Any asteroid or comet whose orbit brings it within 1.3 AU (about 194 million km) of the Sun — and therefore potentially close to Earth's orbit. NASA tracks over 35,000 known NEOs. The vast majority pose no threat, but monitoring them is essential for planetary defense.

An asteroid that meets two official NASA criteria: its orbit brings it within 0.05 AU (~19.5 lunar distances) of Earth's orbit, AND its estimated diameter exceeds 140 meters. PHA status does not mean an impact is imminent — it means the object warrants continuous monitoring due to its size and orbital proximity.

An object that is not classified as PHA by NASA. No Threat does not mean the object is unimportant — it simply means it poses no significant risk during this particular flyby.

The most common NEO class. Apollo asteroids have orbits that cross Earth's orbit, with a semi-major axis greater than 1 AU. Because they cross Earth's path, they are statistically the most likely to cause a future impact. Named after asteroid 1862 Apollo.

Asteroids with orbits mostly inside Earth's orbit, with a semi-major axis less than 1 AU. They still cross Earth's orbit and can be hazardous. Named after asteroid 2062 Aten.

Asteroids that orbit between Earth and Mars. Their orbits approach Earth's orbit from the outside but do not currently cross it. They are not immediately hazardous but can evolve into Earth-crossers over time. Named after asteroid 1221 Amor.

The rarest NEO class — asteroids whose entire orbit lies inside Earth's orbit. They are difficult to observe because they are always close to the Sun from Earth's perspective. Named after asteroid 163693 Atira.

The closest possible distance between an asteroid's orbit and Earth's orbit — regardless of where the two bodies actually are at any given moment. A low MOID means the asteroid's path comes very close to Earth's path, making future close encounters possible. MOID below 0.05 AU is one of the two criteria for PHA classification. It is also the most heavily weighted factor in our Risk Index algorithm (35 points).

Half the longest diameter of an elliptical orbit, measured in Astronomical Units (AU). It defines the average distance of the asteroid from the Sun and determines its orbital period. Earth's semi-major axis is exactly 1 AU.

A measure of how elliptical (stretched) an orbit is. A value of 0 is a perfect circle; values approaching 1 are highly elongated ellipses. Most NEOs have eccentricities between 0.1 and 0.8. High eccentricity means the object's distance from the Sun varies dramatically across its orbit.

A JPL quality metric indicating how well an asteroid's orbit is known. 0 means the orbit is extremely well-determined (many observations over a long arc). 9 means the orbit is highly uncertain. A higher condition code reduces confidence in predicted future positions — and slightly lowers our Risk Index score.

The average distance between Earth and the Moon — approximately 384,400 km. Used as a convenient unit for expressing close approach distances of near-Earth objects. A flyby at 1 LD means the asteroid passes at the same distance as the Moon.

The average distance between Earth and the Sun — approximately 149,597,870 km. The standard unit for measuring distances within the solar system.

A measure of an asteroid's intrinsic brightness, used to estimate its size. Lower H values mean brighter (and usually larger) objects. Since we can't directly measure most asteroid sizes, H combined with an assumed surface reflectivity (albedo) gives us a size range estimate.

Our proprietary algorithm combining five factors to produce a single threat score from 0 to 100. It is not an official NASA metric — it is designed to give a quick, intuitive sense of how noteworthy each flyby is. Scores below 50 are generally safe, 50–75 warrant attention, above 75 are high-risk objects.