What do you get if you cross an asteroid with a comet? An active Trojan of course.
Not the funniest of punchlines, but astronomers really have discovered a new kind of asteroid with a comet-like tail that orbits just ahead of Jupiter in a swarm of rocks, called Trojans. 2019 LD2, as it is known, blurs the boundaries between “dirty snowball” comets and rocky asteroids, as the object spews dust and gas from its tail-end – like a comet. Although other “active asteroids” with similar comet-like properties are not unknown, never before has one also been a Jupiter Trojan.
First observed in June 2019 by the University of Hawai’i’s Asteroid Terrestrial-impact Last Alert System (ATLAS), 2019 LD2 was originally thought to be a faint asteroid that shared Jupiter’s orbit around the Sun. Another look in July 2019 revealed a comet-like tail of dust or gas trailing behind the main body, before it dove out of view behind the Sun. When the Trojan asteroid reappeared last month, ATLAS surveys confirmed that almost a year later it still looked like a comet.
To discover such behavior of a Jupiter Trojan is momentous. Thought to have been captured into orbit by Jupiter’s strong gravity billions of years ago, these asteroids should have already been rid of any surface ice that could vaporize and form “tails”. Astronomers suggest 2019 LD2’s sudden cometary behavior could indicate that it has only recently been captured by Jupiter from a more distant orbit where ice could exist. Alternatively, it could have recently had a landslide or collision to reveal ice under its layers of protective rock.
Sixty degrees ahead and behind Jupiter in its orbit are Lagranian Points, havens of stability where thousands of "Trojan" asteroids congregate.
“We have believed for decades that Trojan asteroids should have large amounts of ice beneath their surfaces, but never had any evidence until now,” Alan Fitzsimmons of Queen’s University Belfast, Northern Ireland, who helped reveal the asteroid’s cometary side, said in a statement. “ATLAS has shown that the predictions of their icy nature may well be correct.”
With the first-ever mission to the Trojan Asteroids set to launch next year, researchers are hoping to get a closer look at the mysterious behavior of 2019 LD2. For now, the attention remains on ATLAS to give a view of the weird and wonderful world of space rocks.
“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,” ATLAS project principal investigator Larry Denneau said in a statement. “It’s a real bonus for ATLAS to make these kinds of discoveries.”
