(ORDO NEWS) — We often think of asteroids and comets as different types of small bodies, but astronomers have discovered an increasing number of “intersections.” At first, these objects appear to be asteroids, and later develop activity, for example, form tails that are typical of comets.
The University of Hawaii’s Asteroid Collision Early Warning System (ATLAS) discovered the first known Trojan asteroid of Jupiter to have a comet-like tail. ATLAS is a NASA-funded project using wide-angle telescopes to quickly scan the sky in search of asteroids that could threaten a collision with the Earth. But, scouring most of the sky every two nights, ATLAS often finds other kinds of objects – objects that are not dangerous, but very interesting.
In early June 2019, ATLAS reported that it seemed to be a weak asteroid near the orbit of Jupiter. The center of minor planets designated this object as 2019 LD2. Checking ATLAS images taken on June 10 by staff members Alan Fitzsimmons and David Young at Queen’s University in Belfast showed its likely cometary character. Subsequent observations by astronomer J. D. Armstrong and his student Sidney Moss on June 11 and 13 using the global network of telescopes at the Las Cumbres Observatory confirmed the cometary nature of this body.
Later, in July 2019, new images made by ATLAS again captured 2019 LD2 – now it really looks like a comet with a faint tail of dust or gas. Then the asteroid passed behind the Sun and was not observed from Earth from the end of 2019 until the beginning of 2020, but after its appearance in the night sky in April 2020, ordinary observations with ATLAS confirmed that it still looks like a comet. These observations showed that the 2019 LD2 facility is probably continuously active for almost a year.
While ATLAS has discovered more than 40 comets, a feature of this object is its orbit. The earliest signs that it was an asteroid near the orbit of Jupiter were now confirmed by accurate measurements of various observatories. In fact, 2019 LD2 is a special kind of asteroid called the Jupiter Trojan that has never been seen before. This is a special type of object that spews dust and gas, like a comet.
Trojan asteroids follow the same orbit as the planet, but remain 60 degrees ahead or 60 degrees behind in the planet’s orbit. Earth has at least one Trojan asteroid, and Neptune has dozens. Jupiter has hundreds of thousands. Jupiter’s Trojan asteroids revolve around the Sun in two huge swarms, one swarm rotates ahead of the planet (where 2019 LD2 was found) and one swarm rotates behind it.
Trojan asteroids were captured on these orbits by the strong gravity of Jupiter. What makes 2019 LD2 so interesting is that we think most of the Jupiter Trojans fell into their current orbits billions of years ago. Any surface ice had to evaporate, spewing gas and dust. As a result, objects orbiting like asteroids, and not behaving like comets, should remain.
“For decades, we believed that Trojan asteroids should have a large amount of ice beneath their surfaces, but never before had any evidence. ATLAS has shown that predictions of their icy nature may well be correct, ”said Fitzsimmons.
What could make 2019 LD2 suddenly show its cometary behavior? Perhaps Jupiter captured it most recently from a farther orbit where surface ice could “survive”. It is also possible that the asteroid has recently undergone a landslide or impact of another asteroid, exposing ice that was previously buried under layers of protective rock.
New observations continue to accumulate and evaluate. There is no doubt that the Universe is full of surprises – and an asteroid collision warning system with the Earth often makes unexpected discoveries of harmless but exciting objects that can reveal more information about the history of our solar system.
“Despite the fact that the ATLAS system is designed to search for dangerous asteroids, ATLAS sees other rare phenomena in our solar system and beyond when scanning the sky,” said Larry Denno, chief researcher of the ATLAS project. “This is a real bonus for ATLAS to make such discoveries.”
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