WILLIAM & DEBORAH HILLYARD

Solar System -

The Asteroid Belt

The Eos family contains several hundred notable asteroids, and more than 4,000 in total.  All orbit the Sun between the 7/3 and 9/4 resonance with Jupiter; that is between 2.99 and 3.03 astronomical units.  Notable members of the group, with average diameters, are Eos (104 km/65 miles), which gives the group its name, Dorothea (38 km), Berenike (39 km), Antikleia and Brigitta (both 33 km).  Analysis reveals that the family originated from the collision between an object about 240 km across with a smaller object with perhaps 10% of the mass of the larger.  This probably occured about one to two billion years ago. 

Asteroid Groups

Eos Family
Flora Family
The Flora family is interesting in that it includes Gaspra (avg diameter 12 km), the first asteroid ever photographed (by Galileo in 1991) and because some scientists believe it may be the origin of the asteroid that hit the Earth and destroyed the dinosaurs 65 million years ago, though this is not a widely held belief.  The group orbits between 2.17 and 2.33 astronomical units from the Sun.  The asteroid Flora is quite large with an average diameter of about 135 km, and contains almost 80% of the mass of the family.  Ariadne is very elongated at approximately 95 x 60 x 50 km.  The family itself contains somewhere around 4% to 5% of the asteroids in the whole belt. 
Koronis Family
The Koronis family of about 300 asteroids orbits in a tight group between 2.83 and 2.91 AU from the Sun.  They are thought to have originated from the breakup of an object about 100 km in diameter at least two billion years ago.  The largest members of the group are Lacrimosa (41km), Urda (39.9 km), Koronis (35.4 km), Ida (31.3 km), Elvira (27.0 km), Florentina (27.0 km), Claudia (24.0 km) and Dresda (23.0 km). 
Hungaria Family
The Hungaria family is a group of small asteroids rather different from the main belt asteroids in that they orbit inside the 4/1 resonance with Jupiter; very close to the Mars orbit at between 1.78 and 2.00 astronomical units from the Sun.  Normally these orbits are unstable and the object is ejected by perturbations from Mars, but the Hungarias avoid this by having orbits highly inclined to the ecliptic.  434 Hungaria, after which the family is named, is the largest at between 13 and 30 km in diameter. 
There are many groups of Asteroids so here we look at some of the notable groups or families. 
Adeona Family
The Adeona family is a group of about 1,000 small asteroids that formed from a parent asteroid 145 Adeona.  145 Adeona is fairly large at 151.1 km across, with a very dark surface, that dwarfs the other members of the familty.  It has a somewhat eccentric orbit that varies between 342 million km and 458 million km from the Sun. 
Baptistina Family
298 Baptistina, a modest asteroid somewhere between 14 and 26 km across (revised 2009 estimate), is believed to be the largest remaining fragment of the progenitor for this family of asteroids.  Originally, it was probably around 170 km across, and collided with a smaller body, perhaps 60 km across.  Until recently, this collision was thought to have occurred about 160 million years ago, but information obtained from the Wide-field Infrared Survey Explorer in 2011 indicates that this occurred only 80 million years ago.  It is likely that the collision produced about 300 asteroids larger than 10 km across, and as many as 140,000 more than 1 km across!  Many of these drifted off, some heading inwards towards the orbits of the inner planets.  A widely held theory was that one of the pieces that broke off in the collision could have been the K-T impactor that caused the extinction of the dinosaurs some 65 million years ago.  This was already controversial as recent research had shown that 298 Baptistina does not have the same chemical composition as whatever caused the K-T boundary, and the revised collision date makes this scenario almost impossible.  Generally, it takes many tens of millions of years for an asteroid to achieve resonance and collide with a planet lying inside the asteroid belt.  Another, perhaps even more tenuous, theory held that the Moon's crater Tycho is the result of another fragment that hit the Moon some 108 million years ago.  The new dating of the collision means that this is almost certainly wrong.   Still, quite an interesting family of asteroids. 
Eunomia Family
15 Eunomia is the largest asteroid in this family.  It is elongated with an average diameter of some 250 km.  It estimated to contain up to 75% of the matter from the progenitor which would have had an average diameter of about 280 km.  This progenitor collided with a smaller body, which was probably about 50 km in diameter, at a speed of about 22,000 km/hr.  The fragmentation produced the family of asteroids.  

258 Tyche is about 65 km across, and is the second largest of the Eunomia family, although it is possible that it is not part of the family and has drifted into the region.  The next largest asteroids ar around 30 km across. 
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