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Largest Earth Trojan Asteroid to Date Discovered by UH Telescope on Maui

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An asteroid bigger than the University of Hawaiʻi Mānoa campus discovered by a UH Institute for Astronomy telescope atop Haleakalā on Maui is only the second-known object of its kind ever found.

According to a press release from UH, the near-Earth object, asteroid 2020 XL5, was discovered by the Panoramic Survey Telescope and Rapid Response System-1 in December 2020. The object is an Earth Trojan asteroid, an asteroid companion to Earth that orbits the sun along the same path. The asteroid is also the largest known Earth Trojan companion.

Pan-STARRS 1 and 2 atop Haleakalā on Maui. Photo courtesy of the University of Hawaiʻi.

“Earth Trojans have been suspected as a source of moon and Earth impactors. Because they have orbits very similar to Earth’s, they have a higher long-term potential for hitting us compared to typical near-Earth asteroids,” said Richard Wainscoat, an IfA astronomer who leads the Near-Earth Object survey project with Pan-STARRS. “Pan-STARRS 2020 XL5 has been shown to have a stable orbit for at least 4,000 years – so it does not pose any threat to Earth for a long time.”

The asteroid, with a diameter of about 0.75 miles, is about 60 million miles from Earth. These kinds of asteroids have largely gone undetected because they are close to the sun in the sky, so observers must search low in the east before sunrise and low in the west after sunset.


In-depth observations with the 4.1-meter Southern Astrophysical Research Telescope in Chile, a program of the National Science Foundation’s NOIRLab, were instrumental in detailing the size and orbit of 2020 XL5. The findings were recently published in Nature Communications. Wainscoat and IfA astronomer Rob Weryk co-authored the study, alongside a team of astronomers from multiple countries including Spain, Germany and Canada.

After its initial discovery, Pan-STARRS acquired additional measurements to refine the asteroid’s orbit. Archival observations from Pan-STARRS dating back to 2012 helped improve the understanding of its orbit and allowed its motion to be predicted.

According to the press release, Earth Trojan asteroids are made of primitive material dating back to the birth of the solar system and could represent some of the building blocks that formed Earth. They are attractive targets for future space missions.


This is just one of many major discoveries for UH’s NASA-funded Pan-STARRS telescopes, including the discovery of ʻOumuamua, the first interstellar object, and capturing the death of a supergiant star.

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