When was the asteroid belt discovered?
"Discovering" the asteroid belt was not a single event, it was a long, slow process.
In 1596, Johannes Kepler predicted that there would be a planet between the orbits of Mars and Jupiter. He based this on the large gap between Mars and Jupiter, which in his opinion was too large and should not be empty.
In 1801, Giuseppe Piazzi discovered Ceres, the largest object in the asteroid belt.
In 1802, Heinrich Olbers discovered Pallas, the third largest object in the asteroid belt.
In 1802, William Herschel claimed that these two objects did not fit the definition of any known object, so he suggested a new class of object and coined the name "asteroid" to describe them.
In the following years of the 19th century, many more asteroids were found. In the early 1850s, the term "asteroid belt" gained popularity. It is unclear who first used the term.