Encyclopedia of the Solar System 2nd ed

Main-Belt Asteroids 351

However, discovery is just the first step. Unless an as-
teroid is tracked and its orbit reliability determined, it will
be “lost.” This tracking process takes weeks and sometimes
months of additional observations. Once an object has an
accurate orbit, it is given a permanent number. The num-
bers are not assigned in order of discovery, but sequentially
by order of orbit determination. With the assignment of a
number, the asteroid’s discoverer has the right to suggest a
name for the object. Asteroids are unique in that they can
be named after persons living or dead, real or imaginary,
mythological characters or creatures, and in several cases,
pets (though this is now discouraged); however, political
and military leaders must have been dead 100 years before
an asteroid can bear their names, and asteroids cannot be
named to advertise commercial products.

1.3 Sizes and Shapes

Shown in Table 1 is a listing of the diameters of the 20
largest main-belt asteroids. Asteroids sizes drop rapidly,
with the largest asteroid 1 Ceres being almost twice as large
as the next largest. There are only 5 asteroids with diame-
ters greater than 400 km and only 3 with diameters between
400 and 300 km. The asteroid population becomes relatively
abundant only below 300 km diameter.

The number of asteroids increases exponentially as the

size decreases in a “power-law” size distribution. This is

consistent with an initial population of strong, solid bodies

that have been ground down by repeated impacts over the

age of the solar system. Today most asteroids are fragments

of larger parent bodies that have been collisionally shattered

into much smaller pieces. This power law is seen not only

in the sizes of asteroids but also in the sizes of the craters

on the Moon, Mars, and the moons of Jupiter and Saturn,

reflecting the population of the asteroids whose impacts

made those craters.

Given the conditions in the Asteroid Belt today, only the

largest asteroids are large enough to have survived from

the beginning of the solar system. The power law predicts,

and observations confirm, that by far the most common

asteroids are the smallest. Asteroid search programs us-

ing powerful telescopes, extremely sensitive CCD sensors,

and state-of-the-art software regularly find asteroids in near-

Earth space with diameters as small as only 5–10 m. The

primary limitation on our ability to find asteroids is their

size. Smaller objects reflect less light and, after a point, a

small object is not observable because the light it reflects

drops below the limiting sensitivity of the telescopic system

trying to detect it. The good news is that we have probably

discovered and tracked all asteroids in the Main Asteroid

Belt larger than 20 km and all those in near-Earth space

TABLE 1 Diameters of the 20 Largest Asteroids

Asteroid Name and

Number Asteroid Class Semimajor Axis Diameter (km)

1 Ceres C 2.767 940

4 Vesta V 2.362 576

2 Pallas B 2.771 538

10 Hygeia C 3.144 430

704 Interamnia D 3.062 338

511 Davida C 3.178 324

65 Cybele C 3.429 308

52 Europa C 3.097 292

87 Sylvia P 3.486 282

451 Patientia C 3.063 280

31 Euphrosyne C 3.156 270

15 Eunomia S 2.644 260

324 Bamberga C 2.683 252

3 Juno S 2.670 248

16 Psyche M 2.922 246

48 Doris C 3.112 246

13 Eugenia C 2.576 244

624 Hector D 5.201 232

24 Themis C 3.133 228

95 Arethusa C 3.068 228