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A rocky body spotted orbiting in Earth's path is a "Trojan asteroid" that escorts our planet around the sun, astronomers have confirmed.
Asteroid 2020 XL5 is the second Trojan asteroid ever discovered. It's three times larger than the only other known Earth Trojan, called Asteroid 2010 TK7, which was confirmed in 2011. These small space rocks orbit along with Earth, but they are hard to spot from our planet — Asteroid 2010 TK7 is sometimes on the other side of the sun from us. They sit in gravitational sweet spots known as Lagrangian points. If Earth and the sun make up two points of an equilateral triangle, the Lagrangian point would be that triangle's third point. Earth and the sun have five of these points.
The newfound Trojan is at a Lagrangian point that has it orbiting ahead of Earth in the two bodies' shared path and the asteroid will stay there for 4,000 more years, researchers reported Tuesday (Feb. 1) in the journal Nature Communications. The asteroid is probably about 0.7 miles (1.18 kilometers) in diameter, though that estimate may be off if the surface of the asteroid is more or less reflective than astronomers assume.
Trojans exist in other planetary orbits as well. They've been spotted around Venus, Mars, Jupiter, Uranus and Neptune, study leader Tony Santana-Ros, a planetary scientist at the University of Alicante in Spain, and colleagues wrote in the new paper. It's harder to see Earth's own Trojans, though, because they often sit in Earth's shadow or orbit at an odd angle from the planet, making them dim and hard to catch. Asteroid 2020 XL5 was first seen in the Pan-STARRS1 survey, which uses a 1.8-meter telescope in Hawaii to take wide-field images of the sky. Follow-up data was gathered from telescopes in Arizona, Chile and the Canary Islands.
The astronomers found that more than 500 years ago, 2020 XL5 was not a Trojan, but that its orbit stabilized in the Lagrangian point around then.
The angles of the orbits of both Asteroid 2010 TK7 and Asteroid 2020 XL5 compared with that of Earth mean it would be infeasible to do a rendezvous mission with either Trojan, the researchers calculated. A flyby — which wouldn't require syncing up a spacecraft with the asteroids' orbits — might be possible, the study found. Of the two, the researchers wrote, the newly discovered Trojan is likely easier to fly by, as its position offers more flexibility for the launch date of a theoretical space probe.
Even if it's not possible to visit either of Earth's orbital buddies, having two orbits to compare will help astronomers understand the orbital math that makes Trojans possible, the researchers wrote.
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"The discovery of a second ET [Earth Trojan] asteroid may enhance our knowledge of the dynamics of this elusive population," they wrote.
Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.
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