Asteroids

The term 'asteroid' is usually used to refer to a minor planet of the inner Solar System, including those that have the same orbit as Jupiter. Most of them occur in the so–called 'asteroid belt', between the orbits of Mars and Jupiter.

A minor planet is an astronomical object in direct orbit around the Sun that is not a planet and is not exclusively classified as a comet. This includes the asteroids.

Ceres, Pallas, Juno and Vesta were the first four asteroids to be discovered – in that order, all between 1802 and 1807. Ceres is the largest asteroid, with a diameter of approximately 945 km (587 miles). Vesta (mean diameter 525 km, 326 miles) is the second largest, and Pallas (mean diameter 512 km) is the third largest. Juno (mean diameter 258 km) is only the tenth largest; the fourth largest is Hygiea, with a mean diameter of about 430 km, which was the tenth to be discovered (in 1849).

Hygiea is what astronomers refer to as a c–type asteroid. This is the most common variety, forming around 75% of all known asteroids. Their composition includes a high proportion of carbon, as compared to rocks and minerals, and this makes them particularly difficult to observe (technically, they have a very low albedo). They occur most frequently at the outer edge of the asteroid belt, and this obviously makes them even more difficult to observe from Earth.

For this question, 'Minor planet' was accepted as a correct answer.