This spherical asteroid, with a diameter less than the direct distance between Auckland and Dunedin, is going to be big news this year. NASA’s spacecraft Dawn will arrive at Ceres in early March 2015. Ceres is the largest member of the Main Asteroid belt between Mars and Jupiter. It may be the nearest dwarf planet to us, but it took Dawn some seven years to get there. To be fair we must note that Dawn has spend more than a year at the third largest object in the Asteroid Belt, Vesta.
We will hear a lot about Ceres come March 2015, and without doubt a lot of major new discoveries will be made during this historic encounter, but let us first review what we presently know about this “not-so-distant” member of our Solar system.
Ceres was discovered in 1801 by Giuseppe Piazzi and was initially thought to be a planet. At the time there was strong suspicion that an unknown planet would exist between Mars and Jupiter. This was actually a quite reasonable assumption based on mathematical considerations, but we now know that objects in the asteroid belt could never have formed one planet due to the strong gravity exerted by giant Jupiter. These tidal forces have flung out most of the original material from the region and the remaining objects were kept scattered throughout the belt preventing them from clumping together. It is estimated that the total mass of the asteroid belt at present is only a fraction of that of our Moon.
Main Asteroid Belt
The Main Asteroid belt is a donut shaped region with a diameter between about 2 and 4 AU. The Minor Planet Center is tasked by the IAU to maintain orbital information of all asteroids and comets in the Main Belt as well as elsewhere in the Solar system. At present there are more than 600,000 objects identified in the Main Asteroid belt, although most of these are very small. The largest objects in the Main Asteroid belt are Ceres (950 km), Pallas (544 km), Vesta (525 km), and Hygiea (oblong shape 300-500 km). See an overview of the relative masses of 12 largest asteroids here.
Scale diagram of the asteroids in the Inner Solar system
The white dots each represent an asteroid in the Main Belt, but it should be noted that their actual size is very much smaller than each pixel size at this scale. The average spacing between asteroids is several tens of thousands of km and several spacecraft have traversed the region on their way to the gas planets without encountering any problems. Statistically it will be very hard to hit an asteroid with an arbitrary trajectory. The Dawn spacecraft which is on its way to Ceres needs accurate navigation to find its target.
Ceres’ name was given by its discoverer Giuseppe Piazzi and is derived from the Roman goddess of Agriculture and Grain. Ceres is the counterpart of the Greek goddess of agriculture Demeter.
In modern records asteroids are named with a number in front that indicates the order of discovery. Therefore it is 1Ceres, because Ceres was the first asteroid discovered in the Main Belt.
Since its name is derived from the goddess of agriculture, the IAU has already worked with the Dawn space programme to name new features to be discovered in the context of Piazzi’s mythological reference. Craters will be named after deities of agriculture and vegetation from world mythology and other features will be named after additional agricultural themes.
Prior to 2006, the International Astronomical Union (IAU) preferred the term "Minor Planet" instead of Asteroid. But the discovery of many more similar objects, everywhere in the Solar System, made the situation increasingly confusing.
In 2006 the IAU introduced the name Small Solar System Body to include asteroids and comets, and the name Dwarf Planet to identify any of those objects that was big enough to have a spherical shape. Since then 1Ceres and Pluto are officially classified as Dwarf Planets.
Yet the names Asteroid, Minor Planet and Comet are still frequently used by astronomers and the general public alike, and it will take some time before we have clear and accepted definitions for all those objects.
But based on the directives of the IAU we can say that every natural object that orbits around the Sun is either:
• a Planet (we have eight)
Comets are in principle different from asteroids as they have a significant amount of volatiles that escape from their surface under influence of solar radiation. Therefore comets usually have a tail and coma when they approach the Sun. However this distinction is not very precise as several asteroids have been detected that also emit volatiles and could be classified as both asteroid and comet. More on the IAU definitions here.
In the sky
Ceres is too faint to see with the naked eye at a magnitude that varies between 6 and 9, but it can be seen with a reasonable telescope. Check our monthly reports for its visibility
Orbit and rotation
It takes Ceres 4.6 years to go around the Sun and its orbit is tilted with respect to Earth’s orbit by just over 10 degrees. The orbit is also slightly elliptical with a distance to the Sun ranging between 2.5 and 3.0 AU. See a diagram of the orbit here.
Ceres rotates about its axis in about 9 hours as can be deduced from time series of images taken by the Hubble telescope.
Ceres has a slightly oblate spherical shape, which suggest that it has a core and mantle structure like the larger rocky planets.
It is probably similar in composition to other asteroids, but may contain a large amount of water in the form of water ice, almost entirely below its surface. Ceres is too small to have a significant atmosphere and there cannot be any liquid water on the surface.
Read information about Ceres in the Hubble Site News Center.
Ceres does not have a high reflectivity and therefore has a fainter magnitude than its smaller “cousin” Vesta. In the Hubble images several surface features can be seen, which probably are mostly impact craters. The most prominent feature is a white spot that has been dubbed “Piazzi” after the discoverer of Ceres. However this “bright” spot still has a relatively low reflectivity and its nature has remained a mystery for now.
Watch this video of an artist's concept of how Ceres looks close-up. Soon we will be able to compare this with real images by Dawn.
First images from Dawn
Dawn is now in its approach phase towards Ceres and travels towards it at a speed of 725 kmph. Recently the spacecraft re-appeared from behind the Sun (from Solar conjunction) and can again communicate properly with Earth. Mission control has now programmed the maneuvers for this approach phase.
The very first image of Ceres taken by Dawn is of May 3 - 2011. More recently this image December 1 – 2014 was taken at a distance of 1.2 million km, when cameras were calibrated in preparation for the final approach.
Dawn will arrive at Ceres on 6 March 2015.
For the latest information about the Dawn mission go here.