May 26, 2025 (News On Japan) - A small celestial body made headlines earlier this year when it was briefly thought to pose a threat to Earth. But the true stars of this story are not those 60-meter objects zipping past our planet, but two massive asteroids—Ceres and Vesta—each hundreds of kilometers wide and orbiting between Mars and Jupiter.
Ceres was discovered on January 1, 1801, by Giuseppe Piazzi at the Palermo Observatory in Sicily. It marked the beginning of the 19th century with the first identification of an asteroid, a category of celestial body that now exceeds 1.4 million known examples. Ceres was later reclassified as a dwarf planet in 2006 by the International Astronomical Union, no longer technically an asteroid under current definitions.
Following Ceres, a series of large asteroids were discovered over the next decades—Pallas, Juno, Vesta—mostly by German astronomers. By the end of the 19th century, 463 had been cataloged. Among them, Vesta, discovered in 1807 by Heinrich Olbers, remains particularly significant not only for its brightness but for its continued relevance in planetary science.
While Ceres is the largest asteroid, Vesta is the brightest seen from Earth. This is due not only to its proximity and size but also to its high albedo—a measure of surface reflectivity. Ceres reflects just 9% of the light that hits it, while Vesta reflects 42%, making it 4.7 times more reflective than Ceres and sometimes visible to the naked eye under dark skies.
Both Ceres and Vesta were visited by NASA’s Dawn mission, which launched in September 2007. After using Earth and Mars for gravity assists, Dawn entered orbit around Vesta in July 2011, conducting detailed observations for over a year. In September 2012, it departed for Ceres, arriving in March 2015. Though initially scheduled for a six-month survey, Dawn spent more than three years studying the dwarf planet until contact was lost in October 2018 when it ran out of fuel.
The Dawn mission revealed significant differences between the two bodies. Vesta, roughly 570 km across, has a fast rotation period of 5 hours and 20 minutes, giving it a short "day" compared to Earth. Ceres, at about 960 km in diameter, rotates every 9 hours and takes 4 years and 7 months to orbit the Sun. Ceres also exhibits unusual bright spots on its surface, later confirmed to be salt deposits that reflect sunlight strongly.
Currently, Vesta is approaching a favorable position for observation. From around May 23rd, it will appear in the southern sky near the constellations Virgo and Libra at around 10 p.m., particularly from locations with low light pollution. Though faint, it can be seen with binoculars or small telescopes, and astronomy enthusiasts may even spot it without aid under ideal conditions.
Vesta’s continued visibility and scientific importance underscore its unique role. With Ceres now classified as a dwarf planet, Vesta and Pallas remain the largest official asteroids. Some researchers suggest that Vesta may also be reclassified in the future as studies of its structure and behavior continue.
As the number of identified asteroids surpasses 1.44 million, and with more than 750,000 officially cataloged, the solar system’s so-called "minor planets" are proving to be anything but minor—for scientists and stargazers alike.
Source: テレ東BIZ
