Ceres

Ceres is the largest body in the asteroid belt between Mars and Jupiter, and the only dwarf planet in the inner solar system.^[1][2] It was the first member of the belt to be discovered, by Giuseppe Piazzi in 1801, and was called an asteroid for many years; it is so much bigger and so different from its rocky neighbors that scientists classified it as a dwarf planet in 2006.^[1] With a radius of 476 km, about one thirteenth of Earth's, it holds about a quarter of the belt's total mass, yet Pluto is still 14 times more massive.^[1] It is the one dwarf planet that belongs on this site's Asteroids pages as well as here.

Discovery and name

JPL's record gives the discovery as 1 January 1801 by Piazzi at Palermo, in Sicily.^[3] It was at first classified as a planet, then as an asteroid, before the International Astronomical Union designated it a dwarf planet in 2006 along with Pluto and Eris.^[4] Ceres is named for the Roman goddess of agriculture and harvests.^[1]

Orbit

Ceres orbits the Sun at an average distance of 2.8 astronomical units, about 413 million km, once every 1,682 days, or 4.6 years; sunlight takes 22 minutes to reach it.^[1] JPL's catalogue gives a nearly circular path from 2.55 to 2.99 astronomical units, tilted 10.6 degrees to the plane of the planets.^[3] It rotates once every 9 hours, one of the shortest days in the solar system, with its axis tilted only 4 degrees to its orbit.^[1]

Surface and interior

Ceres probably has a solid core and a mantle of water ice, and could be composed of as much as 25 per cent water.^[1] Its surface is covered in countless small, young craters, none larger than 280 km across.^[1] The bright spots that NASA's Dawn spacecraft found in several craters, brightest of all in Occator Crater, are deposits of carbonates and other salts, and Dawn returned evidence of recent and possibly ongoing activity in several places.^[5] Gravity and topography data show that its density increases with depth, and the discovery of ammonia on its surface suggests that its materials, or Ceres itself, came from the outer solar system.^[5] Whether any liquid remains underground, and whether Ceres ever had conditions for life, are open questions; NASA notes that if it has no living things today, there may be signs it harbored life in the past.^[1] Ceres has no moons and no rings.^[1]

The visit

NASA's Dawn mission launched on 27 September 2007, spent a year at the asteroid Vesta, and went into orbit around Ceres on 6 March 2015, about four months before New Horizons flew past Pluto, making Ceres the first dwarf planet to be visited and orbited by a spacecraft.^[6][4] Dawn had gathered everything needed for its original objectives by the middle of 2016 and kept observing from ever lower orbits.^[5] The mission ended on 1 November 2018, when the spacecraft ran out of the hydrazine that kept it pointed; it remains in orbit around Ceres.^[6]

Can I see it?

Not with the unaided eye except under the darkest skies; binoculars show it as a star-like point when it is well placed. Its position on any night is in JPL's catalogue.^[3]