Editor’s Note: Correction to the May 20 story, ATLAS telescope discovers first-of-its-kind asteroid.
Recently discovered object 2019 LD2, originally believed to be the first cometary “Jupiter Trojan” asteroid by astronomers at the University of Hawaiʻi at Mānoa Institute for Astronomy turns out to be an interloper comet masquerading as a member of the Trojan population. The distinction was first suggested by amateur astronomers Sam Deen and Tony Dunn on the Minor Planet Mailing List on May 21. The discovery was confirmed by UH’s Asteroid Terrestrial-impact Last Alert System (ATLAS) colleagues Alan Fitzsimmons and Henry Hsieh that 2019 LD2 is in fact a comet with a chaotically changing orbit currently resembling that of a Trojan asteroid.
The cometary nature of this object was announced in a Minor Planet Electronic Circular on May 22, giving it the slightly different new name of P/2019 LD2 that designates it as a comet. On May 23, additional analysis by Japan astronomer Syuichi Nanako confirmed the evolving orbit was published by the Central Bureau of Astronomical Telegrams.
Comet P/2019 LD2 turns out to be a Jupiter-family comet, a different population of objects also under the influence of Jupiter, with typical comet-like orbits that can extend from the outer solar system beyond Saturn all the way to the inner solar system. True Jupiter Trojan asteroids have orbits that closely follow Jupiter’s orbit around the Sun but are “clumped” ahead and behind Jupiter. These asteroids are essentially locked into clumps because they are in a delicate permanent gravitational balance between Jupiter and the Sun.
Comet P/2019 LD2 regularly comes close enough to Jupiter every few decades so the pushes and pulls from close gravitational interactions with Jupiter can change the comet’s orbit dramatically. In the case of P/2019 LD2, its location and orbit currently approximates the position and near-circular orbit of Jupiter Trojan asteroids. The current orbit is not stable, meaning Jupiter will alter it again in the coming decades and comet P/2019 LD2 will no longer be easily confused with a Jupiter Trojan asteroid.
We often think of asteroids and comets as distinct types of small bodies, but astronomers have discovered an increasing number of “crossovers.” These objects initially appear to be asteroids, and later develop activity, such as tails, that are typical of comets.
Now, the University of Hawaiʻi ‘s Asteroid Terrestrial-impact Last Alert System (ATLAS) has discovered the first known Jupiter Trojan asteroid to have sprouted a comet-like tail. ATLAS is a NASA-funded project using wide-field telescopes to rapidly scan the sky for asteroids that might pose an impact threat to Earth. But by searching most of the sky every two nights, ATLAS often finds other kinds of objects – objects that aren't dangerous, but are very interesting.
Early in June 2019, ATLAS reported what seemed to be a faint asteroid near the orbit of Jupiter. The Minor Planet Center designated the new discovery as 2019 LD2. Inspection of ATLAS images taken on June 10 by collaborators Alan Fitzsimmons and David Young at Queen’s University Belfast revealed its probable cometary nature. Follow-up observations by UH astronomer J.D. Armstrong and his student Sidney Moss on June 11 and 13 using the Las Cumbres Observatory global telescope network confirmed the cometary nature of this body.
Later, in July 2019, new ATLAS images caught 2019 LD2 again – now truly looking like a comet, with a faint tail made of dust or gas. The asteroid passed behind the Sun and was not observable from the Earth in late 2019 and early 2020, but upon its reappearance in the night sky in April of 2020, routine ATLAS observations confirmed that it still looks like a comet. These observations showed that 2019 LD2 has probably been continuously active for almost a year.
While ATLAS has discovered more than 40 comets, what makes this object extraordinary is its orbit. The early indication that it was an asteroid near Jupiter's orbit have now been confirmed through precise measurements from many different observatories. In fact, 2019 LD2 is a special kind of asteroid called a Jupiter Trojan – and no object of this type has ever before been seen to spew out dust and gas like a comet.
Trojan asteroids follow the same orbit as a planet, but stay either around 60 degrees ahead or 60 degrees behind along the orbit. Earth has at least one Trojan asteroid, and Neptune has dozens. Jupiter has hundreds of thousands. The Jupiter Trojan asteroids orbit the Sun in two huge swarms, one swarm orbiting ahead of the planet (where 2019 LD2 was found) and one swarm orbiting behind it. The Trojan asteroids have been captured into these orbits by Jupiter's strong gravity. What makes 2019 LD2 so interesting is that we think most Jupiter Trojans were captured billions of years ago. Any surface ice that could vaporize to spew out gas and dust should have done so long ago, leaving the objects quietly orbiting as asteroids – not behaving like comets.
“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” Fitzsimmons said.
What could have made 2019 LD2 suddenly show cometary behavior? Maybe Jupiter captured it only recently from a more distant orbit where surface ice could still survive. Maybe it recently suffered a landslide or an impact from another asteroid, exposing ice that used to be buried under layers of protective rock. New observations to find out are being acquired and evaluated. What's certain is that the Universe is full of surprises – and surveys to guard the Earth from dangerous asteroids often make unexpected discoveries of harmless but fascinating objects that can reveal more about our Solar System’s history.
“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,” said ATLAS project principal investigator Larry Denneau. “It’s a real bonus for ATLAS to make these kinds of discoveries.”
ATLAS is funded by the Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office.
For more information, visit: https://www.nasa.gov/planetarydefense
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