3 Juno

Juno is a massive asteroid located within the asteroid belt. It is a significant object in our solar system. Juno contains approximately 1% of the total mass of the entire asteroid belt.

It is categorized as an S-type asteroid, which means it is a stony object. Among these stony asteroids, Juno is the second most massive. Only 15 Eunomia is more massive in this specific category. Despite its size, Juno only has 3% of the mass of Ceres. Ceres is the largest asteroid in the belt.

The movement of Juno follows a very specific path called an orbit. This orbit is characterized by high eccentricity. Eccentricity describes how much an orbit deviates from a perfect circle. Juno's orbit is very stretched out. This shape is even more extreme than the orbit of the dwarf planet Pluto. The orbit is also tilted at an inclination of about 12 degrees. Because the path is so elliptical, Juno's distance from the Sun changes greatly. At perihelion, it moves closer to the Sun than Vesta. At aphelion, it moves further away than Ceres.

Juno's physical properties make it stand out to astronomers. It has an unusually high albedo. Albedo is a measure of how much light a surface reflects. Because it is so reflective, Juno can appear very bright. At a favorable opposition, it can reach a magnitude of +7.5. This makes it appear brighter than Neptune or the moon Titan. This brightness is why it was discovered before larger asteroids like Hygiea or Europa. However, at most oppositions, it only reaches a magnitude of +8.7. At that brightness, it requires binoculars to see.

History shows that our understanding of Juno has changed over time. Karl Ludwig Harding discovered Juno on September 1, 1804. He was a German astronomer. At the time of discovery, Juno was considered a planet. It was grouped with Ceres, Pallas, and Vesta. As more asteroids were found, scientists changed their classification. During the 1850s, Juno was reclassified as an asteroid. The name comes from Juno, the highest Roman goddess. The astronomical symbol for Juno is a scepter topped by a star.

Scientists have used many tools to study the surface of Juno. In 2004, researchers used the Hale Telescope for Mid-Infrared Asteroid Spectroscopy. This is a method called MIDAS. These studies helped determine its average radius is about 135.7 kilometers. Infrared images show a large crater on the surface. This crater is approximately 100 kilometers wide. It is the result of a geologically young impact. This impact likely created a dark feature on the asteroid.

Astronomers also study Juno through a process called occultation. An occultation happens when an object passes in front of a star. Juno was the first asteroid for which an occultation was observed. This occurred on February 19, 1958, when it passed in front of star SAO 112328. Another major event happened on December 11, 1979. During that event, Juno occulted the star SAO 115946. Eighteen different observers registered that event. More occultations were recorded as recently as July 2013.

Juno is also linked to other celestial bodies and materials. It is the main body in the Juno family of asteroids. Spectroscopic studies suggest Juno could be the progenitor of chondrites. Chondrites are common stony meteorites. These meteorites are composed of iron-bearing silicates like olivine and pyroxene. The mass of Juno has even been estimated using radio signals. These signals come from spacecraft near or orbiting Mars. Scientists measured tiny perturbations, or changes, in the motion of Mars. These changes were caused by the gravitational pull of Juno.