Scientists are busy studying comet 3I/ATLAS, which has hogged the headlines because of its unusual characteristics. What’s even more fascinating is its interstellar origin and astronomers want to use this opportunity to learn about the environment outside our solar system. But what does ‘interstellar’ even mean and why is 3I/ATLAS more special than comets that we usually see?
What is an interstellar comet?
First things first. 3I/ATLAS is the third known interstellar comet [thus 3I] and it’s named after the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope which discovered it. Now, the word ‘interstellar’ means space between the stars, and it’s associated with an object which formed outside our solar system. So naturally, an interstellar comet is a comet that originated elsewhere in the universe and is just passing through.
The solar system’s boundary is marked where the heliosphere ends. Heliosphere is a giant bubble around everything in our solar system – the planets, asteroids, comets – and it’s created by the solar wind flowing away from the Sun. NASA explains it as the region of space under the influence of our Sun. Anything outside this region is considered a part of interstellar space.
The Voyager 1 and 2 probes, which launched in 1977, became the first man-made objects to enter interstellar space when they escaped the heliosphere in 2012 and 2018, respectively.
How do we know 3I/ATLAS is an interstellar comet?
Astronomers determine whether an object is interstellar by studying its trajectory. Normal comets follow elliptical orbits that are tied to the Sun whereas interstellar comets travel on hyperbolic paths, meaning our solar system is just a landmark in their journey and they will never return.
Their hyperbolic trajectories are identified by calculating the comet’s eccentricity and excess speed. Interstellar comets move too fast to be gravitationally bound to the Sun and this is a key indicator of their origin. These characteristics have helped scientists discover three interstellar comets so far- 1I/Oumuamua, 2I/Borisov and the latest 3I/ATLAS.
Why is 3I/ATLAS special?
Interstellar comets such as 3I/ATLAS are special because they act like time capsules, carrying material from the earliest stages of their star systems. Besides, their chemistry is totally different from native comets such as Comet Lemmon and hint at chemical environments very different from ours.
For instance, 2I/Borisov reportedly had unusually high levels of carbon monoxide compared to typical solar system comets whereas 3I/ATLAS is dominated by carbon dioxide and has a very high concentration of nickel. The latter’s age is also estimated at around seven billion years, making it significantly older than the Sun. It is also said to have many anomalies including a lack of cometary tail despite reaching perihelion (closest distance to the Sun) on October 30.
These outsiders are natural probes of other planetary systems and they offer limited time to study how organic chemistry vary across the universe.
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