Comets are ancient cosmic icebergs. They are roughly 4.6 billion years old and formed at the same time as the Sun, Earth and the other planets.
Gravitational interactions fling them towards the inner planets from out beyond the orbit of Neptune, making them some of the most chaotic and dynamically interesting objects in the Solar System.
A comet’s nucleus is typically between 1 and 50 km across with the structure of a dirty snowball. They are made of dust and ice, which partly goes from solid to gas when the comet is warmed by the Sun. Cometary and asteroid impacts may be responsible for a fraction of the water present on Earth and for the icy reservoirs that are thought to exist at the bottom of permanently shadowed craters on the Moon.
The defining characteristic of a comet is its tails. Tails come in two main types, a dust tail and an ion (or plasma) tail. When sunlight warms the ice on a comet, gas and dust are released into space.
The dust falls into orbit around the comet and is gently pushed into fan-shaped tail by incoming sunlight.
The gas is ionised by ultraviolet sunlight and the particles of the solar wind, and these charged particles (ions) form a narrow ion tail that is pushed directly away from the Sun by the solar wind.
Comet tails are the reason for Earth’s annual meteor showers, during which Earth passes through clouds of dust left behind by comets.
Comet 12P/Pons-Brooks is a ‘near-Earth comet’, which means it orbits the Sun in less than 200 years and comes close enough to the Sun that it could cross Earth’s orbit. ESA’s Planetary Defence Office keeps a close eye on near-Earth comets to monitor any risk of collision.
Near-Earth comets are much rarer than near-Earth asteroids – their less icy cousins. We know of only 122 near-Earth comets, but of almost 35 000 near-Earth asteroids.
ESA missions to comets and asteroids
Seeing comets from afar can be beautiful, but to really get to know them, you have to study them up close.
ESA has developed and flown a number of spacecraft to comets and asteroids to improve our understanding of the formation and evolution of the Solar System, the role of these objects in delivering the ingredients of life to Earth, how comet activity works, and the risks these fast-moving space rocks pose to our planet.
Giotto
Giotto was ESA’s first deep-space mission. In 1986, it passed closest to the nucleus of comet Halley. Its images showed for the first time the shape of a comet nucleus and found the first evidence of organic material in a comet. In 1992, after a long cruise through space, Giotto was directed to Comet Grigg–Skjellerup, which it passed just 200 km from the nucleus.
Rosetta
Rosetta was the first spacecraft to rendezvous with a comet when it arrived at 67P/ Churyumov–Gerasimenko on 6 August 2014 and entered orbit around it. It was the first spacecraft to follow a comet on its journey around the Sun and measure the increase in activity as the icy surface warmed up. On 12 November 2024, Rosetta’s Philae probe became the first to land on the surface of a comet. On the way, it also flew by two asteroids, Steins and Lutetia.
Hera
Launching later this year, the Hera mission is part of the world’s first test of asteroid deflection. The main spacecraft and its two CubeSats will perform a detailed post-impact survey of the asteroid Dimorphos following the impact of NASA’s DART mission in September 2022. Hera will turn the grand-scale experiment into a well-understood and repeatable planetary defence technique.
Mission to Apophis
Meanwhile, ESA’s Planetary Defence Office is also considering a number of options for a mission to asteroid Apophis. In April 2029, Apophis will come closer to Earth than our geostationary satellites and will be visible to the naked eye. The trajectory of Apophis is very well known, and it poses no risk to Earth, but it offers a unique chance to study such a large asteroid up close and better prepare for one that is.
Comet Interceptor
Looking further ahead, ESA’s future Comet Interceptor will build on the findings of Giotto and Rosetta. Comet 12P/Pons-Brooks is a returning comet that has made a number of previous appearances over the centuries. Launching in 2029, the unique aspect of the Comet Interceptor mission is that it will target a newly discovered comet entering the inner Solar System for the first time. Such a ‘pristine’ comet would carry material unaltered since the formation of the planets.
Special mention: SOHO
While not a dedicated comet mission, no list of prolific comet observers would be complete without mentioning the ESA/NASA Solar Heliospheric Observatory (SOHO). SOHO is used to monitor the unpredictable activity of the Sun, but previously undiscovered comets often pass through its field of view on the way to their fiery demise. While Pons and Brooks still hold the top spots for visual discoveries, we now spot most new comets using large telescopes either on Earth or on satellites in orbit. SOHO has spotted almost 5000 comets.
Source: European Space Agency
