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“Dark comets” could make up 60 percent of near-Earth objects

“Dark comets” could make up 60 percent of near-Earth objects
“Dark comets” could make up 60 percent of near-Earth objects

According to a new study, up to 60 percent of near-Earth objects could be “dark comets,” a strange class of objects that have characteristics of both asteroids and comets.

Comets are remnants of dust, ice and rock left over from the formation of the solar system. They are typically 1 to 10 kilometers in diameter and tend to orbit the sun in long orbits. They are much easier to spot than asteroids because of their tails, which can stretch for tens of millions of kilometers.

As comets get closer to the Sun in their orbit and heat up, they outgas and lose gas and later (when they are even closer to the Sun) dust, which forms their characteristic tail trail or coma. This outgassing acts like a thruster, slightly changing the comet’s trajectory, rotation and speed. This is called “non-gravitational acceleration” because it is acceleration that is not produced by falling into a gravitational funnel of objects in the Solar System.

Asteroids are also rocky (and metallic) remnants from the early days of the solar system, but they do not have the ice of comets or their characteristic tails. They can vary in size from about 530 kilometers (329 miles) to less than 10 meters (33 feet) in diameter, and they generally have shorter orbits.

In practice, however, space objects are not so easy to categorize. The interstellar object ‘Oumuamua, for example, showed signs of non-gravitational acceleration without an associated coma, and a recent study found that the comet Tsuchinshan-ATLAS also did not brighten as expected as it approached the Sun and may break up into smaller “clumps,” which may not persist for very long.

The new study examined seven objects known as “dark comets,” which appear to accelerate without the assistance of gravitational fields, the Sun’s radiation pressure, or the Yarkovsky effect.

“Dark comets are in low-Earth orbits and lack detectable comae (hence the name ‘dark’), but experience non-gravitational accelerations that are inconsistent with radiative effects,” the team explained in their study. “Outgassing is consistent with both reported accelerations and undetected comae. In addition to their accelerations and lack of comae, dark comets exhibit several other notable properties – they are generally (i) small (on the order of 10 − 100 m), (ii) rotate rapidly (0.046 − 1.99 h), and (iii) accelerate in non-radial directions.”

Upon closer examination of these objects, the team found that they are likely loose rock accumulations that evolved from smaller objects due to gravitational effects.

“During this transition, and especially in the NEO environment, these progenitor objects undergo significant non-gravitational accelerations that trigger a rotational fragmentation cascade,” the team explains. “The resulting fragments would continue to rotate and volatilize their volatiles.”

When rock fragments are thrown away from the main body, the torque is reduced.

“At some point, the torque is no longer sufficient to rotate the object beyond its critical speed, and it is stable against further destruction by rotation. At this point, the fragments are small, rotating rapidly, and largely degassed.”

By studying the past orbits of objects with the non-gravitational acceleration observed in these objects, the team found that they likely originated in the inner band of the asteroid belt, which could help explain the origin of water on Earth.

“We believe these objects originated in the inner and/or outer asteroid belt, and it follows that this is another mechanism by which ice enters the inner solar system,” Aster Taylor, an astronomy graduate student at the University of Michigan and lead author of the study, said in a statement. “There may be more ice in the inner main belt than we thought. There may be more objects like this out there. This could be a significant portion of the next population. We don’t really know, but these findings have raised many more questions for us.”

The team estimates that between 0.5 and 60 percent of near-Earth objects could be dark comets.

“These pieces are also covered in ice, so they spin faster and faster until they break into more pieces,” Taylor added. “You can just keep doing that until you get smaller and smaller. What we’re proposing is that you get these small, fast-spinning objects by taking a few larger objects and breaking them into pieces.”

As always, further studies are needed, but while they provide clues to Earth’s water mystery, they may also raise a new one.

“Near-Earth objects don’t stay in their current orbits for very long because the near-Earth environment is chaotic,” Taylor said. “They only stay in the near-Earth environment for about 10 million years. Because the solar system is much older, that means that near-Earth objects come from somewhere — that we’re constantly being fed near-Earth objects from another, much larger source.”

The study was published in the journal Icarus.

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