Juno (Asteroid) Facts & Information

Juno is the third asteroid to be discovered, and it happened in 1804. It is located in the asteroid belt, and it is the 11th-largest asteroid, and one of the two largest stony S-type asteroids.

Key Facts & Summary

  • Designated as 3 Juno, this asteroid contains around 1% of the total mass of the asteroid belt.
  • The asteroid was discovered in 1804 by German astronomer Karl Harding. At first, it was considered a planet.
  • Its aphelion takes it at around 3.5 AU away from the Sun, while its closest approach is at 1.9 AU.
  • The semi-major axis of Juno is 2.6 AU, and its eccentricity is 0.2.
  • The average orbital speed of Juno has been estimated to be at 17.93 km / 11.14 mi.
  • Juno is the second most massive S-type asteroid. It only has 3% of Ceres’s mass.
  • The orbital period of Juno has been calculated to take around 4.3 years.
  • Juno is unusually reflective amongst S-type asteroids. This is because of distinctive surface properties.
  • Juno’s spectral classification is S, and its apparent magnitude varies from 7.4 to 11.55. Thus, it is the brightest asteroid ever to be discovered after 4 Vesta.
  • The absolute magnitude of Juno is 5.33
  • Juno’s diameter is speculated to be at 200 by 165 by 125 miles / 320 by 267 by 200 kilometers.
  • This big asteroid can be viewed with the use of binoculars.
  • S-type asteroids are the second-most common types of asteroids in the Solar system.
  • Many S-type asteroids have similar compositions and spectral characteristics with certain meteorites.
  • The initial S- stands for siliceous. Such asteroids make up around 17% of all the known asteroids in the solar system.

Initially considered as a planet, Juno was named after the mythological highest Roman goddess. The adjectival form of Juno is Junonian.

It was the third asteroid to be discovered. This happened in 1804, and the one to find it was the German astronomer Karl Harding. It is the 11thlargest asteroid, and one of the two largest stony S-type asteroids, along with 15 Eunomia.


All asteroids are rocky remnants from 4.6 billion years ago. They are leftovers from the early solar system. Most of them are located in the central asteroid belt between Mars and Jupiter. The thickness of the asteroid belt is believed to be at around  1 AU.

They orbit the Sun, and among them, Juno is one of the largest asteroids. In the asteroid belt, there are several groups of different estimated ages. The Karin family, for example, is believed to have formed around 5.7 million years ago out of a single object. The Veritas family around 8.3 million years ago, and the Datura family at only 53.000 years ago.


If we were to take 4 Vesta’s unusual brightness, Juno would be the brightest asteroid ever discovered. It has an apparent magnitude of 7.4 to 11.55. Its absolute magnitude is 5.33.

Based on Juno’s mass, it is the 11th most massive asteroid in the solar system, though some estimates put it in the 10th place. In comparison with the largest asteroid, Ceres, Juno represents only 3% of the giant’s total mass. When it comes to the asteroid belt, Juno makes up a total of 1% of the asteroid belt’s total mass.

Even among S-type asteroids, Juno is unusually reflective. Its high albedo is sometimes brighter than Neptune or Titan, and this is also the reason why it was discovered before the larger asteroids: Hygiea, Europa, Davida, and Interamnia.

Along with 1 Ceres, 2 Pallas, and 4 Vesta, it was considered a planet for some time. When facing the Sun, Juno has been determined to reach temperatures of up to 293 K, and at perihelion 301 K. Estimates of Juno’s size varies, but the most accepted one is that it has a diameter of 271.4 km / 168.8 mi.

Orbit and Rotation

Juno orbits around the Sun slightly closer than Ceres and Pallas. It has a moderately inclined orbit at around 12 degrees to the ecliptic, but it has an extreme eccentricity, even greater than that of Pluto.

Because of this, Juno gets closer to the Sun at perihelion than Vesta, and further out at aphelion than Ceres. Until 1854, Juno was considered to have the most eccentric orbit of any known celestial body.

The asteroid rotates in a prograde direction with an axial tilt of about 50 degrees. It completes one orbit around the Sun in 4.36578 years, and it rotates once every 7.21 hours.

Classification & Other Features

Some studies determined that Juno might be the progenitor of chondrites, a common type of stony meteorite composed of iron-bearing silicates such as olivine and pyroxene. 

Infrared images revealed that Juno has a 100 km / 62 mi crater or ejecta feature, the result of a geologically young impact. The asteroid is of spectral type S for salicaceous.

The unusually bright asteroid reaches opposition from the Sun every 15.5 months or so. Its minimum distance varies greatly depending on whether it is near perihelion or aphelion. Favorable positions occur once every 13 years.

The Future

Though there are millions of asteroids that have survived for millions of years, one day, the entire asteroid belt will be gone, and Juno will suffer the same faith. It will happen when the Sun approaches the end of its life. The Sun’s powerful last light will shatter the asteroids with radiation and destroy them.

Did you know?

  • The orbit of Juno appears to have changed slightly around 1839. This was likely due to gravitational perturbations caused from a passing asteroid.
  • Juno was the first asteroid in which an occultation was observed. It passed in front of a dim star. Many more occultations have since occurred with the last one happening in 2013.
  • The previous time Juno was in a favorable opposition was in 2005, at a distance of 1.063 AU, having a magnitude of 7.55.
  • Juno’s next favorable opposition will happen on the 30th of October, 2031. It will be at a distance of 1.044 AU, and it will have an apparent magnitude of 742.


  1. Wikipedia
  2. Astronoo
  3. NASA

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