Well, that's something boffins haven't seen before: A strange alien streaks around Jupiter

We know this sounds like a tall tail but...

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2019_LD2
Tail end ... The 2019 LD2 asteroid-slash-comet. Credit: JD Armstrong/IfA/LCOGT

Pic Astronomers scanning the sky for potentially hazardous space rocks have discovered a first – a presumed trojan asteroid around Jupiter that is looking increasingly like a comet.

The cosmic enigma was spotted by the Asteroid Terrestrial-impact Last Alert System (ATLAS), which employs two telescopes in Hawaii to snap wide-field images. If the conditions are favorable, the system is able to cover the whole observable sky every two nights. ATLAS looks out for near-Earth objects that could crash into our planet, and warns of an impact weeks to a few days in advance.

Now, after more than five years of observations, the system has clocked something it’s never seen before: what appears to be a Jupiter trojan asteroid that has suddenly spawned a tail of dust and gas, something that’s only seen in comets. Scientists believe it has stayed that way for about a year. A trojan in this case is a rock that librates around one of the gas giant's two stable Lagrange points.

The weird object, known as 2019 LD2, was spotted in June: images taken by ATLAS that month were analyzed by Alan Fitzsimmons and David Young, both of the Astrophysics Research Center at the Queen’s University Belfast, who noticed 2019 LD2’s comet-like behavior.

Days later, James Armstrong and Sidney Moss, of the University of Hawaii, which operates ATLAS, performed followup observations using the Las Cumbres Observatory's global network, and confirmed the other duo's suspicions. The dual sightings confirmed something unusual was up.

Ch-ch-changes

It’s not the first time asteroids have crossed over to comets, though it’s the first time Earth's scientists have seen it occurring for a space rock with such a wide orbit.

Although our own home world has its own trojan asteroids, Jupiter attracts hundreds of thousands due to its larger gravitational field.

Artist Concept of Jovian Trojans (credit: NASA/JPL-Caltech)

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Crucially, the tail of gas could reveal the rock's inner secrets.

“We have believed for decades that trojan asteroids should have large amounts of ice beneath their surfaces, but never had any evidence until now. ATLAS has shown that the predictions of their icy nature may well be correct," said Fitzsimmons.

2019 LD2 is in the group orbiting at Lagrange point L4, lying 60° ahead of Jupiter in its orbit. Scientists aren’t sure why this particular rock among all the others in the large flock of Jupiter trojans has started vaporizing to form a tail as it streaks across the sky.

Some at the University of Hawaii believe it’s possible 2019 LD2 is a recent addition to Jupiter's trojan family, and it was captured from a more distant orbit where temperatures were low enough for ice to survive on its surface and that it’s only vaporizing now that it's closer to the Sun. Or perhaps it suffered a landslide or crashed into another asteroid, which scraped its top protective layer off and now its icy core is exposed to the Sun, causing it to vaporize.

Larry Denneau, co-principal investigator at ATLAS, said: "Even though the ATLAS system is designed to search for dangerous asteroids, ATLAS sees other rare phenomena in our solar system and beyond while scanning the sky. It's a real bonus for ATLAS to make these kinds of discoveries." ®

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