The Lucy Trojan Asteroid Mission

NASA’s Lucy mission is unique in that it’s the first to study a certain type of asteroid, called Trojans. Trojan asteroids share Jupiter’s orbit around the Sun. These asteroids may have formed farther out in the solar system than other asteroids we have studied. Ultimately, they have been caught by Jupiter’s gravity. They likely have different blends of the solar system’s starting materials than other asteroids we’ve previously visited. Consequently, Trojan asteroids have been called “fossils of planet formation” because scientists think they hold clues to understanding the formation of the solar system.

Launched on October 16, 2021, Lucy (the spacecraft) will visit seven of Jupiter’s Trojan asteroids between 2027 and 2033 (and one asteroid in the asteroid belt). These Trojan asteroids are situated at gravitationally stable points ahead of and behind Jupiter (Lagrangian points). There are two groups of Trojan asteroids. One group orbits the Sun in Jupiter’s orbit, preceding the planet. The other group trails Jupiter in its orbit. As more and more of these asteroids were discovered in the early 1900s, they were named after participants in the Trojan War as told in Homer’s epic work the Iliad. Asteroids orbiting ahead of Jupiter are named after Greek warriors, and the asteroids trailing Jupiter are named after Trojan warriors.

An artist’s impression of Lucy and two asteroids. Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab/Adriana Gutierrez

Lucy will visit seven Trojan asteroids and one asteroid in the asteroid belt. The mission relies on gravity assists to do this. The trailing Trojans are at L5, and the Trojans ahead of Jupiter are at L4. Image credit: NASA

You may be wondering if “Lucy” is a typical NASA acronym. It isn’t. The mission is named after the fossilized skeleton that helped scientists learn where humans fit into the evolutionary chain of life. The skeleton was discovered in Ethiopia in 1974, and she was named Lucy. Scientists hope that the Lucy asteroid mission will revolutionize our knowledge of planetary origins and the formation of the solar system.

Lucy will fly by Earth twice using gravity assists to speed up and head to Jupiter. These gravity assists will also be used to direct Lucy toward its targets. (See Gravity Assists below.)

Notice how Lucy speeds up as it goes around Earth. That’s the gravity assist. The first gravity assist will happen on October 16, 2022. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab

Lucy will first make one huge loop out to Jupiter to visit some of the preceding Trojans. Then this looping path will take it back to Earth for another gravity assist, which will take it out to the trailing Trojans.

Lucy will continue cycling between the two groups of asteroids every six years. Lucy will not orbit these asteroids but will fly by them.

Note to Parents and Educators

There are some activities available at, such as colouring activities and a Lucy Paper Snowflake download.

Gravity Assists

A gravity assist uses the gravity of a planet (or the Sun) to slow a spacecraft down or speed it up. For instance, if a spacecraft is heading toward the Sun, the Sun’s gravity just starts pulling it in, and the spacecraft goes faster and faster. A gravity assist from Earth, Venus, or Mercury can be used to slow it down—the gravity assist acts as a brake, slowing the spacecraft down. Going the other way, say, to Jupiter, the spacecraft can use Earth’s gravity to increase its speed. With the gravity assist, there is a change in momentum, which changes the energy of the spacecraft. The gravity assist technique is the result of over 50 years of development of spacecraft navigation techniques. This technique is used to save fuel, which matters when designing an efficient spacecraft.