Asteroid A small object that orbits the Sun. Asteroids range in size from 1 metre (3.28 ft) across to roughly spherical objects about 1000 km (600 miles) in diameter. Most asteroids are made of rock, some are made of metal and some are made of both rock and metal. 

Asteroid Belt The region between Mars and Jupiter where most of the known asteroids in the Solar System lie.

Bolide An unusually bright meteor that explodes in the sky.

Centaurs A group of small objects made of rock and ice that orbit the Sun between the planets Jupiter and Neptune.

Ceres The smallest known dwarf planet in the Solar System and the only one in the asteroid belt. Ceres measures just over 1000 km (600 miles) across.

Chelyabinsk meteor The Chelyabinsk meteor was a large meteoroid (or small asteroid) that entered Earth's atmosphere over the southern Ural region of Russia on 15th February 2013.  The light from the meteor (classified as a superbolide) was briefly brighter than the Sun. Approximately 18 m (59 ft) in diameter, it exploded about 30 km (20 miles) above ground, in what is known as a meteor air burst, generating a bright flash and a powerful shock wave. Nearly 1500 people were injured. 

Coma The cloud of dust and gas that surrounds a comet as it nears the Sun. The Sun melts the frozen gases and water that make up the comet’s nucleus. A combination of sunlight and the solar wind then “blow” the gases back into a pair of tails.

Comet A lump of dust and ice that orbits the Sun. On nearing the Sun a comet develops two tails of gas and dust. A cometary nucleus may measure up to a few kilometres across. Their tails stretch behind them for hundreds of millions of kilometres.

Dwarf planet A relatively large, round object that orbits the Sun. The orbits of dwarf planets are littered with rubble. True planets have sufficient mass (and therefore gravity) to sweep clear their orbits of rubble.

Eris The second largest known dwarf planet in the Solar System, after Pluto. Eris measures 3000 km (about 2000 miles) across. Located far beyond Pluto, it is one of the most distant large objects in the Solar System.

Giotto Probe A European space probe that flew by Halley’s Comet in 1986, sending back pictures and sampling the gases and dusts given off by the comet.

Halley’s Comet A bright comet that appears in our skies every 76 years.

Halley, Edmond (1656–1742) English astronomer who was the first person to realize that comets are objects that orbit the Sun.

Hayabusa2 A space probe launched by Japan Aerospace Exploration Agency (JAXA) which landed two robotic rovers on asteroid Ryugu on 27th June 2018. 

Kuiper Belt A region of the Solar System beyond the planet Neptune. It consists of icy bodies, including the dwarf planet Pluto. The Kuiper Belt is thought to be about 20 times as wide as the Asteroid Belt.

Meteor A streak of light in the sky that occurs when a meteoroid enters the Earth’s atmosphere and becomes very hot. Meteors are sometimes called shooting stars.

Meteorite A meteoroid that lands on the surface of a planet or moon. Meteorites come in three types: stony, iron and stony-iron

Meteoroid A small lump of rock that hurtles across the Solar System. It was once part of an asteroid, comet, moon or planet.

Meteor shower An event that occurs when a large number of meteors appear in a short time. The Perseid meteor shower occurs every year between 9th and 13th August when Comet Swift–Tuttle crosses the Earth's orbit.

NEOs (Near-Earth Objects) An asteroid, comet or large meteoroid whose orbit brings it close to the Earth.

Oort Cloud A cloud of comets surrounding the Solar System beyond Pluto. Astronomers believe that the Oort Cloud may be the place where some comets come from. Its outer edges are about a quarter of the distance between our Solar System and the nearest star, Proxima Centauri.

Pluto Previously thought to be the ninth planet, Pluto is now classified as a dwarf planet. It measures on average 2274 km (1412 miles) across and is the largest known object in the Kuiper Belt.

Rosetta mission A mission launched, by the European Space Agency (ESA) in 2004, in which a space probe, called Rosetta, carried a smaller spacecraft, Philae, which landed on the surface of Comet 67P/Churyumov–Gerasimenko in May 2014.

Solar System The Sun and all of the objects that orbit (go round) it: the eight planets, their moons, dwarf planets, asteroids, comets and meteoroids, and vast amounts of gas and dust. The Sun’s mass, compared to the rest of the Solar System, gives it the gravitational pull that keeps these objects in orbit around it. The planets all orbit the Sun in an anticlockwise direction (if viewed from above the Sun's north pole), following elliptical (oval) paths, rather than circular ones.

Stardust space probe A US space probe that captured dust grains, or “stardust”, floating in space, as well as from a comet.

Trojan An asteroid that shares the same orbital path as Jupiter. They are named after characters from the Trojan War of Greek mythology.

Tunguska fireball The name given to the huge explosion in the Tunguska region of Siberia, Russia on 30th June 1908. The Tunguska fireball was probably a large meteoroid (or small asteroid or comet) exploding at an altitude of about 6 km (about 4 miles), a phenomenon known as a meteor air burst.

Consultant: Mike Goldsmith