A recently discovered asteroid appears to be an Earth Trojan, orbiting a gravitationally stable area with only one other known occupant.

Earth has a second Trojan asteroid sharing its orbit, reports amateur Tony Dunn on the Minor Planet Mailing List. The asteroid, dubbed 2020 XL5, is a few hundred meters across and its orbit is tied to a gravitationally stable ahead of the Earth in its orbit.

Trojans are asteroids gravitationally locked to stable Lagrange points either 60° ahead (L4) or behind (L5) the planets in their orbits around the Sun. 2020 XL5 was found around the L4 point. Massive Jupiter has more than 9,000 Trojans. In theory, Trojan orbits would be stable around every planet except Saturn, where Jupiter’s gravity pulls them away. So far, Trojans have been found sharing orbits — at least temporarily — with Neptune, Uranus, Mars, Venus, and Earth.

Earth Trojans are hard to find because during most of their orbits, they appear close to the Sun in the sky. Not only that, but the gravitational resonance does not hold them in lockstep at 60° ahead and behind of the Earth, explains Dunn. Instead, the objects trace paths around the L4 and L5 points, which are themselves moving as Earth orbits the Sun.

NASA’s Wide-field Infrared Survey Explorer spacecraft spotted the first Earth Trojan, 2010 TK7, also locked to the L4 point, in October 2010 when it scanned the infrared sky 90° from the Sun. Two other observers recovered it a few months later with the Canada-France-Hawaii Telescope. It’s slightly smaller than 2020 XL5.

The orbits of our two Trojans are best visualized along with that of Earth’s and, in the case of 2020 XL5, the orbits of all the inner planets. When viewed relative to Earth, 2010 TK7 drifts between a spot close to Earth to the L3 point on the other side of the Sun from Earth, but it doesn’t pass through the L4 point. The orbit of 2020 XL5 ranges more widely, drifting inward to inside Venus’s orbit and outward almost to Mars.

The wide-ranging orbit shows “[2020 XL5] is almost certainly a garden-variety bit of rock that went [close to] Venus and got perturbed into an orbit with a period very close to one year,” says amateur Bill Gray of the Pluto Project.

Aldo Vitagliano, a retired Italian chemist and author of the Solex orbital software, said on the MPML that the orbit should remain stable for 2,000 to 4,000 years, but  gravitational tugs would eventually move it to another orbit. So far 2020 XL5 has only been observed for only a few weeks, and amateur astronomer Sam Deen says we may have to wait until November or December until more observations can be made to pin down its orbit.

The first Earth trojan, 2010 TK7, comes within 20 million kilometers (12 million miles) to Earth every few hundred years; it is currently drifting away. Models show its orbit is stable enough to stay in a one-to-one resonance with Earth for about a quarter million years. While there are Earth Trojan orbits that are stable for the life of the solar system, no objects have been found occupying them.

Two spacecraft on their way to visit near-Earth objects searched Trojan regions in 2017, but NASA’s Osiris-Rex found nothing at L4 and the Japanese Hayabusa 2 found nothing at L5. However, the observations were not definitive, and in 2019 Renu Malhotra (University of Arizona) wrote that the Earth could still have up to several hundred Trojans at least a few hundred meters in diameter, amounting to several percent of the some 10,000 near-Earth objects of that size.

A population of Earth Trojans should have survived since the planet formed if its orbit hasn’t changed since then, she says. Their existence — or lack thereof — has other implications, too. Searching for ancient Trojans could help explain why the leading hemisphere of the Moon has about 70% more young craters than the trailing side, a difference current models can’t explain. Earth Trojans slowly escaping from their orbits might account for the extra young craters.

Now, Malhotra says, astronomers are stepping up their search for Earth trojans. The Catalina Sky Survey has expanded the area it covers, and a group at the Vera Rubin observatory is also planning observations once that observatory comes online in a year or two.

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