Scientists have discovered four families of extremely young asteroids -- less than 7 million years old -- in orbit between Mars and Jupiter.
The key dating parameters used were the longitudes of the pericenter and ascending node, according to researchers from Sao Paulo State University (UNESP) in Brazil.
For a planet, comet or asteroid moving around the Sun in an elliptical orbit, the pericenter is the point at which it comes closest to the Sun.
The ascending node is the point at which the orbit crosses from the southern side of a reference plane, typically the ecliptic plane, to the northern side.
"When an asteroid family is formed, all the asteroids' pericenters and ascending nodes are aligned, but as the family evolves, the alignment is lost owing to gravitational disturbances produced by planets and possibly by some massive asteroids," said Valerio Carruba, a professor at UNESP.
In addition to the four new families they themselves identified, the group studied 55 new families identified by other scientists.
They established a diagram that, with considerable precision, distinguishes between families formed by collisional events and families formed by fission of a precursor body.
When two asteroids collide, one or both may fragment, giving rise to a family with several objects.
Fission, on the other hand, consists of the ejection of matter by a precursor body, either because it acquired very rapid rotation on its own axis and suffered a collision or because it recently expelled a secondary body that broke up.
"One of the four families we identified was undoubtedly formed by a collisional event. Collision is very likely to have been the origin of another. The rest were identified very recently, and we need more studies to formulate a hypothesis regarding their formation," Carruba said.
The Main Belt is an extraordinary niche of asteroids, with more than 700 known objects. The number is rising steadily thanks to improving methods of detection, and it can be estimated at million.
According to Carruba, the asteroids in the Main Belt are far from evenly distributed. Various different regions have formed within the belt owing to the highly complex gravitational interaction among so many bodies and, above all, to Jupiter's powerful gravitational field.
"Between 33 and 35 per cent of the asteroids in the Main Belt are members of families," Carruba said.
"There are over 120 recognisable families and dozens of less statistically significant groups. Large families comprise hundreds of members, whereas small families may have some ten members," he said.
Estimates of the age of the asteroid families in the belt range from a few million to hundreds of millions of years.
The origin of the oldest family has been dated to 4 billion years ago, so it participated in the first stage of the solar system's formation.
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