Their highly elongated orbits only bring them close to the Sun and the Earth for a short period of time, which heats them up and produces their signature tails.
Asteroids, on the other hand, are usually found between Mars and Jupiter in the asteroid belt and have thought to be in the warm part of our system for nearly five billion years – and typically, these objects contain no ice.
There are a few unusual exceptions to this dichotomy, however, known as “main belt comet”. This one, known as Asteroid 248370, is one of approximately 20 objects that have currently been confirmed or are suspected to exist.
“248370 can be thought of as both an asteroid and a comet, or more specifically, a main-belt asteroid that has just recently been recognised to also be a comet,” Planetary Science Institute senior scientist Henry Hsieh said.
“It fits the physical definitions of a comet, in that it is likely icy and is ejecting dust into space, even though it also has the orbit of an asteroid. This duality and blurring of the boundary between what were previously thought to be two completely separate types of objects – asteroids and comets – is a key part of what makes these objects so interesting.”
The asteroid has a solid head approximately two miles across surrounded by a dust cloud, with a 450,000 mile-long tail but only 900 miles wide.
“This extremely narrow tail tells us that dust particles are barely floating off of the nucleus at extremely slow speeds and that the flow of gas escaping from the comet that normally lifts dust off into space from a comet is extremely weak. Such slow speeds would normally make it difficult for dust to escape from the gravity of the nucleus itself, so this suggests that something else might be helping the dust to escape. For example, the nucleus might be spinning fast enough that it’s helping to fling dust off into space that has been partially lifted by escaping gas,” Hsieh said.
The discovery of this strange object will spark further study, as it is thought that a substantial part of Earth’s water was delivered via impacts from asteroids in the main belt. These rocks offer a potential way to test that hypothesis and gain more information about the origin of life on Earth.
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