A Hirayama family of asteroids is a group of minor planets that share similar orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. 253 Mathilde, a C-type asteroid. ... Minor planets, or planetoids are minor bodies of the Solar system orbiting the Sun (or of other planetary systems orbiting other stars) that are larger than meteoroids (the largest of which might be taken to be around 10 meters or so across) but smaller than major planets (Mercury having a... The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ... In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas. ... In astrodynamics, under standard assumptions any orbit must be of conic section shape. ... Inclination in general is the angle between a reference plane and another plane or axis of direction. ...

Strictly speaking, families and their membership are identified by analysing the so-called proper orbital elements rather than the current osculating orbital elements, which regularly fluctuate on timescales of tens of thousands of years. The proper elements are related constants of motion that are thought to remain almost constant for times of at least tens of millions of years. The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale. ... In Astronomy, and in particular in Astrodynamics, the osculating orbit of an object in space is the gravitational Keplerian orbit about a central body which best approximates the (more complex) motion of the object at a given instant in time. ... The elements of an orbit are the parameters needed to specify that orbit uniquely, given a model of two ideal masses obeying the Newtonian laws of motion and the inverse-square law of gravitational attraction. ...

The Japanese astronomer Kiyotsugu Hirayama (1874-1943) pioneered the estimation of proper elements for asteroids, and first identified several of the most prominent families in 1918 [1]. An astronomer or astrophysicist is a person whose area of interest is astronomy or astrophysics. ... Kiyotsugu Hirayama (平山清次) (1874–1943) was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called Hirayama families in his honour. ... 1874 (MDCCCLXXIV) was a common year starting on Thursday (see link for calendar). ... 1943 (MCMXLIII) was a common year starting on Friday (the link is to a full 1943 calendar). ... 1918 (MCMXVIII) was a common year starting on Tuesday of the Gregorian calendar (see link for calendar) or a common year starting on Wednesday of the Julian calendar. ...

Kiyotsugu Hirayama initially identified the Koronis, Eos, and Themis families, and later recognized also the Flora and Maria families. The Koronis Family is a family of asteroids in the Main Belt. ... Eos family is a prominent family of asteroids that are believed to have formed as a result from an ancient catastrophic collision between asteroids. ... The Themis Asteroid Family The Themis Asteroid Family is a Hirayama family of asteroids found in the outer portion of the main asteroid belt, between the orbits of Mars and Jupiter. ... The Flora family of asteroids is a large grouping of S-type asteroids in inner main belt, whose origin and propeties are relatively poorly understood at present. ...

A list of the recognized families can be found here.67.71.58.225 18:47, 21 November 2006 (UTC)burns fraser. Minor planet is the official term for asteroids and trans-Neptunian objects. ...