Earth is about to have a close encounter with a comet, possibly passing through its ion tail 鈥� a rare occurrence. If we are lucky, it could show up as a blue streak in the northern hemisphere night sky.
at University College London and at the European Space Agency say there is a good chance that tonight, 10 October, our planet will intersect the path of the ion tail of comet C/2023 A3 (Tsuchinshan-ATLAS). 鈥淚t鈥檚 a particularly unusual situation,鈥� says Grant. 鈥淚t鈥檚 certainly the first time in the last 30 to 40 years this has happened to Earth.鈥�
Most comets have two main tails. The first is a relatively short dust tail 鈥� debris that falls away from the comet as it orbits the sun 鈥� which can produce meteor showers as the small grains enter our atmosphere should Earth pass through this material. For example, the Perseid meteor shower, which appears every August, is the result of debris from Comet Swift-Tuttle.
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The second type is an ion tail, made up of charged molecules that are pushed away from the comet by the solar wind. Despite extending for hundreds of millions of kilometres, these tails are narrow in the grand scheme of space, just a few million kilometres wide, making encounters with them rare.
Modelling the motion of Earth and Tsuchinshan-ATLAS, the pair predict we will fly through its ion tail overnight. Spacecraft near Earth may be able detect ions from the comet which, if they can be identified, would allow us to work out its composition. 鈥淲e can say if there鈥檚 water or amino acids,鈥� says Grant. 鈥淚t鈥檚 basically a free sample of the comet.鈥�
The ions could also have an impact on our planet. 鈥淲e might see a sudden increase in the auroral density, but it will be very brief,鈥� says Grant, as well as geomagnetic effects, something seen in an encounter with the ion tail of Halley鈥檚 comet in 1910.
What might be more apparent is the ion tail itself, which could appear as a wispy blue streak in the sky extending away from the comet. 鈥淚f you have low enough light pollution, you might see this ion tail stretching from the west to the north across the sky,鈥� says Grant. 鈥淎nyone in the northern hemisphere should have a chance.鈥�
If the encounter happens as expected, the tail will be visible long after tonight鈥檚 encounter, probably until the end of October. Data from a NASA spacecraft called STEREO-A 鈥� which has already got close to the comet 鈥� suggests 鈥渢here is a nice strong ion tail present鈥�, says Grant. 鈥淚 always play this stuff down. But this does look good.鈥�
arXiv
