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The brightest asteroids are those that can reach an apparent magnitude brighter than or equal to +8.3 based on 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.

Asteroids in the outer part of the asteroid belt cannot obtain brightness at these levels. Hygiea and Interamnia rarely reach magnitudes brighter than 10.0. This is due to the different distribution of spectral types within different sections of the asteroid belt being such that the highest-albedo asteroids are all concentrated closer to Mars, and much lower albedo C and D types being common in the outer belt.

Asteroids with very high eccentricities will only reach their maximum magnitude on unusual occasions when their perihelion is very close to a heliocentric conjunction with Earth, or (in the case of 99942 Apophis) when the asteroid passes very close to Earth.

Asteroid Magnitude
when
brightest
Semi-major axis (AU) Eccentricity
of orbit
Diameter
(km)
Year of
discovery
99942 Apophis 3.4[1][2][3] 0.922 0.191 0.32 2004
4 Vesta[4] 5.20 2.361 0.089172 529 1807
2 Pallas 6.49 2.773 0.230725 544 1802
1 Ceres 6.65 2.766 0.079905 952 1801
7 Iris 6.73 2.385 0.231422 200 1847
433 Eros 6.8 1.458 0.222725 34 × 11 × 11 1898
6 Hebe 7.5 2.425 0.201726 186 1847
3 Juno 7.5 2.668 0.258194 233 1804
18 Melpomene 7.5 2.296 0.218708 141 1852
15 Eunomia 7.9 2.643 0.187181 268 1851
8 Flora 7.9 2.202 0.156207 128 1847
324 Bamberga 8.0 2.682 0.338252 229 1892
1036 Ganymed 8.1 2.6657 0.533710 32 1924
9 Metis 8.1 2.387 0.121441 190 1848
192 Nausikaa 8.2 2.404 0.246216 103 1879
20 Massalia 8.3 2.409 0.142880 145 1852

NotesEdit

  1. Apophis typically has an apparent magnitude of 20–22. It will only achieve its strongest brightness of 3.4 on April 13, 2029.
  2. "(99942) Apophis Ephemerides for 13 Apr 2029". NEODyS (Near Earth Objects - Dynamic Site). http://newton.dm.unipi.it/neodys/index.php?pc=1.1.3.1&n=99942&oc=500&y0=2029&m0=4&d0=13&h0=18&mi0=0&y1=2029&m1=4&d1=14&h1=0&mi1=0&ti=10&tiu=minutes. Retrieved 2011-05-05. 
  3. "(99942) Apophis Ephemerides for 13 Apr 2029". Minor Planet Center Ephemeris Service - Dynamic Site). http://www.minorplanetcenter.net/cgi-bin/mpeph2.cgi?ty=e&TextArea=Apophis&d=2029+Apr.+13+20%3A30&l=60&i=1&u=m&uto=0&c=&long=&lat=&alt=&raty=a&s=t&m=m&adir=S&oed=&e=-2&resoc=&tit=&bu=&ch=c&ce=f&js=f. 
  4. Only Vesta regularly attains a brightness sufficient to be visible to the naked eye.