probably form part of a belt of asteroids intersecting the Earth's orbit and moving with planetary velocity". Another early appearance occurred in Robert James Mann's A Guide to the Knowledge of the Heavens: "The orbits of the asteroids are placed in a wide belt of space, extending between the extremes of [...]". The American astronomer Benjamin Peirce seems to have adopted that terminology and to have been one of its promoters. Over 100 asteroids had been located by mid-1868, and in 1891, the introduction of astrophotography by Max Wolf accelerated the rate of discovery still further. A total of 1,000 asteroids had been found by 1921, 10,000 by 1981, and 100,000 by 2000. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing numbers. On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding was unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids". Origin Formation In 1802, shortly after discovering Pallas, Olbers suggested to Herschel that Ceres and Pallas were fragments of a much larger planet that once occupied the Mars–Jupiter region, with this planet having suffered an internal explosion or a cometary impact many million years before, while Odessan astronomer K. N. Savchenko suggested that Ceres, Pallas, Juno, and Vesta were escaped moons rather than fragments of the exploded planet. The large amount of energy required to destroy a planet, combined with the belt's low combined mass, which is only about 4% of the mass of Earth's Moon, does not support these hypotheses. Further, the significant chemical differences between the asteroids become difficult to explain if they come from the same planet. A modern hypothesis for the asteroid belt’s creation relates to how, in general for the Solar System, planetary formation is thought to have occurred via a process comparable to the long-standing nebular hypothesis; a cloud of interstellar dust and gas collapsed under the influence of gravity to form a rotating disc of material that then conglomerated to form the Sun and planets. During the first few million years of the Solar System's history, an