Last year a serendipitous encounter took place. The European Space Agency’s Solar Orbiter flew through the tail of Comet C/0219 Y4 (ATLAS), usually known as just Comet ATLAS. This chance crossing provided unique insights into cometary tails.

The findings of the celestial rendezvous were presented at the National Astronomy Meeting 2021. The data allowed the team to model the environment of the cometary tail. The magnetic field from the solar winds – the stream of charged particles released by the Sun – appears to drape around the comet nucleus creating a weaker magnetic field in the tail.

This is very important. Comets usually have a bright and visible tail made of dust. This comes from the dust released by the comet as it approaches the Sun. The second tail is often fainter and is known as the ion tail. Its formation is due to the interaction between gas from the comets and the particles in the solar wind.

To add even more uniqueness to the event, by the time Solar Orbiter encountered the space body, Comet ATLAS was already fragmented. It broke apart into about 30 pieces, each the size of a house. Solar Orbiter crossed the tail and was able to measure the changes in magnetic field from the solar wind, into the comet’s tail, and then back into the solar wind.

Solar Orbiter is the sixth spacecraft to have flown through the tail of a comet but it is the first to do so within the orbit of Venus. It is also one of the few to conduct direct measurement from a fragmented comet.

“This is quite a unique event, and an exciting opportunity for us to study the makeup and structure of comet tails in unprecedented detail,” lead author Lorenzo Matteini, a solar physicist at Imperial College London, said in a statement. “Hopefully with the Parker Solar Probe and Solar Orbiter now orbiting the Sun closer than ever before, these events may become much more common in future!”

Solar Orbiter and NASA’s Parker Solar Probe are orbiting the Sun in very different orbits to deliver diverse but complementary observations of the Sun, including the closest data from our star as well as the first observation of the Sun’s polar regions.

That said, work such as this shows just how versatile such spacecraft can be, expanding our knowledge of the heavens beyond what their official missions entailed. Solar Orbiter’s science mission begins in earnest in November of this year.