Asteroids and Comets

Rocky asteroids and icy comets are leftovers from the formation of the Solar System.

Comet NEOWISE with its faint tail in the dark sky before sunrise.
Comet NEOWISE Over Washington. Credit: NASA/Bill Ingalls. Source, NASA media guidelines.

Asteroids and comets are small bodies left over from the formation of the Solar System, about 4.6 billion years ago. Studying them tells us what the building blocks of the planets were like.

Asteroids

Asteroids are rocky or metallic bodies, from a few metres to hundreds of kilometres across. Most of them orbit the Sun in the main belt between Mars and Jupiter.

  • In 2022 NASA's DART spacecraft hit the small asteroid Dimorphos and changed its orbit, the first test of planetary defence.
  • In 2023 the OSIRIS-REx mission delivered to Earth a sample collected from the asteroid Bennu.

Comets

A comet is a "dirty snowball" of ice, dust and rock. When it comes close to the Sun, the ice turns into gas and forms a glowing head and long tails that always point away from the Sun.

Comet NEOWISE, in the image above, was discovered in March 2020 and was one of the brightest comets visible from the northern hemisphere in decades.