Astronomers Spot First Trojan Asteroid with Comet-Like Tail: 2019 LD2

Watch Jupiter Trojan asteroid 2019 LD2’s orbit the sun for 25 years in this orbit animation. New imagery from the University of Hawaiʻi’s Asteroid Terrestrial-impact Last Alert System (ATLAS) reveals that it has an unusual comet-like tail.

Credit: / Orbit animation: NASA/JPL-Caltech / image: ATLAS/A. Heinze/IfA / produced & edited by [Steve Spaleta](

2019 LD2 is the first known Jupiter trojan asteroid to display cometary activity with a visible coma and tail, according to a team of astronomers from the University of Hawaii and Queen’s University Belfast.

This image shows the Jupiter trojan asteroid 2019 LD2. Image credit: ATLAS.

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 one trojan asteroid, 2010 TK7. Mars hosts at least nine, Uranus has two, and Neptune has 22 trojans.

Jupiter has more than one million trojan asteroids larger than 1 km. These Jupiter trojans orbit the Sun in two huge groups, one group orbiting ahead of the planet (2019 LD2 belongs to this group) and one group orbiting behind it.

“What makes 2019 LD2 so interesting is that we think most Jupiter trojans were captured billions of years ago,” said Queen’s University Belfast’s Professor Alan Fitzsimmons and colleagues.

“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.”

2019 LD2 was discovered on June 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), an asteroid impact early warning system being developed by the University of Hawaii and funded by NASA.

A detailed analysis of the discovery images of 2019 LD2 by Professor Fitzsimmons and his colleague, Dr. David Young of Queen’s University Belfast, revealed its probable cometary nature.

Follow-up observations by University of Hawaii astronomers Dr. James ‘J.D.’ Armstrong and Sidney Moss on June 11 and 13, 2019, using the Las Cumbres Observatory global telescope network confirmed the cometary nature of the asteroid.

In July 2019, new ATLAS images caught the object again — now truly looking like a comet, with a faint tail made of dust or gas.

2019 LD2 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 2020, 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.

“We have believed for decades that trojan asteroids should have large amounts of ice beneath their surfaces, but never had any evidence until now,” Professor Fitzsimmons said.

“ATLAS has shown that the predictions of their icy nature may well be correct.”

“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,” the astronomers said.

“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.”