Lucy, Exploring Jupiter's Trojan Asteroids

Facts Worth Sharing

  • NASA's Lucy mission is our first visit to a group of asteroids called Trojans that share Jupiter's orbit around the Sun.
  • Lucy will visit 7 Trojan asteroids between 2027 and 2033, plus a bonus main-belt asteroid in 2025.
  • Asteroids are leftover planet-building materials from the formation of our solar system. By studying a variety of these small worlds, we learn more of our origin story.

Why We Need Lucy

Where did we come from? That big question could be answered by studying some relatively small worlds: asteroids.

Asteroids, comets, and other small worlds are leftover planet-building materials from the formation of our solar system. Around 4 billion years ago, a chaotic chain of planet migrations threw some into Earth, possibly bringing here water and organic materials—the building blocks of life as we know it.

We don't know exactly what happened, and on Earth, processes like weather and geologic activity have erased much of our past. Asteroids, however, have floated through space mostly untouched, serving as fossils ready to unlock the secrets of our origin story.

NASA's Lucy mission is our first visit to a particular group of asteroids called Trojans that share Jupiter's orbit around the Sun. The Trojans may have formed farther away before getting caught by Jupiter's gravity, and likely have different blends of the solar system's starting materials than other asteroids we've previously visited.

Lucy, named after the fossilized skeleton that helped scientists learn where humans fit into the evolutionary chain of life, launches in 2021 and will visit 7 Trojan asteroids between 2027 and 2033. Just like its ancestral namesake, Lucy aims to give us a more complete picture of our origin story.

Lucy mission asteroid targets
Lucy mission asteroid targets NASA's Lucy mission will visit 7 of Jupiter's Trojan asteroids and one main belt asteroid. Top row, from left: Patroclus and Menoetius, which orbit each other as a binary asteroid, and Eurybates (not pictured: Eurybates' small moon Queta). Bottom row, from left: Orus, Leucus, Polymele, and main belt asteroid DonaldJohanson. The largest asteroid is Patroclus, with a diameter of about 113 km (70 miles). The smallest is DonaldJohanson, with a diameter of about 4 km (2-3 miles).Image: NASA's Goddard Space Flight Center

How Lucy Works

Jupiter’s Trojan asteroids are divided into two groups that flank the giant planet as it orbits the Sun. To visit both groups, Lucy must fly to an elongated orbit around the Sun that roughly carries it between Earth and Jupiter.

After launching in 2021, Lucy will fly past Earth twice to bend its trajectory. On the way out to the Trojan asteroids, the spacecraft will fly past a bonus science target in April 2025: main belt asteroid DonaldJohnson, named after one of the co-discoverers of the Lucy fossil.

Lucy’s first trip out to Jupiter will carry it through the leading swarm of Trojan asteroids collectively known as the Greek camp. The spacecraft will fly past Eurybates and its moon Queta in August 2027, Polymele in September 2027, Leucus in April 2028, and Orus in November 2028.

Then, Lucy will fall back towards Earth before whipping back out to Jupiter. On its second visit, Lucy will fly through the trailing swarm of Trojan asteroids. Confusingly, the trailing group is called the Trojan camp! On this trip, Lucy will visit Patroclus and companion asteroid Menoetius in March 2033.

This will complete Lucy’s primary mission. However, the spacecraft will remain in a stable orbit between Earth and Jupiter, providing opportunities for an extended mission and more asteroid visits.

NASA's Lucy Spacecraft
NASA's Lucy Spacecraft NASA's Lucy mission will explore six Trojan asteroids, a unique family of asteroids that orbit the Sun in front of and behind Jupiter.Image: NASA

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