What is the green comet bedazzling the night for the first time since Stone Age

Nearly five to six comets are discovered by professional astronomers every month. The number is increasing as the instruments and detecting software become more advanced. For instance, in 2019, 66 comets were discovered. This number was 73 in 2020, 105 in 2021 and 76 in 2022. However, few were bright enough to be visible to the naked eye or with amateur binoculars. The green comet — comet C/2022 E3 (ZTF) — is one of those rare comets that bedazzle the night sky. Further, the comet is exotic with its greenish glow.

What are comets?

Comets are ‘dirty ice balls’ composed of small rocks, dust, ice and frozen volatile substances like dry ice (solid CO2), methane, ammonia, and carbon monoxide (CO). Just like the remnants of building material strewn around a construction site even after its completion, astronomers believe that the icy rocks in the Kuiper belt are leftovers from the solar system’s origin. Further, cometary objects in the Oort cloud are remains of the material from which the Sun evolved. Hence by exploring comets, astronomers uncover the secrets of the formation and evolution of the Sun and the solar system.

Where do comets come from?

The light coming out of the Sun, travelling at a breakneck speed of 3 lakhs kilometres per second, takes little more than eight minutes to reach Earth and about 4.5 hours to arrive at Neptune. There is a Kuiper Belt region beyond Neptune and Pluto, home to short-period comets. Light from the Sun takes 7 to 8 hours to reach this region. Far beyond into space is a cloud that envelopes the solar system called the Oort cloud, which is home to long-period comets like the Green Comet C/2022 E3. Light from the Sun takes 10 to 28 days to reach the inner edge of the Oort cloud and may take about a-year- and-a-half to reach the outer edge. Like all other solar system objects, all cometary objects orbit the Sun with orbital periods ranging from hundreds to thousands of years. Once in many years, a cometary object visits the inner solar system, passing close to Earth and making a celestial show in the night sky.

Why do the comets have tails?

When the comets are home, lodged in the Kuiper belt or Oort clouds, they sport no tails. The tails appear only when they nose-dive inward towards the Sun. When they cross the orbit of Jupiter, the ultraviolet rays from the Sun are powerful to ionise the molecules such as carbon monoxide. Due to ionisation, the molecules lose an electron and become CO+ ions. These ions stream away from the Sun and turn into a narrow blue ion tail. This is the first tail that astronomers can notice with a powerful telescope, even when the comet is near Saturn or Jupiter’s orbit.

As the comet gets closer, nearing the orbit of Mars, the sunlight begins to warm up the comet. The ice melts, and dust particles are energised. The melted ice’s gas and dust surround the cometary core like a haze. Sunlight reflected by this haze makes it appear brighter. This mixture of gas and diffuse dust region is known as the coma of a comet. The rays of the Sun smack the coma and propel the dust and gas particles away from the Sun, resulting in a second tail, usually white or yellow. Since some of the dust particles are small, some big, the energy from the sunlight drives them at different speeds. The lighter ones whizz past while the heavier one moves sluggishly. This makes the dust fan-shaped.

Once the comet goes behind the Sun, gravity slings it back outward. The size of the tail decreases as the distance between the comet and the Sun increase in its outward journey.

Why does it have the mysterious third tail?

Two tails of the comets are well known and, in fact, anticipated. However, an enigmatic third tail appeared in the images of Green Comet C2022 E3 taken around January 21, baffling the observers. The bizarre third tail was flowing in front of, instead of behind, the comet, adding mystery to the third tail. The astronomers call the third tail ‘anti-tail’ as its orientation appears away from the other two tails. The anti-tails are an optical illusion. Dust and gas from the comet’s tail are scattered along its path. When the Earth moves through this path, Sun’s rays illuminate these and make them glow. The materials that make the anti-tail are far from the comet but viewed from an angle from the Earth, they appear to be coming out of the comet.

Why this comet glows in green?

The reason behind the exotic green colour of the comet is the abundant presence of carbon and cyanogen in the core of this comet. The carbon molecules come in many forms, from simple graphite to diamond. The comet has a substantial amount of a rare type of carbon called diatomic carbon, which consists of two carbon atoms bound together. Just like the chlorophyll in the plant leaf absorb all the colours of VIBGYOR of sunlight except Green, making the leaf appear green. Likewise, the diatomic carbon and the cyanogen in the coma and the comet’s tail reflect the green light making it glow green.

How newly discovered comets are named?

The comet C/2022 E3 (ZTF) was discovered by the Zwicky Transient Facility on March 2, 2022.

The first letter of the name indicates the main character of the solar system object. If the value is P/, then the comet is periodic. Its orbital velocity is less than the escape velocity of the Sun, and the comet will make a return visit to the inner solar system in future. If the value is C/, then the comet is non-periodic. In this case, the comet’s velocity is more than the escape velocity. Hence, the comet may never return or return after thousands of years.

The second part of the name is the year when the comet was discovered; in this instance, the comet was discovered in 2022. The third part has a combination of a character and a number. The symbols indicate the fortnight in which the comet was discovered. Comets discovered between January 1 to 15 are assigned A; those found between January 16 and 31 are B; those located between February 1 and 15 are C, and so on. Therefore in this instance, E indicates that it was discovered between March 1 and 15. Further, the number ‘3’ indicates that it was the third comet to be found during this fortnightly period. ZTF stands for Zwicky Transient Facility, which made the discovery.

Where is the comet heading?

From the comet’s orbit, astronomers computed that comet C2022 E3 must have commenced its journey from the Oort cloud. Further, the path indicates that the comet must have passed close to Earth about 50,000 years ago. Perhaps the stone age culture viewed this comet; we may never know. The current velocity of the comet is more than the crucial escape velocity of the Sun. This means after swinging behind the Sun on January 12, 2023, the comet is speeding away from the solar system. If this is unchecked in tens of thousands of years, the comet will leave the solar system and escape into interstellar space. Nevertheless, the gravitational tugs from the planets orbiting the Sun could pull from behind and break its speed, keeping it within the confines of the solar system. Only time will tell.

How can I observe it?

The comet will become brighter as it comes near the Earth while orbiting in its path. On February 2, 2023 — which is today — the comet will make the closest approach to the Earth. At that time, it would be 42 million kilometres from where the light coming out of the comet will take just 2.5 light minutes to reach Earth. During these nights, experts anticipate the brightness may increase, and it may become a magnitude five object. Stellar objects of magnitude five would be visible to the naked eye in dark skies. Therefore, it is imperative to choose a spot without light pollution.

During the end of January and the first week of February, the comet’s path is very close to the pole star. Therefore, we need to identify a location with an unhindered view of the northern horizon.

Dust, low-hanging clouds often mar the view of the faint stellar objects close to the horizon. The comet would rise high around midnight when it would subtend around 25 degrees. Therefore, it would be wise to observe the comet around midnight.

Once you are in such a position, look around the pole star and spot the faint glow of green light. That is the comet. With the aid of a binocular, we can see its dust tail.