Were you to ask anyone to give a simple, off-the-cuff description of a comet, the likely response would be something like “….a brightly glowing ball of ice and dust sporting a long tail streaming out behind it as it sweeps gracefully across the night sky”.
While this is a fairly good description on a superficial level, omitting, as it does, any reference of the comet’s nucleus and coma, it turns out it is not the only description of a “comet”. What if I were to tell you that there is now a category of celestial objects referred to as “dark comets”. Intrigued? I certainly was, so I delved into this topic further.
What are Dark Comets?
Dark comets are a new classification of celestial objects that look like asteroids, but behave and move through space much like comets. It all began when “asteroid” 2003 RM was discovered by NASA’s Near-Earth Asteroid Tracking (NEAT) program on Sept. 2, 2003.
Additional optical observations of the object (visible only every 5 years) were made in 2003, 2008, 2013, and 2018. A 2016 study revealed that changes in the observed plane-of-sky positions and acceleration of 2003 RM were inconsistent with perturbations typical of asteroids.
The trajectories of comets are typically affected by non-gravitational forces due to the sublimation (the direct conversion of a solid to a gas using heat) and the outgassing of volatiles. The motion of asteroids, on the other hand, are primarily driven by gravitational forces.
Asteroids are also subject to non-gravitational forces such as solar radiation pressure and the Yarkovsky effect, but these effects are much weaker, by several magnitudes, than forces induced by outgassing.
The Yarkovsky effect
The Yarkovsky effect, named after Polish-Russian civil engineer Ivan Osipovich Yarkovsky (1844 – 1902), is the force produced by the manner in which one side of an asteroid absorbs energy from the Sun during the daytime, and re-radiates it back into space as heat at night; the unbalanced thermal radiation, by way of the emission of thermal photons, adds a small amount of acceleration to the asteroid, propelling it through space, albeit at a very slow rate.
Over time, tens of millions of years, this assisted acceleration can significantly alter an asteroid’s trajectory, causing some of the asteroids in the main asteroid belt between Mars and Jupiter to become near-Earth objects, and potential threats to our planet. Astronomers estimate that between 0.5% and 60% of all near-Earth objects could be dark comets from the main asteroid belt.
The astronomers theorized that 2003 RM’s observed atypical accelerations and slight deviations from its expected orbit were the result of unseen cometary outgassing. However, as they could not find, and still have not, to date, found any visual evidence of such activity, 2003 RM currently retains its classification of an asteroid.
The Oumuamua (1l/2017 U1) and Other Discoveries
In 2017, the discovery of the celestial object Oumuamua (1l/2017 U1), the first documented interstellar object to pass through the inner solar system, only added to the conundrum of these unexplainable objects.
As with 2003 RM, the astrometric positions of Oumuamua, which had appeared as a single point of light, typical of an asteroid, changed, and the object rapidly accelerated, as if due to the comet-like outgassing of volatile material from its surface.
However, again, like 2003 RM, no comet-like coma or outgassing activity was detected around Oumauma. You may remember the media hype as to whether or not this strange object might have been an alien spacecraft.
By 2023, 12 additional similar objects had been discovered, bringing the total to 14, enough to generate the new celestial object classification of “dark comets”. By analyzing the reflectivity or albedo of these objects, plus plotting their orbits, astronomers have been able to divide them into two different types.
The first are referred to as “outer dark comets” – comets which have similar characteristics to the Jupiter-family of comets (comets with orbital periods of less than 20 years, passages modified by Jupiter, highly eccentric or elliptical orbits; and which are very large in size (hundreds of metres in diameter).
The second type are referred to as “inner dark comets” – comets that reside in the inner solar system (Mercury, Venus, Earth, and Mars), travel in nearly circular orbits, and are fairly small (only tens of metres or less in diameter).
Concluding Thoughts
Dark comets are of great interest to astronomers and other earth scientists, as it is theorized that they may have delivered volatiles and organic materials, and possibly water, to the inner solar system billions of years ago, possibly leading to the development of life on our planet.
Many questions remain about these enigmatic celestial objects – Where do they originate? What causes their atypical accelerations? Do they contain ice and/or water? Are they responsible for life on earth? Perhaps future space missions to one or more of these dark comets may, one day, provide the answers.
This Week’s Visibility
Mercury (mag. +0.8, in Scorpius – the Scorpion) rises around 6 a.m. this coming week, visible when it reaches 10 degrees above the southeast horizon, before fading as dawn breaks around 7:20 a.m. Look for bright Mercury to the upper left of Antares, the brightest star in Scorpius – the Scorpion, low on the southeast horizon around 7 a.m. on Dec 23. On the 25th, Mercury reaches its greatest elongation (visual angle of separation as seen from Earth) 22 degrees west of the Sun.
Venus (mag. -4.2, in Capricornus – the Sea Goat) becomes visible around 4:50 p.m., 23 degrees above the southern horizon as dusk fades to dark, before sinking towards the horizon and setting around 8:10 p.m.
Mars (mag. -0.8, in Cancer – the Crab) becomes visible 7 degrees above the east-northeast horizon around 7:40 p.m., reaching 66 degrees above the southern horizon around 2:30 a.m., before getting lost in the dawn twilight around 7:30 a.m., 26 degrees above the western horizon.
Jupiter (mag. -2.8, in Taurus – the Bull) becomes accessible in the eastern evening sky 15 degrees above the horizon around 4:50 p.m., reaching 65 degrees above the southern horizon around 10:50 p.m., before becoming inaccessible when it sinks below 7 degrees above the northwest horizon by about 5:30 a.m.
Saturn (mag. +1.0, in Aquarius – the Water-bearer) becomes accessible about 5:15 p.m., 35 degrees above the southern horizon as dusk yields to darkness, its highest point of the evening, before sinking towards the horizon and setting by about 10:30 p.m.
Uranus (mag. +5.6, in Taurus) becomes accessible 37 degrees above the eastern horizon by about 5:50 p.m., reaching a height of 62 degrees above the southern horizon by about 9:25 p.m., before becoming inaccessible around 2;35 a.m. a.m., when it drops below 21 degrees above the western horizon.
Neptune (mag. +7.9, in Pisces – the Fish) becomes accessible by about 5:50 p.m., 41 degrees above the southern horizon, its highest point of the evening, and remaining observable until about 9:25 p.m., when it drops below 21 degrees above the southwest horizon.
Until next week, Merry Christmas and Happy Holidays, and, as always, clear skies.
Events:
Dec. 23 – Mercury to upper left of Antares, in Scorpius; just above SE horizon; 7 a.m.
Dec 25 – Waning crescent Moon to lower left of Spica, in Virgo; SE; just before sunrise
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