Ceres 

The first asteroid 

A view of Ceres in natural colour, pictured by the Dawn spacecraft. Images were acquired by Dawn at 04:13 UT May 4, 2015, at a distance of 13,641 km. At the time, Dawn was over Ceres' northern hemisphere. The prominent, bright crater at right is Haulani. The smaller bright spot to its left is exposed on the floor of Oxo. Ejecta from these impacts appears to have exposed high albedo material similar to deposits found on the floor of Occator Crater.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Justin Cowart

On January 1, 1801, Italian astronomer Giuseppe Piazzi (1746–1826)  from the observatory in Palermo, Sicily, discovered the first known asteroid, a dwarf planet (1) Ceres. Originally considered a planet, it was reclassified as an asteroid in the 1850s after the discovery of dozens of other objects in similar orbits. In 2006, it was reclassified again as a dwarf planet.

   Piazzi spotted the object while conducting observations for his star catalogue. A few months later he wrote:

   "...on the evening of the 1st of January of the current year, together with several other stars, I sought for the 87th of the Catalogue of the Zodiacal stars of Mr la Caille. I then found it was preceded by another, which, according to my custom, I observed likewise, as it did not impede the principal observation. The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. In the evening of the 2d I repeated my observations, and having found that it did not correspond either in time or in distance from the zenith with the former observation, I began to entertain some doubts of its accuracy. I conceived afterwards a great suspicion that it might be a new star. The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited 'till the evening of the 4th, when I had the satisfaction to see it had moved at the same rate as on the preceding days."

   Piazzi had measured the position of the object on a total of 24 nights between January 1 and February 11. On January 24, he had announced his discovery in letters to fellow astronomers, among them his fellow-countryman, Barnaba Oriani (1752–1832). In it he wrote:

   "I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public." 

Portrait of Giuseppe Piazzi by Costanzo Angelini, circa 1825. Credit: Wikimedia Common

   Additional observations could not be taken for several months thereafter because the position of Ceres in the sky was too close to the Sun as it moved from the evening sky to the morning sky. Without an accurate orbit and ephemeris (predicted positions at specific times), attempts to reobserve the new planet in August proved unsuccessful. 

   To recover Ceres, the German mathematician Carl Friedrich Gauss (1777–1855), then 24 years old, developed an efficient method of orbit determination. Within a few weeks, he predicted the path of Ceres. Using an ephemeris for Ceres provided by Gauss, a Hungarian astronomer Franz Xaver von Zach (1754–1832) observed Ceres on December 7,  and, after bad weather cleared, again on December 31, 1801, and January 11, 1802. Using Gauss’s ephemeris, Wilhelm Olbers (1758–1840) also observed Ceres from Bremen on January 2, 1802.

A view of Ceres, taken by NASA's Dawn spacecraft on December 10, 2015, from low orbit, an approximate distance of 385 kilometers. It shows an area in the southern mid-latitudes of the dwarf planet, around a crater chain called Gerber Catena. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

   Ceres is the largest body in the main asteroid belt, 939 km in diameter. It follows an orbit between Mars and Jupiter, near the middle of the asteroid belt, with an orbital period of 4.60 years, at a mean distance of 2.77 au (414 million km), a perihelion 2.55 au (381 million km) and an aphelion 2.98 au (446 million km). Its inclination to the ecliptic is 10.6°. 

   Dawn spacecraft found Ceres's surface to be a mixture of water ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice-rock) mantle/core and a less dense but stronger crust that is at most 30% ice by volume. Although Ceres likely lacks an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form. The surface composition of Ceres is homogeneous on a global scale, and is rich in carbonates and ammoniated phyllosilicates that have been altered by water, though water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions. Organic compounds were detected in Ernutet Crater, and most of the planet's near surface is rich in carbon, at approximately 20% by mass.

   Dawn revealed that Ceres has a heavily cratered surface, though with fewer large craters than expected. The largest confirmed crater on Ceres, Kerwan Basin, is 284 km across. Three large shallow basins (planitiae) with degraded rims are likely to be eroded craters. The largest, Vendimia Planitia, at 800 km across, is also the largest single geographical feature on Ceres. Ceres has one prominent mountain, Ahuna Mons, 4.1 km high; this appears to be a cryovolcano and has few craters, suggesting a maximum age of 240 million years. Hundreds of bright spots (faculae), covered by bright salt deposits, have been observed by Dawn, the brightest in the middle of 92 km Occator Crater.

   Ceres is named for the Roman goddess of corn and harvests, whose earthly home, and oldest temple, lay in Sicily.

   Giuseppe Piazzi (16 July 1746 – 22 July 1826) was an Italian priest, mathematician, and astronomer. In 1790, he established an observatory at Palermo. His most famous discovery was the first dwarf planet, (1) Ceres, in 1801. He also measured the positions of 7646 stars, discovering that the star 61 Cygni had a large proper motion. 

Occator Crater, measuring 92 kilometers across and 4 kilometers deep, contains the brightest area on Ceres. Image taken by Dawn spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

 © 2025, Andrew Mirecki