Largest by diameter Estimating the sizes of asteroids from observations is difficult due to their irregular shapes, varying
albedo, and small
angular diameter. Observations by the
Very Large Telescope of most large asteroids were published 2019–2021. The number of bodies
grows rapidly as the size decreases. Based on
IRAS data there are about 140
main-belt asteroids with a diameter greater than 120 km, For a more complete list, see
List of Solar System objects by size. The inner asteroid belt (defined as the region interior to the 3:1
Kirkwood gap at 2.50 AU) has few large asteroids. Of those in the above list, only
4 Vesta,
19 Fortuna,
6 Hebe,
7 Iris and
9 Metis orbit there. (Sort table by mean distance.)
Most massive Below are the sixteen most-massive measured asteroids.
Ceres, at a third the estimated mass of the
asteroid belt, is half again as massive as the next fifteen put together. The masses of asteroids are estimated from
perturbations they induce on the orbits of other
asteroids, except for asteroids that have been visited by spacecraft or have an observable moon, where a direct mass calculation is possible. Different sets of
astrometric observations lead to different mass determinations; the biggest problem is accounting for the aggregate
perturbations caused by all of the smaller
asteroids. The proportions assume that the total mass of the asteroid belt is , or .
Brightest from Earth Only Vesta is regularly bright enough to be seen with the naked eye. Under ideal viewing conditions with very dark skies, a keen eye might be able to also see Ceres, as well as Pallas and Iris at their rare perihelic oppositions. The following asteroids can all reach an
apparent magnitude brighter than or equal to the +8.3 attained by
Saturn's moon
Titan at its brightest, which was discovered 145 years before the first asteroid was found owing to its closeness to the easily observed Saturn. None of the asteroids in the outer part of the asteroid belt can ever attain this brightness. Even Hygiea and Interamnia rarely reach magnitudes of above 10.0. This is due to the different distributions of spectral types within different sections of the asteroid belt: the highest-albedo asteroids are all concentrated closer to the orbit of
Mars, and much lower albedo
C and
D types are common in the outer belt. Those asteroids with very high eccentricities will only reach their maximum magnitude rarely, when their perihelion is very close to a heliocentric conjunction with Earth, or (in the cases of
433 Eros,
99942 Apophis, , , and
367943 Duende) when the asteroid passes very close to Earth. • Apophis will only achieve that brightness on April 13, 2029. It typically has an apparent magnitude of 20–22.
Slowest rotators This list contains the
slowest-rotating known minor planets with a period of at least 1000 hours, or 41 days, while most bodies have
rotation periods between 2 and 20 hours. Also
see Potentially slow rotators for minor planets with an insufficiently accurate period ().
Fastest rotators This list contains the
fastest-rotating minor planets with a period of less than 100 seconds, or 0.0277 hours. Bodies with a highly uncertain period, having a
quality of less than 2, are highlighted in dark-grey. The fastest rotating bodies are all unnumbered
near-Earth objects (NEOs) with a diameter of less than 100 meters
(see table). Among the
numbered minor planets with an unambiguous period solution are , a 60-meter sized stony NEO with a period of 352 seconds, as well as and , two
main-belt asteroids, with a diameter of 0.86 and 2.25 kilometers and a period of 1.29 and 1.95 hours, respectively (
see full list). == Orbital characteristics ==