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Astronomers Chase Shadows From Jupiter's Mysterious Trojan Asteroids

Ahead of the Lucy mission, astronomers are gearing up to observe eclipses of Trojan asteroids as they appear over Europe and parts of North Africa.

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An illustration of the Trojan asteroids around Jupiter.
An illustration of the Trojan asteroids as they lead and follow Jupiter.
Illustration: NASA

Astronomer Marc Buie has been tracing the shadows of a swarm of rocky bodies as they pass in front of stars, leaving behind a distinct imprint of their shape. These space rocks are known as the Trojan asteroids, two groups of asteroids that lead and follow Jupiter as they orbit the Sun.

NASA’s Lucy spacecraft is on its way to Jupiter’s Trojan asteroids for a chance to get a closer look at the primordial rocks. But before its scheduled arrival at the Trojans in 2027, Buie and his team have been providing the mission scientists with strange and exciting details about the elusive space rocks. The astronomer has been traveling the world, leading a team of over 500 professional and amateur astronomers observing the Trojans. The team members are gearing up for an opportunity to gather even more detail about the asteroids later this month, for which they will travel across Europe to track the Trojans across the sky.

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Buie has been observing the Trojan asteroids since 2018 as part of the occultation campaign for the Lucy mission. Occultations, or eclipses, happen when an object passes in front of a star and temporarily blocks its light, allowing the observer to trace out features like its shape and size. “We’re basically looking at this star with a telescope, and then at the appointed time, the star disappears. It’s just gone, hidden for a number of seconds,” Buie told Gizmodo over the phone. “And what has happened to you sitting on the ground is that the shadow of the asteroid just passed over you.”

So far, Buie has seen 21 occultations of the Trojans from different parts of Europe, Australia, Senegal, and the United States, but he’s not done yet. As the Lucy occultation science lead at the Southwest Research Institute, Buie has assembled an international team of astronomers to gear up for a big observation event coming up on October 23.

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The shadow of the Trojan asteroids will eclipse Morocco, go up through Southern Spain, then cross just to the east of Paris, and up into Belgium, Norway and Sweden. “This particular campaign coming up is the first time I’m sending teams out without [me] physically being there,” Buie said. “It’s easier for me to just stay home and coordinate everything.” There’s a group of French astronomers being deployed out of Paris and another group of astronomers out of Seville, Spain, that will essentially drive around and chase the shadow of the asteroids, seeking clear skies to get a better look at the Trojans as they pass overhead.

“I’m a little nervous, I like to be there. But I gotta learn how to coordinate them from afar,” Buie confessed. The astronomer has been chasing the asteroids all over the world, and this year has been particularly tiring. Usually, Buie will have one occultation every year or two that’s worth observing, “but the pace has picked up with Lucy,” he said. In the past 12 months, Buie and his team have monitored 13 occultations, with all but one being outside the U.S.

That’s due to the Milky Way, where most of the stars are concentrated in the galactic plane. The Trojan asteroids are currently passing across the equator of the galaxy, where the chances of them hitting stars are much higher than if they were passing the galactic poles. By next year, the number of occultations of the Trojan asteroids will start to drop as they shift their position. “I’ve learned over the years that you never say no to a good occultation opportunity,” Buie said. “You don’t get to say, “I’m too busy, I’ll do it next year.”

The acquired occultations data will directly contribute to the Lucy mission; the spacecraft will attempt to confirm these findings while exploring the Trojans from up close. Each dataset, whether from the occultations or the spacecraft, will compliment the other to create a fuller picture of the celestial object being observed.

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The data coming out of the occultations so far has been well worth it. During one of the occultations in March, the team discovered a moonlet around asteroid Polymele, the smallest of the Trojan targets. The moonlet is around 3 miles wide (5 kilometers), orbiting Polymele, which is itself around 17 miles wide (27 km), at a distance of 125 miles (200 km), according to NASA. Although it is yet to be named, the tiny asteroid was added to the list of Lucy’s targets, along with another small satellite around Eurybates that was discovered in January 2021 by the Hubble Space Telescope.

The Trojan occultations have also revealed the shapes of these objects to a level never seen before. The occultations team has noted substantial topography on the asteroids, where there could be impact craters or large canyons. “That’s surprising,” Buie said. “Personally, I thought the bigger ones were going to just be like billiard balls—very smooth, and featureless, or nearly featureless, spheres.”

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The Trojan asteroids merit their own mission since they may contain clues to how the solar system formed. These space rocks are held in a gravitational balancing act between Jupiter and the Sun and are believed to be the remnants of the primordial material that formed the outer planets. The Trojans have likely existed for the past four billion years, but scientists have yet to get a close look at this gang of asteroids.

Enter Lucy. The spacecraft launched in October 2021 on an unprecedented mission to explore the Trojan asteroids. The $981 million spacecraft is slated to visit one asteroid in the Main Belt between Jupiter and Mars and eight Trojan asteroids. Lucy was originally meant to visit seven asteroids, but now its target count is up to nine. “Lucy’s tagline started out: 12 years, seven asteroids, one spacecraft,” Lucy’s program scientist Tom Statler said in a statement from August. “We keep having to change the tagline for this mission, but that’s a good problem to have.”

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Lucy is gearing up for its upcoming Earth gravity assist on October 16, which will accelerate the spacecraft and direct its trajectory beyond the orbit of Mars. From there, Lucy still has about five years to go before reaching its first target, Donaldjohanson. In August 2027, Lucy will begin its Trojan tour by visiting Eurybates and its binary partner Queta, followed by Polymele and its binary partnery, Leucus, Orus, and the binary pair Patroclus and Menoetius.

Once Lucy reaches the asteroids, the occultation team will get a bit of a breather. “You can’t do this for free, it takes money to support the people renting cars, airplane flights, hotel rooms,” Buie said. “So when the mission was proposed, I got a budget to do these campaigns. But as soon as I’m finished doing all this occultation work for Lucy, and the money is gone from Lucy, then it’s going to go back to being quiet.”

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