Minimum orbit intersection distance
Encyclopedia
Minimum orbit intersection distance (MOID) is a measure used in astronomy
to assess collision risk between astronomical objects. It is defined as the distance between the closest points of the osculating orbit
s of the two bodies in question. Of greatest interest is the risk of a collision with Earth; the MOID between an object and Earth is called Earth MOID. Earth MOID is often listed on comet
and asteroid
databases such as the Jet Propulsion Laboratory
's Small-body Database. However, MOID can be defined with respect to other bodies as well: Jupiter MOID, Venus 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 larger bodies than Earth, there is a risk of collision with a somewhat larger MOID; for instance, Jupiter MOIDs less than 1 AU are considered noteworthy.
A low MOID does not mean that a collision is inevitable, however. It is also necessary that the two bodies reach that point in their orbits at the same time, which will happen eventually unless the two objects are gravitationally locked in orbital resonance
. However, this point may be very far in the future, and the orbit of the smaller body may be disturbed by other planets. Calculations become increasingly inaccurate as trajectories are projected further forward in time, especially beyond times where the body is disturbed in passing a planet. MOID has the advantage that it is obtained directly from the orbital elements
of the body and no projection into the future is required. Because MOID is used as a snapshot of current risk, it is calculated from the simple Keplerian orbits of the bodies with no adjustment for perturbations.
The only object that has ever been rated at 4 on the Torino scale
(since downgraded), the Aten asteroid
99942 Apophis
, has a MOID of 0.00017 AU. This is not the smallest MOID in the catalogues; many bodies with smaller MOID are not classed as PHO's because of their small size. For comparison, the only object that has been detected and had its MOID calculated before it hit the Earth was the small meteoroid
2008 TC3
. This object was listed with a MOID of 0.00001 AU in the Minor Planet Center
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).
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
to assess collision risk between astronomical objects. It is defined as the distance between the closest points of the osculating orbit
Osculating orbit
In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space is the gravitational Kepler orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space (at a given moment of time) is the gravitational Kepler orbit In astronomy,...
s of the two bodies in question. Of greatest interest is the risk of a collision with Earth; the MOID between an object and Earth is called Earth MOID. Earth MOID is often listed on comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
databases such as the Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
's Small-body Database. However, MOID can be defined with respect to other bodies as well: Jupiter MOID, Venus MOID and so on.
An object is classified as a potentially hazardous object
Potentially Hazardous Object
A potentially hazardous object is an asteroid or comet with an orbit such that it has the potential to make close approaches to the Earth and a size large enough to cause significant regional damage in the event of impact....
(PHO) – that is, posing a possible risk to Earth – if, among other conditions, its Earth MOID is less than 0.05 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
. For larger bodies than Earth, there is a risk of collision with a somewhat larger MOID; for instance, Jupiter MOIDs less than 1 AU are considered noteworthy.
A low MOID does not mean that a collision is inevitable, however. It is also necessary that the two bodies reach that point in their orbits at the same time, which will happen eventually unless the two objects are gravitationally locked in orbital resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
. However, this point may be very far in the future, and the orbit of the smaller body may be disturbed by other planets. Calculations become increasingly inaccurate as trajectories are projected further forward in time, especially beyond times where the body is disturbed in passing a planet. MOID has the advantage that it is obtained directly from the orbital elements
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
of the body and no projection into the future is required. Because MOID is used as a snapshot of current risk, it is calculated from the simple Keplerian orbits of the bodies with no adjustment for perturbations.
The only object that has ever been rated at 4 on the Torino scale
Torino Scale
The Torino Scale is a method for categorizing the impact hazard associated with near-Earth objects such as asteroids and comets.It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known...
(since downgraded), the Aten asteroid
Aten asteroid
The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered . They are defined by having semi-major axes of less than one astronomical unit...
99942 Apophis
99942 Apophis
99942 Apophis is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on...
, has a MOID of 0.00017 AU. This is not the smallest MOID in the catalogues; many bodies with smaller MOID are not classed as PHO's because of their small size. For comparison, the only object that has been detected and had its MOID calculated before it hit the Earth was the small meteoroid
Meteoroid
A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite...
2008 TC3
2008 TC3
thumb|right|300px|Ground path of the meteoroid; red line is the path, terminating where it would have hit the ground; green line is the infrasound detection of the explosion; orange crosshairs show METEOSAT IR fireball location; predicted altitudes are listed; exact path and fireball altitude not...
. This object was listed with a MOID of 0.00001 AU in the Minor Planet Center
Minor Planet Center
The Minor Planet Center operates at the Smithsonian Astrophysical Observatory , which is part of the Center for Astrophysics along with the Harvard College Observatory ....
database, and is the smallest MOID calculated for an Apollo asteroid
Apollo asteroid
The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...
. It is even smaller at the more precise JPL Small Body Database (0.0000078 AU).
See also
- List of Mercury-crossing minor planets
- List of Venus-crossing minor planets
- List of Earth-crossing minor planets
- List of Mars-crossing minor planets
- List of Jupiter-crossing minor planets
- List of Saturn-crossing minor planets
- List of Uranus-crossing minor planets
- List of Neptune-crossing minor planets