An asteroid is acting like a comet — confirming what scientists have speculated for years

Image of asteroid 2019LD2 taken on 11 June 2019, using the Las Cumbres Observatory Global Telescope (LCOGT) NetworkJD Armstrong/IfA/LCOGT
  • A new kind of Jupiter Trojan asteroid has been spotted streaking through the skies by the National Aeronautics and Space Administration’s ATLAS system.
  • This asteroid has a comet-like tail that’s never been seen before.
  • It confirms what scientists have speculated for years — that Trojan asteroids have large amounts of ice beneath their surface.
Astronomers have spotted the first known ‘Jupiter Trojan’ asteroid, which isn’t really the run-of-the-mill asteroid that us Earthlings are used to hearing about. Instead of being a composite ball of rock, the asteroid with the official designation of 2019 LD2 seems to have sprouted a comet-like tail.

This is a first for the scientific community and for ATLAS — a project funded by the National Aeronautics and Space Administration (NASA) which uses wide-field telescopes scan the skies for asteroids that might pose a threat to Earth. It has detected over 40 asteroids so far, but none like the Jupiter Trojan.

This finally confirms what astronomers have speculated for years — that Trojan asteroids have large amounts of ice beneath their surface. “ATLAS has shown that the predictions of their icy nature may well be correct" Alan Fitzsimmons, one of the astronomers who made the initial discovery, said.

Stars seem to streak by in the left panel because the images have been shifted and added to follow 2019 LD2, but the comet itself (indicated by two red lines) is almost lost in the crowded field of stars. The same data are shown at right, but with the stars subtracted. ATLAS uses this star-subtraction process (called difference imaging) for all asteroid search images. ATLAS

What made this Jupiter Trojan suddenly act like a comet?

Trojan asteroids are those, which follow the same orbit as a planet — either 60 degrees ahead of 60 degrees behind. Even Earth is known to have at least one Trojan asteroid.

A Jupiter Trojan is one that follows Jupiter’s orbit, not uncommon for the gaseous giant considering its size and strong gravitational pull. There’s one swarm of Jupiter Trojans orbiting behind the planet, and another orbiting ahead of it, where 2019 LD2 was found.

2019 LD2 suddenly spewing ice, dust and gas from its surface could be for a number of reasons. Scientists speculate one reason may that it was only recently captured by a more distant orbit, increasing the likelihood of ice being able to survive on its surface.

Another reason could be a recent landslide or impact that brought out ice, which would normally be below, to the surface.


The discovery of 2019 LD2 and its comet-like tail

2019 LD2 was first detected on 10 June 2019. When ATLAS saw it again in July, it saw the peculiar trail of dust and gas running behind the asteroid.

Before astronomers had a chance to study the asteroid further, it disappeared behind the Sun wasn’t seen again until April 2020. This time, ATLAS was able to confirm that the asteroid’s tail wasn’t just an anomaly, but a feature that had been with it for at least a year.

Not just ATLAS, but multiple observatories — including Las Cumbres Observatory Global Telescope (LCOGT) Network — have been able to confirm its tail using precise measurements.


Normally, astronomers distinguish between asteroids and comets by their characteristics and composition. But, increasingly, they have been detecting ‘crossovers’ where the distinction is no longer as obvious.


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