We’ve all seen those gorgeous pictures of comets streaking by Earth, resplendent with tails of gas and dust flowing potentially millions of miles behind. But have you ever seen a tail without a comet?
Scientists recently had the rare chance to study the disembodied tail of Comet C/2019 Y4 (ATLAS), a comet that everyone hoped would turn into a beautiful naked-eye opportunity for amateurs. But those hopes were literally shattered in April 2020 when the comet strayed too close to the sun and was torn to pieces.
While the nucleus or heart of the comet dissipated, however, the tail was still intact enough that a European Space Agency mission got to do some rare observations. Solar Orbiter, whose primary mission is to better study the sun and its “space weather”, made a chance flypast of the tail, allowing the astronomers to peek at the remnants.
Such studies are important because they can tell us more about the origin story of the solar system. Billions of years ago, before the planets and moons grew to the size we see today, our solar system was a scattering of small rocks and icy objects. Comets thus represent a chance to see what our neighborhood was made of shortly after it was born, and reveal a lot about how much the environs changed in the ensuing eras.
Comets usually have two tails. The one that comets display to casually observing Earthlings is a dust tail, which reflects a lot of light and can be bright if the comet gets close enough to our planet. (Such close approaches are rare, so it’s best to invest in a moderate telescope if you want to spot comets on the regular, though.) The other, less visible tail is the ion tail, which is the set of charged particles that happen due to interactions between the gas coming off the comet and another set of irradiated bits coming from the sun’s “solar wind” that blows through the solar system.
The observing team discovered a weak magnetic field around the comet tail. This happened when the solar wind, carrying a magnetic field with it due to its cloud of charged particles, “draped” around the zombie tail, researchers said in a release.
“When the solar wind interacts with a solid obstacle, like a comet, its magnetic field is thought to bend and ‘drape’ around it,” the Royal Astronomical Society stated. “The simultaneous presence of magnetic field draping, and cometary ions released by the melting of the icy nucleus, then produces the characteristic second ion tail — which can extend for large distances downstream from the comet’s nucleus.”
The rare encounter will help scientists better understand the formation and structure of the ion tail of comets, and luckily more opportunities may sprout up in the future. There’s a new fleet of spacecraft set to operate close to the sun in the coming decade, with several targeting Venus and two prominent sungazing ones (ESA’s Solar Orbiter and NASA’s Parker Solar Probe) regularly flying through the neighborhood, too. ESA stated that with so many spacecraft in this zone, we’ll likely have more chances to look at the tails of comets straying close to the sun to learn more about their origin story.
The work was led by Lorenzo Matteini, a solar physicist at Imperial College London, and presented at the United Kingdom’s National Astronomy Meeting 2021.