Minimum orbit intersection distance (MOID) is a measure used in

astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...

to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits of two bodies. Of greatest interest is the risk of a collision with

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

. Earth MOID is often listed on

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...

and

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

databases such as the JPL Small-Body Database. MOID values are also defined with respect to other bodies as well:

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

MOID,

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

MOID and so on. An object is classified as a potentially hazardous object (PHO) – that is, posing a possible risk to Earth – if, among other conditions, its Earth MOID is less than 0.05 AU. For more massive bodies than Earth, there is a potentially notable close approach with a larger MOID; for instance, Jupiter MOIDs less than 1 AU are considered noteworthy since Jupiter is the most massive planet.Bruce Koehn,
Minimum Orbital Intersection Distance
, Lowell Observatory, retrieved online 14 May 2009

15 July 2015. A low MOID does not mean that a collision is inevitable as the planets frequently perturb the orbit of small bodies. It is also necessary that the two bodies reach that point in their orbits at the same time before the smaller body is perturbed into a different orbit with a different MOID value. Two objects gravitationally locked in

orbital resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...

may never approach one another.

Numerical integration In analysis, numerical integration comprises a broad family of algorithms for calculating the numerical value of a definite integral, and by extension, the term is also sometimes used to describe the numerical solution of differential equatio ...

s become increasingly divergent as trajectories are projected further forward in time, especially beyond times where the smaller body is repeatedly perturbed by other planets. MOID has the convenience that it is obtained directly from the

orbital elements Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...

of the body and no numerical integration into the future is used. The only object that has ever been rated at 4 on the Torino Scale (since downgraded), the Aten asteroid (99942) Apophis, has an Earth MOID of 0.000316 AU. This is not the smallest Earth MOID in the catalogues; many bodies with a small Earth MOID are not classed as PHO's because the objects are less than roughly 140 meters in diameter (or

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

, ''H'' > 22). Earth MOID values are generally more practical for asteroids less than 140 meters in diameter as those asteroids are very dim and often have a short

observation arc In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...

with a poorly determined orbit. As of June 2022, there have been five objects detected and their Earth-MOID calculated before the Earth impact. The first two objects that were detected and had their Earth-MOID calculated before Earth impact were the small asteroids and 2014 AA. was listed with a MOID of 0.00001 AU in the

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function T ...

database, and is the smallest MOID calculated for an Apollo asteroid. It is even smaller at the more precise JPL Small Body Database (0.0000078 AU).2008 TC3
at JPL Small Body Database. Retrieved 14 May 2009
archived
15 July 2015.

References

External links

Fast Geometric Method for Calculating Accurate Minimum Orbit Intersection Distances
(PDF)
MOID for all NEOs (Near-Earth Objects) for Mercury to Jupiter (Updated Daily)

{{Small Solar System bodies Near-Earth objects Space hazards Orbits Astrodynamics

See also

References

External links