International Space Agencies – Missions and Discoveries

Green Comet appears close to Earth after 50,000 years


From UPSC perspective, the following things are important :

Prelims level: Green Comet

Mains level: Not Much


The rare green comet that last came to Earth about 50,000 years ago has returned to the skies of Earth. C/2022 E3 (ZTF) can be seen with the naked eye if the conditions in the sky are just right.

What are Comets?

  • Comets are frozen rocky or gas-filled objects that are remnants of the formation of the solar system.
  • Due to their composition, characteristics and the path they move in, they tend to leave a light “behind them”.
  • Here, the comet itself is green (called the head of the comet) and emits a whitish light behind it (often called the tail of the comet).
  • Just like other bodies in space, comets also have orbits.
  • They are sometimes pulled in close to the sun because of the sun’s gravity acting on them.
  • As they orbit near the Sun, they heat up and spew gases and dust into a glowing head that can be larger than a planet.
  • The remains of dust following this burning up, from a distance, look like a trail of light to humans on Earth.

What is Green Comet C/2022 E3 (ZTF)?

  • Comet C/2022 E3 (ZTF) was first discovered in March last year by the wide-field survey camera at the Zwicky Transient Facility when it was already inside the orbit of Jupiter.
  • While it was initially believed to be an asteroid, it began developing a tail as the Sun’s influence began vapourising the ice.
  • At the time of its discovery, it was shining with a magnitude of 17.3.

Why is it green in colour?

  • Comets have often been seen giving out blue or whitish light, or even green.
  • In this case, the green glow “is thought to arise from the presence of diatomic carbon – pairs of carbon atoms that are bound together – in the head of the comet.
  • The molecule emits green light when excited by the ultraviolet rays in solar radiation.

When and where can the green comet be seen?

  • Observers in the Northern Hemisphere will find the comet in the morning sky, as it moves swiftly toward the northwest during January.
  • It’ll become visible in the Southern Hemisphere in early February.
  • In Indian skies, when looking in the northwest direction, one might spot it 16° above the horizon in the Bootes constellation.
  • But with lights from buildings and streetlights on, it can be difficult to make it out without equipment.

Is the green comet rare?      

  • It last came in the skies above Earth during the Upper Paleolithic period, a time when Neanderthals roamed the planet and early homo sapiens had just come around.
  • Coming under the category of long-period comets, which take more than 200 years to orbit the Sun, the green comet is not easily spotted.
  • With a highly elliptical orbit, the comet will head back to the Oort cloud and make its next appearance roughly 50,000 years later.
  • But given their orbits, it’s not unique for comets to reappear close to Earth only after many, many years.


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