Comet Lemmon C/2012 F6: The green visitor of the southern hemisphere

Comet Lemmon with its characteristic green hue — Comet Lemmon photographed, on January 9, 2013, from the Siding Spring Observatory (Australia). Its blue-green color (due to diatomic carbon C₂) and its dust tail are clearly visible.
Image source: Rolando Ligustri / Siding Spring Observatory (new window)

Scientific summary

The Astronoo article presents comet Lemmon C/2012 F6, an icy object from the Oort cloud, famous for its green hue observed in 2012-2013. Spectroscopic observations reveal that this coloration comes from the fluorescence of diatomic carbon (C₂), excited by solar ultraviolet radiation and re-emitting at 518 nm. The sublimation of cometary ices, composed of water, CO₂ and organic compounds, generates an extended coma and dust and plasma tails. Its hyperbolic orbit suggests a single passage through the inner solar system, offering a privileged case study for understanding the composition of primitive bodies.

Why does comet Lemmon appear green in the night sky?

This article is dedicated to comet Lemmon C/2012 F6, an icy visitor to the solar system discovered in March 2012 by the Mount Lemmon Survey in Arizona. But why does this comet appear green in the night sky? The answer lies in its chemical composition. When the comet approaches the Sun, the sublimation of its ices releases molecules of diatomic carbon (C₂) which, excited by solar ultraviolet radiation, re-emit a characteristic green light at 518 nm. This fluorescence, visible in the coma, earned Lemmon the nickname “green visitor of the southern hemisphere.” In 2013, its magnitude reached 5.5, making it briefly visible to the naked eye from Australia and South America. Its hyperbolic orbit indicates that it will probably never return to the inner solar system. This article explores its discovery, its composition, the mechanisms of its color and the conditions of its observation. It also details the role of cyanogen (CN) in its spectral emissions, as well as the observation methods recommended for amateur astronomers.

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An unusual comet

A green visitor of the southern hemisphere

Comet Lemmon C/2012 F6 made an impression in 2012-2013 with its bright green color and its exceptional visibility in the southern hemisphere. Discovered by the Mount Lemmon Survey program in Arizona, it offers a perfect example of the chemical and physical phenomena that animate these icy travelers of the solar system.

What is a comet?

The composition of a cometary nucleus

A comet is an icy celestial body composed of:

Ices (water, CO₂, methane, ammonia)
Silicate dust (similar to meteorites)
Organic compounds (including diatomic carbon C₂)

The formation of the coma and tails

When a comet approaches the Sun (within less than 3-4 AU), the sublimation of ices creates:

A coma (temporary atmosphere around the nucleus)
One or more tails:
- Dust tail (curved, white/yellowish)
- Plasma tail (straight, bluish)

N.B.:
Unlike asteroids, comets have often very eccentric orbits, taking them from the confines of the solar system (Oort cloud) to the vicinity of the Sun.

The table below summarizes the main orbital and physical characteristics of comet Lemmon:

Identity card of comet Lemmon C/2012 F6
Characteristic	Value/Details
Official designation	C/2012 F6 (Lemmon)
Discovery date	March 23, 2012 (Mount Lemmon Survey, Arizona, USA)
Orbit type	Hyperbolic (non-periodic)
Perihelion	0.73 AU (March 24, 2013)
Maximum magnitude	~5.5 (January 2013)
Distinctive color	Green (C₂ emission at 518 nm)
Visibility	Southern hemisphere (December 2012 - April 2013)
Minimum distance from Earth	0.98 AU (February 5, 2013)
Discoverer	A.R. Gibbs
Notable composition	Diatomic carbon (C₂), cyanogen (CN), silicate dust

N.B.:
Comets with hyperbolic orbits like Lemmon probably come from the Oort cloud and return only once near the Sun before permanently leaving the inner solar system.

Why this green color?

The fluorescence mechanism of diatomic carbon

The emerald hue of comet Lemmon comes from a fluorescence phenomenon:

Solar ultraviolet radiation excites the diatomic carbon (C₂) molecules in the coma.
These molecules then emit light at 518 nm (green) when returning to their ground state.
Cyanogen (CN) also contributes to this coloration (emission at 388 nm, violet).

Timeline of observations

Observations of comet Lemmon extended over several months, with a peak of visibility in January 2013:

Timeline of key observations
Date	Event	Magnitude	Position
March 23, 2012	Discovered by A.R. Gibbs	20.8	-
December 2012	Becomes visible with binoculars	<10	Southern hemisphere
January 9, 2013	Peak of visibility	5.5	Sagittarius
March 24, 2013	Perihelion passage	~6	0.73 AU from the Sun
April 2013	Gradual disappearance	>10	-

N.B.:
Magnitude 5.5 corresponds to the limit of naked-eye visibility under a perfectly black sky. In the city, a magnitude of 4 or less is generally necessary for observation without an instrument.

How to observe a comet like Lemmon?

Recommended observation equipment

The choice of equipment depends on the desired level of detail:

Suitable observation equipment
Equipment type	Characteristics	Possible observations
Naked eye	Dark sky (limiting magnitude >6)	Fuzzy coma (no details)
Binoculars	10×50 or 20×80	Coma and short tail visible
Telescope	≥ 150 mm aperture	Details of the coma, dust tail
Photography	DSLR + long exposure (20-60 sec)	Extended tails, colors (with processing)

N.B.:
Swan-Band or C₂ filters can accentuate gaseous structures in photography, but reduce overall light by 30 to 50%.

Next passages of comet Lemmon

A hyperbolic orbit with no expected return

Comet Lemmon C/2012 F6 follows a hyperbolic trajectory, meaning it will probably not return to the inner solar system regularly. A periodic comet (like Halley, Period ≈ 76 years) returns regularly at precise intervals. A hyperbolic comet has an open trajectory: it can pass near the Sun one or more times, or move away permanently into interstellar space. In reality, the comet is moving away from the Sun and its trajectory does not allow a classical “period” to be defined.

N.B.:
Hyperbolic comets, unlike periodic comets, are not expected for regular close passages. Any future observation would be exceptional and unpredictable on a human scale.

Table of estimated orbital predictions for comet Lemmon

Orbital predictions for comet Lemmon
Year	Minimum distance to the Sun	Estimated distance from Earth	Remarks
2012-2013	0.73 AU	0.98 AU	Passage observed from the southern hemisphere
2025	0.72 AU (close approach to the Sun)	≈ 0.95 AU	Possible naked-eye observation in the southern hemisphere
2100 (simulation)	~5 AU	~4.8 AU	Distant passage, visibility impossible to the naked eye

N.B.:
Hyperbolic comets, unlike periodic comets, do not have regular close passages. Any future observation would be exceptional and highly uncertain on a human scale.

References

FAQ: Everything about comet Lemmon

What is comet Lemmon C/2012 F6?

Comet Lemmon C/2012 F6 is a non-periodic comet discovered on March 23, 2012 by the Mount Lemmon Survey in Arizona. It is famous for its bright green color, due to the emission of diatomic carbon (C₂) in its coma. It reached a magnitude of 5.5 in January 2013, making it briefly visible to the naked eye from the southern hemisphere.

Why is comet Lemmon green?

Its green color comes from the fluorescence of diatomic carbon (C₂). Under the effect of solar ultraviolet radiation, these molecules are excited and then re-emit light with a wavelength of 518 nm, in the green. Cyanogen (CN) also contributes to this hue through an emission in the violet at 388 nm.

When and how was comet Lemmon discovered?

It was discovered on March 23, 2012 by astronomer A.R. Gibbs as part of the Mount Lemmon Survey, in Arizona (United States). At its discovery, its magnitude was about 20.8, making it invisible without a powerful instrument. Its hyperbolic trajectory was quickly confirmed by observatories around the world.

Will comet Lemmon ever return near Earth?

Probably not. Its hyperbolic orbit means that it will not return regularly to the inner solar system. It could move away permanently into interstellar space or, in an exceptional case, return after a very long period, but no reliable prediction is possible on a human scale.

Can comet Lemmon still be observed today?

No, comet Lemmon has moved away from the Sun and Earth since 2013. Its current magnitude is far too low to be observed, even with amateur instruments. Only large professional telescopes could possibly detect it in the coming decades.

What is the difference between a periodic comet and a hyperbolic comet?

A periodic comet, like Halley's Comet (period of about 76 years), returns regularly near the Sun according to a closed elliptical orbit. A hyperbolic comet, like Lemmon, follows an open trajectory: it passes only once near the Sun before permanently leaving the inner solar system. This fundamental difference results from the origin and dynamics of each object.