2005 XY4

Asteroid sizes are estimated primarily from their absolute magnitude (H), which measures intrinsic brightness. Since brightness depends on both size and surface reflectivity (albedo), scientists use assumed albedo values to convert H into a diameter range. A typical near-Earth asteroid has an albedo of 0.05 to 0.25. Radar observations from facilities like Goldstone can provide more precise size measurements by bouncing radio waves off the asteroid and analyzing the returned signal. Spacecraft flybys and occultation events (when an asteroid passes in front of a star) provide the most accurate measurements but are rare.

Orbital parameters describe the shape, size, and orientation of an asteroid's path around the Sun. The semi-major axis is half the longest diameter of the elliptical orbit, measured in astronomical units (AU). Eccentricity describes how elongated the orbit is (0 = perfect circle, closer to 1 = highly elliptical). Inclination is the angle between the asteroid's orbital plane and Earth's orbital plane. Perihelion is the closest point to the Sun, aphelion is the farthest. The orbital period is how long one complete orbit takes. The Jupiter Tisserand invariant helps classify the object's dynamical relationship with Jupiter.

An asteroid is classified as "potentially hazardous" (PHA) by NASA when it meets two specific criteria: its estimated diameter is 140 meters or larger (absolute magnitude H of 22.0 or less), and its minimum orbit intersection distance (MOID) with Earth is 0.05 AU (about 7.5 million kilometers) or less. This classification is based on orbital geometry, not an imminent collision threat. It means the asteroid's orbit could theoretically bring it close enough to Earth to be concerning if its trajectory were to change due to gravitational perturbations. Most PHAs have well-understood orbits with no significant impact probability in the foreseeable future.

Close approach predictions vary in accuracy depending on the quality and quantity of observations. Well-observed asteroids with long data arcs (many years of tracking) have extremely precise orbit calculations -- their positions can be predicted to within a few kilometers over decades. Newly discovered asteroids with short data arcs may have uncertainties of thousands of kilometers. The "orbit uncertainty" parameter (0-9 scale) indicates this precision, with 0 being the most certain. NASA's Sentry system continuously recalculates impact probabilities as new observations refine each asteroid's orbit.