Lucy: Mission to the Trojan Asteroids – Solar System Fossils

Lucy is a NASA space probe on a twelve-year journey to eight different asteroids, visiting a main belt asteroid as well as seven Jupiter trojans.

This time-lapsed animation shows the movements of the inner planets (Mercury, brown; Venus, white; Earth, blue; Mars, red), Jupiter (orange), and the two Trojan swarms (green) during the course of the Lucy mission. Source: NASA

What are Trojan asteroids?

For any given Two body system in space (both bodies are having mass) orbiting each other, there exist 5 points (called Lagrange points) in their orbital plain where net gravitational pull is zero due to these two bodies. Any third body if placed on any of these points will get zero gravitational force from other two bodies, and it will remain there suspended freely unless an external force is applied on it.

Now in our case, we have Sun and Jupiter system. And two of these Lagrange points are on orbit of Jupiter. That’s where you will find these Trojan Asteroids. Other Lagrange points are: one between Sun and Jupiter, one on far side of Jupiter and one on far side of Sun. Now since Jupiter is orbiting Sun, all these 5 Lagrange points will also move, that’s why these Trojan asteroids don’t change their position relative to Jupiter.

Some other Examples of Objects freely suspended in Lagrange point L2:

  1. China has suspended an orbiter on far side of moon, on Lagrange point L2 of Earth Moon System, to establish a communication with its rover on far side of Moon.
  2. NASA will place James Webb Telescope on far side of Earth, on Lagrange point L2 of Earth Sun system. To observe further deep in space.
  3. Gegenschein is sunlight scattered by interplanetary dust. Most of this dust is orbiting the Sun in about the ecliptic plane, with a possible concentration of particles at the L2 Earth–Sun Lagrangian point.
Lagrange points L4 and L5 are where Trojan asteroids are found.

Here we can see all 5 Lagrange points. 5 and 4 is where all Trojan Asteroids are suspended. At 2 is where all objects from above three examples are.

Along with 7 Trojan asteroids, Lucy will also be visiting one main belt asteroid which happens to be in it’s path of journey towards L4 Trojans.

Lucy mission trajectory. The first visit, in 2025 and the last in 2033. Source: NASA

Instruments onboard Lucy and what will we learn?

Density determination – To determine density, we need to find the mass of the asteroid. For that, as Lucy space probe will pass through an asteroid it will experience a slight decrease in its velocity ~1/100th of mile/hr slower. With calculating the change in flyby speed we can determine how massive an asteroid is.

L’Ralph, L’LORRI and L’TES instruments on Lucy. Source: NASA

Two instruments – L’Ralph and L’LORRI – onboard Lucy are successors of those currently being used on New Horizons space probe.


Panchromatic and color visible imager (0.4-0.85 μm) and infrared spectroscopic mapper (1-3.6 μm). L’Ralph is based on the Ralph instrument on New Horizons and was built at Goddard Space Flight Center. It will be used to measure silicates, ices, and organics at the surface. To understand composition of Trojans and how they are related to geology of Trojans.


High-resolution visible imager. L’LORRI is derived from the LORRI instrument on New Horizons and was built at the Johns Hopkins University Applied Physics Laboratory. It will provide the most detailed images of the surface of the Trojans.

L’LORRI has no moving parts, no filters and is of same design as that of Hubble Space Telescope. It is capable of studying surfaces of trojans in high resolution.

Source: NASA

What will we find out on Trojan asteroids? Mountains, basins, pits, valleys? We don’t know and we will surely be surprised!


Thermal infrared spectrometer (6-75 μm). L’TES is similar to OTES on the OSIRIS-REx mission and was built at Arizona State University. It will reveal the thermal characteristics of the observed Trojans, which will also inform the composition and structure of the material on the surface of the asteroids.

L’TES will use long wave infrared light to see detailed mapping of temperatures of these Trojans. By studying the changes throughout the day night on Trojans in temperature, we can figure out the material it is made of.

Temperature map of Bennu asteroid. Source: NASA
Temperature mapping of the Mars. Source: NASA
Lucy is also taking diamonds into space – Lucy beam splitter! Source: NASA

The spacecraft is also adorned with a golden plaque containing its launch date, the positions of the planets at the launch date, the continents of Earth at the time of launch, its nominal trajectory, and twenty speeches, poems, and song lyrics from people such as Martin Luther King Jr., Carl Sagan, The Beatles, and more.

Lucy Plaque

Because the spacecraft will not leave the Solar System or be ditched into a planetary body, there is a chance that future generations of humanity will be able to recover it and with this mission “what was the foot of the walker becomes the path” of the human lineage.


Published by Anand Krishna

Amateur astronomer and astrophotographer. Interested in astrophoto processing, astrostatistics, comet hunting, visual and radio astronomy.

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