By Radmila Topalovic
Comet 3I/ATLAS taken in August 2025 by the Gemini South Telescope in Chile.
By International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=174444353
So much happens on Earth it’s easy to forget everything that happens around our planet. We can see five planets with the naked eye, they are constantly in motion and move in and out of our night sky. The farthest two – Uranus and Neptune, the icy giants, are too faint and require binoculars or a telescope. Beyond Neptune (more than 30 times the distance between the Earth and the Sun) lies the Kuiper Belt, a doughnut shaped region of icy rocks, including our beloved little Pluto, now a dwarf planet. The frosting on the doughnut is provided by the vast Oort Cloud, a ball of trillions of icy objects stretching out a light-year from the Sun. This is the true edge of the Solar System, beyond which the Sun’s gravity is too weak to hold onto anything else.
Comets are rocky and icy bodies that get knocked out of the Kuiper Belt or Oort Cloud and are sent hurtling through the Solar System. They may end up in an elongated (eccentric) orbit around the Sun, they could be highly inclined relative to the flat pancake-like plane of our planets and if we’re lucky we may even see them in the night sky. I remember spotting Comet Hale-Bopp in 1997 in the light-polluted skies of Birmingham, it was around that time I was applying to study astrophysics at university, if comets are an omen this one was an awesome life enhancer! I loved it, it looked weird – I’d never seen anything like it before, it was fuzzy with a long tail, absolutely gorgeous. I felt special having seen it, a rare viewing of an unexpected guest having travelled from its home in the Oort Cloud 50 000 million kilometres away. It will return to our skies but not for another 2400 years, I’m so pleased I saw it.
Whatever your thoughts are about unexpected guests, get used to them because we get visitors from outer space! The rock currently in the news is a comet called 3I/ATLAS, first seen in July this year. Comets usually come from our Kuiper Belt or Oort cloud, this comet comes from beyond the Solar System, the “I” in its name stands for “interstellar”. The “3” is even more interesting, this alien object is the third comet from interstellar space, following the cigar-shaped 1I/’Oumuamua (Hawaiian for ‘first distant messenger’) seen in 2017 and 2I/Borisov in 2019. Does this mean we have had only three visitors? Of course not, however detecting these objects is a challenge and it is getting easier with the continual advancement in telescopes and imaging.
3I/ATLAS could be up to 5 and a half km across, however at its closest approach to Earth it will lie 240 million km away and it will therefore be too faint to see without a telescope. It will also lie behind the Sun at that point but will reappear for another sighting in late November.
The Sun’s energy is warming the surface, the ices are transforming into gases, creating a hazy coma around the comet. When this happens astrophysicists can forensically analyse the comet and deduce what lies on the surface. This provides us with precious insight into the comet’s stellar system home, before it left to pop over and say hello to us.
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