticipated level of destruction. Most discoveries start at 1 or 2 on the scale (green or yellow) but rapidly drop to 0 (white) as further observations are made and the orbit is refined. No objects have yet been assigned higher numbers than this.