NASA scientists have lifted the veil of secrecy over a new class of comets – they are called dark. Such comets do not have the usual tails, but they are not devoid of frozen volatile matter, which, when approaching the star, can imperceptibly evaporate from their surface and act like a jet engine. Such comets can pose a danger to the Earth because they are not visible, and their trajectories become unpredictable when approaching the Sun.
Artist’s representation of the asteroid ‘Oumuamua. Image source: ESA
Scientists first discovered such an object in March 2016. Asteroid 2003 RM deviated slightly from its trajectory. In principle, asteroids can change their trajectory due to the so-called Yarkovsky effect, when a weak reactive impulse arises from the heating of its sides. In the case of 2003 RM, the deviation was much stronger than the uneven heating of the asteroid by the Sun would allow. At the same time, it never developed a visible tail, which did not allow the asteroid to be classified as a comet.
«When you see such a deviation in the behavior of a celestial object, it usually means that it is a comet whose surface is releasing volatiles that give it a little thrust, said study co-author Davide Farnocchia of NASA’s Jet Propulsion Laboratory in Southern California. “But no matter how hard we tried, we could not detect any sign of the comet’s tail. It looked like any other asteroid—just a dot of light. So, for a short time, we had one strange celestial object whose [entity] we could not fully unravel.”
The strange behavior of asteroid 2003 RM was remembered a year later, when the interstellar asteroid Oumuamua (1I/2017 U1) burst into our system. The asteroid ‘Oumuamua accelerated so much as it approached the Sun that some saw it as an interstellar cruiser. He didn’t have a tail either. And it definitely wasn’t a comet. Later, it was suggested that the asteroid had reserves of frozen molecular hydrogen, the evaporation of which gave the object additional acceleration.
«‘Oumuamua was amazing in so many ways,” Farnocchia said. “The fact that the first object we detected [as an alien] from interstellar space exhibited behavior similar to 2003 RM made 2003 RM even more intriguing.”
By 2023, researchers had already identified seven solar system objects that looked like asteroids but behaved like comets. This was enough for the astronomical community to assign them their own category of celestial objects: “dark comets.” Today, after discovering seven more such objects, researchers have created the first classification of these objects.
After analyzing the orbits and reflectivity of 14 dark comets, scientists divided them into two classes. One class is large (hundreds or more meters in diameter) celestial bodies with highly elongated orbits. In some ways they are similar to the Jovian family of comets. The second class of dark comets are small celestial bodies up to several tens of meters or less. They have circular orbits, and they lie within the orbits of the inner planets of the system: Mercury, Venus, Earth and Mars.
Scientists are interested in dark comets as objects for the potential delivery of life to Earth from space. Fortunately, they are not large enough to threaten life already existing on our planet. On the other hand, comets are still dangerous objects, and it is better to always keep an eye on them.