Sen—NASA’s Dawn space probe is currently on the final leg of its long journey to the dwarf planet Ceres, and we will soon learn secrets of yet another of the minor bodies of the Solar System.
Having already spent months surveying one lesser world, Vesta, Dawn will go into orbit around Ceres on Mar. 6 and begin to map its surface.
Scientists are eager to know, for example, what is producing mysterious bright spots on its surface. These regions were first noted from afar by the Hubble Space Telescope, but images during Dawn’s approach have already shown them to be small and intense.
It is the latest example of the success of robotic exploration that I described last week. And this time it opens the door to revealing more about a class of object in the Solar System called the asteroids.
For as well as being a dwarf planet, Ceres is also the largest of the asteroids. These are objects that sometimes make the news when they come close to Earth and are presented as potential threats. But the vast majority—hundreds of thousands of them—circle the Sun in a zone between the orbits of Mars and Jupiter.
I am lucky to be able to say that one of them, 6726 Suthers, was named for me a few years back—it is no threat, I promise—so asteroids feel like family to me. “Mine” was discovered only relatively recently, in 1991, but Ceres was the first to be found a little over 214 years ago on New Year’s Eve 1800.
The discovery came after astronomers of the time had already decided that there must be a missing planet in the Solar System, because the gap between Mars and Jupiter seemed too great, so they set out to find it.
A Hungarian nobleman, Baron Franz von Zach, organised a search party of 24 observers in 1800 who were given the wonderful name of the Celestial Police. They each had their own “beat” by dividing the sky into a number of individual areas to scour.
But astronomy’s equivalent of a manhunt was pipped at the post by an independent searcher. Sicilian monk Giuseppe Piazzi was not a member of the cosmic cops, but it was he, while checking the sky from Palermo Observatory, who spotted an unrecorded “star” in the constellation of Taurus.
That seemed to be it—a new planet. When he finally revealed his discovery, some months into 1801, Piazzi wanted to call it Ceres Ferdinandea after the patron goddess of Sicily and its ruler King Ferdinand, but only the classical name was permitted.
Images of dwarf planet Ceres, complete with mysterious bright spots, taken on Feb. 19, 2015, by the Dawn spacecraft. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
However, it soon became clear that Ceres was just the first in a number of bodies out there. In 1802, Heinrich Olbers, of the Celestial Police, found another close to Ceres in the sky and named it Pallas. Karl Harding discovered a third, Juno, in 1804, and Olbers spotted a fourth, Vesta, in 1807. Vesta, though only second largest, is the brightest of them, incidentally, and can just be glimpsed with the unaided eye in a clear dark sky.
It was William Herschel, discoverer of Uranus, who came up with the term asteroids, meaning starlike, once astronomers realised that they were too small to be normal planets.
Following Vesta’s discovery, there was a lengthy gap of 38 years before another asteroid was found, but since then, the numbers have soared to over 400,000. More than 200 are more than 100 km (60 miles) wide and there are more than 750,000 bigger than 1km (3/5 mile) across. There must be millions of smaller fragments.
Because astronomers had expected to find a planet in the Mars-Jupiter gap, they used to assume that the asteroids were the remains of just such a world that had broken up. But today computer models have shown that they have to be fragments from the early Solar System that were never able to collect together in the first place, due to Jupiter’s disruptive influence.
Ceres was given its new dwarf planet status by what must appear to be today’s equivalent of the Celestial Police, the International Astronomical Union. Their jurisdiction over the heavens led to the controversial demotion of Pluto from being a full planet in 2006, and it became, with Ceres, a member of the dwarf planet club.
Readers might have observed that Suthers is a somewhat less classical name than that which Piazzi was obliged to give Ceres. In 1947 the IAU’s Minor Planet Center was set up to log information about the growing list of discoveries and to catalogue their orbits. It became traditional for the myriad of smaller rocks to be given the names of living people in recognition of their achievements in different fields.
Thus you will find space rocks named after scientists, but also actors, musicians, comedians, and the occasional populariser of astronomy like myself.
Since 1991, space probes have paid a number of asteroids a visit and observed them close-up. First was NASA’s Galileo which got close to Gaspra while on its way to Jupiter. Subsequent flypasts have been made of many others. NASA's NEAR Shoemaker spacecraft and the Japanese space agency JAXA’s Hayabusa actually managed to touch down on Eros, in 2001, and Itokawa (briefly), in 2005, respectively.
Visiting asteroids is a priority for NASA astronauts and for private companies wanting to get involved in space mining for resources. So there is huge interest in what Dawn will tell us about the first and largest asteroid, Ceres.