NEW DAWN ON CERES

We’re about to reach a dwarf planet that could possess more water than Earth.

Next month, NASA’s Dawn spacecraft will gently slip into orbit around Ceres, the largest asteroid in our Solar System. In 2011 and 2012, Dawn spent almost 14 months studying Ceres’s smaller cousin, Vesta.

If the science results obtained at Vesta are any indication, astronomers can expect a bonanza of exciting new insights about Ceres in the months to come. Moreover, Dawn’s arrival at Ceres will mark the start of the close-up examination of dwarf planets: four months later, on 14 July, New Horizons will fly past Pluto in the cold, outer reaches of the Solar System.

Ceres circles the Sun every 4.6 years in the wide gap between the orbits of Mars and Jupiter. In the late 19th Century, astronomers already believed a new planet might lurk there – an idea put forward in a 1772 book by German astronomer Johann Elert Bode.

Building on the work of Johann Daniel Titius, Bode described a mathematical sequence that predicted the sizes of the known planetary orbits, assuming the existence of an as-yet-unknown planet between Mars and Jupiter. When, in 1781, the newly discovered planet Uranus also turned out to abide to Bode’s Law, astronomers became convinced that the gap between the fourth and fifth planets could not be empty.

In 1800, Hungarian astronomer and nobleman Baron Franz Xaver von Zach started a search programme, to be carried out by 24 astronomers all over Europe. In fact, the ‘Himmelspolizei’ (Celestial Police), as Von Zach called the initiative, was the first example of large-scale international cooperation in astronomy. One of the scientists asked to cooperate in the search was Giuseppe Piazzi, the 54-year-old director of the Palermo Observatory, on the north coast of Sicily.

The ‘planet’ in the gap

For some reason, Von Zach’s letter never reached Piazzi, but while the other 23 celestial sleuths were scanning