The Jupiter Trojan asteroids are a population of small bodies, the largest about 160 miles (250 kilometers) wide, that are leftover raw materials from the formation of our solar system’s giant planets (Jupiter, Saturn, Uranus, and Neptune). They share an orbit with Jupiter as it goes around the Sun, although, on average, they are as far away from Jupiter as Jupiter is from the Sun! They may not be as famous as the objects in the main asteroid belt between the orbits of Mars and Jupiter, but they are almost as numerous. Caught between the gravity of the Sun and the gravity of Jupiter, they stay in the vicinity of Jupiter’s L4 and L5 Lagrange points, two points that form an equilateral triangle with Jupiter and the Sun, leading and following Jupiter in its orbit.

Lagrange points are positions in space where the gravitational forces of a two-body system like the Sun and the Earth produce enhanced regions of attraction and repulsion. These can be used by spacecraft to reduce fuel consumption needed to remain in position.

In a situation where two large bodies are orbiting each other (for instance, Jupiter orbiting the Sun, the Moon orbiting the Earth, etc.), there are five places where a third body can freely orbit along with the other two, always staying the same distance from, and in the same relative orientation with, each of them. These five places are the Lagrange points.

The Trojan asteroids are time capsules from the earliest days of our solar system, more than 4 billion years ago; but they have never been explored by a space mission. They are a diverse group. Scientists hypothesize that they are a mixture of bodies that formed all over the outer solar system, and later became trapped around Lagrange points. Whatever Lucy finds at the Trojan asteroids will give us vital clues about the formation of our solar system.

Surprisingly for NASA, Lucy doesn’t stand for anything! The name Lucy was chosen in honor of the fossilized human ancestor, named "Lucy," that was found in Ethiopia in 1974. This Lucy mission will refine our understanding of our solar system’s origins and evolution, just as the Lucy fossil revolutionized our understanding of the human species.

Keep up to date with fun ways to interact with the Lucy mission by following @NASASolarSystem on social media and looking for the hashtag #LucyMission. If you are an undergraduate science or engineering student enrolled in an accredited U.S. institution and want to learn more about how NASA develops missions, space technology, and the next generation of space explorers, apply to join the free online L’SPACE Academy at https://lspace.asu.edu.

Lucy launched on Oct. 16, 2021 at 5:34 a.m. EDT (9:34 UTC) on a United Launch Alliance (ULA) Atlas V rocket from Space Launch Complex 41 at Cape Canaveral Space Force Station, Florida?

Lucy’s prime mission is nearly 12 years long. During that time, Lucy will travel almost 4 billion miles (6 billion kilometers), making three-and-a-half giant loops around the Sun. Lucy’s average cruising speed is about 39,000 miles (62,000 kilometers) per hour. It will slow down to 15,000 miles (24,000 kilometers) per hour on average as it passes each Trojan asteroid. Lucy will be the first spacecraft to travel a bit beyond Jupiter and return to the vicinity of Earth for a third and final gravity assist that will send it out to its final Trojan encounters.

Lucy will fly by eight Trojan asteroids and three main belt asteroids, visiting a record number of objects on separate orbits around the Sun.

Most of the Trojan asteroids are named for heroes in Homer’s "The Illiad." Below are some of the pronunciations most frequently used by the Lucy team. Note: these are anglicized versions, not what a modern (nor classical) Greek speaker would say.

Asteroid and Pronunciation:

Eurybates: “yoo-RIB-a-teez” or “you-ri-BAY-teez”
Queta: “KEH-tah” (named in honor of modern Olympian Enriqueta Basilio)
Polymele: “pah-li-MEH-lee” or “pah-LIM-ah-lee”
Leucus: “LYOO-kus” or “LOO-kus”
Orus: “OR-us”
Patroclus: “pa-TROH-klus” or “PAT-roh-klus”
Menoetius: “men-EE-shus”

Lucy's trajectory is dictated by celestial mechanics, which is the science of gravity's affect on the movement of everything in the solar system. In essence, engineers found that Lucy's trajectory brings it close to a number of interesting asteroids. Lucy’s first Trojan target, Eurybates, was selected because it differs from most of the other Trojan asteroids in two ways: it is both the largest remnant of a rare massive collision, and it has a neutral-colored C-type spectrum. Scientists hypothesize these two facts may be related, and that by exploring Eurybates we might get a glimpse of the “insides” of asteroids which could be exposed on the surface. However, every good scientist knows that comparisons are important, so, with careful planning and a little luck, the team found a trajectory passing objects spanning the full range of colors (including redder-colored D and P spectral-types) and sizes, from less than a mile (approximately 1 kilometer) to around 70 miles (over 100 kilometers) in diameter.

It takes a lot of rocket fuel to slow down and orbit an asteroid, and even more fuel to do it multiple times. As the first explorer of the Trojan asteroids, Lucy serves as a scout, gathering lots of visual, compositional, and physical information with robotic cameras, spectrographs, and Doppler tracking (to measure the mass). In future missions, that information can be used to plan more exploration.

After the mission ends in 2033, Lucy will continue on a stable orbit, traveling from near Earth’s orbit out to the Trojan swarms. The team has carefully designed the trajectory to ensure that Lucy will not hit Earth or contaminate any place that could have signs of life for more than 100,000 years. If no future humans collect Lucy as a historical artifact of the early days of solar system exploration, then Lucy’s orbit will eventually become unstable, and Jupiter's gravity will most likely send the spaceship into the Sun or fling it out of the solar system.

Lucy has three science instruments:

L’Ralph is two instruments in one: a color camera (the Multi-Spectral Visible Imaging Camera, MVIC) and an infrared imaging spectrometer – an instrument to determine the composition of materials on the surfaces of the asteroids, called the Linear Etalon Imaging Spectral Array, LEISA.

L’LORRI, the Long Range Reconnaissance Imager, is the high spatial resolution panchromatic (black and white) camera.

L’TES is the far infrared Thermal Emission Spectrometer, basically a thermometer to remotely measure the temperature and thermal properties of the surface.

Additionally, Lucy will use its high-gain antenna to communicate with Earth for an additional radio science experiment to determine the masses of its asteroid targets using the Doppler shift of the radio signal. Lucy will also be able to use its terminal tracking cameras (T2CAM) to track the asteroids during encounters to keep them in the field of view and take wide-field images of the asteroids to better ascertain their shapes, and perhaps discover new asteroids.

Scientists first hypothesized that the Trojan asteroids formed in their current location, and thus represent the same population that formed the central core of Jupiter billions of years ago. However, when astronomers obtained more information about the early evolution of the solar system and noticed the subtle but significant diversity in the Trojan population, they revised this view. The evidence now suggests that these objects might have come from all over the outer solar system and settled in their current locations after being scattered by the gravitational pulls of Saturn, Uranus, and Neptune during a period of great instability early in our solar system’s evolution.

The Lucy spacecraft isn’t actually going close to Jupiter at all. Though they are called the Jupiter Trojan asteroids, the asteroids average the same distance from Jupiter as Jupiter is from the Sun. Fun fact: The Lucy spacecraft will be closer to Jupiter during its Earth gravity assists then it will be when it encounters the Trojan asteroids.

With the solar arrays completely unfurled, the spacecraft is nearly 52 feet (16 meters) wide. Each solar array is nearly 24 feet (7 meters) in diameter.

There are two main types of power sources for space missions: solar arrays and radioisotope thermoelectric generators (RTGs). Lucy will break records by operating farther from the Sun than any previous solar-powered spacecraft. While Lucy’s large solar arrays do pose some challenges for the mission (they are large and make maneuvering more difficult), they are made of materials that are both easily available and easy to work with. Lucy’s solar arrays deliver significantly more power for the same amount of weight, leaving room for the spacecraft and instruments. The solar-powered Lucy spacecraft is allowing us to explore farther into the solar system than ever before and fostering the development of more efficient solar cells.