NASA’s Lucy: voyage to the Trojan asteroids

Space probes tend to be aimed at a single major target, or its moons. It is rare for a spacecraft to be aimed at several equally important objects in different orbits around the Sun. This, however, is exactly what is doing the currently-underway Lucy mission. Aimed at Jupiter’s Trojan asteroids, this NASA spacecraft will open a window into the ancient Solar System.

Trojan asteroids

While we usually think of planets and asteroids as orbiting under the influence of the Sun alone, the reality is far more complex. All objects with mass attract each other. While usually these interactions are negligible, they sometimes lead to interesting consequences: one of these is the phenomenon of Trojan asteroids.

These are asteroids that orbit 60° ahead or behind a given planet on the same orbit, always staying near the L4 and L5 Lagrange points. The planet with the most Trojans is Jupiter, thanks to its enormous mass: it has more than 9800. The asteroids are all named after heroes of the Trojan War, according to their location: the Greek camp ahead of Jupiter and the Trojan one behind.

Artist impressions of seven of the eight targets Lucy will visit. First two Trojan asteroids from the Trojan camp, then four from the Greek camp, and an inner main belt asteroid. For reference, Menoetius is approximately 100 km across. Credits: NASA

According to leading theories, Trojan asteroids are remnants of the outer portions of the protoplanetary disk from which the planets formed. After that period, things did not stay put for long. Instead, planets slowly changed their orbits, ending up where they are today. During this process, called migration, much of the leftover raw materials were ejected outward, but some became trapped in the gravitational interactions of Jupiter and the Sun. That material became the Trojan asteroids, which thus offer a window into the early days of our planetary system.

Based on Earth observations, Trojan asteroids are very diverse. This hints that they were formed in different parts of the outer protoplanetary disk and that they had different histories. Lucy will map and characterize surface geology and composition, interior structure, and natural satellites. The spacecraft is named after the Lucy hominin fossil since Trojan asteroids can be thought of as fossils of planetary formation.

Trajectory and spacecraft

Lucy was launched on October 16, 2021, on a ULA Atlas V 401 rocket. Since then, the spacecraft has performed an Earth flyby in 2022, boosting its orbit around the Sun. It will do so again in 2024 when it will get enough energy to reach the Jupiter Trojans. Meanwhile, it will also visit two asteroids in the inner main belt. After arriving in the Greek Camp in 2027, Lucy will spend one year flying by four asteroids, then it will fall back towards the Earth. Using another gravity assist in 2030, it will propel itself toward the Trojan camp. Its target there is the double asteroid Patroclus-Menoetius. 617 Patroclus was named before the Greek/Trojan camp convention and thus constitutes a “Greek spy” in the Trojan camp.

The launch of NASA's Lucy mission on a United Launch Alliance Atlas V 401 rocket. October 16, 2021. Credits: NASA — *The launch of NASA’s Lucy mission on a United Launch Alliance Atlas V 401 rocket. October 16, 2021. Credits: NASA*

The main body of the spacecraft is a cube approximately two meters in size. This mainly houses the chemical propulsion systems and the electronics. Several features stick out: the high-gain antenna, the Instrument Pointing Platform, and two solar panels. The latter are huge, 7 meters in diameter.

The size is needed to ensure power production at Jovian distances, where solar irradiation is just 4% of that on Earth. At that point, the panels will produce 504 W. Lucy will be the second mission to use solar panels at Jovian distances, after Juno. ESA’s JUICE will also be the third.

The trajectory Lucy will follow. All animations are from Jupiter’s point of view, leaving the Trojan asteroids almost stationary. Credits: NASA

While one of the solar panels has deployed correctly, the other deployed only in part. After many attempts, the panel was unfolded almost completely, but it could not be latched in place. Mission controllers announced in December 2022 that they think further efforts will have no results, and that the solar panels should allow the mission to continue nominally. Doubts should be cleared in February 2024, when the first main engine burn will take place. The likely cause for the failure is the deployment cable slipping from its pulley and winding on the motor axle.

Instruments

Lucy is fitted with a suite of remote-sensing instruments, all mounted on a platform for pointing. Many of these are based on instruments flown on New Horizons or OSIRIS-REx.

L’LORRI is Lucy’s most sensitive camera. It will provide high-resolution black-and-white images, sensing light from the entire visible spectrum. It is capable of detecting a 70 m crater from 1000 km away, even if the surface is very dark. These pictures will help understand the Trojan asteroids’ geology, and also assist in the search for potential moons and ring systems. The camera has no moving parts, reducing chances for failure. Most of the optical system is made of silicon carbide, which allows greater precision thanks to its minimal thermal dilation.

In March 2023 Lucy used its L'LORRI instrument to image the Greek camp Trojans it will visit in 2027-2028. Even if they appear as little dots, this data will help the team choose exposure times for the flybys. Credits: NASA — In March 2023 Lucy used its L’LORRI instrument to image the Greek camp Trojans it will visit in 2027-2028. Even if they appear as little dots, this data will help the team choose exposure times for the flybys. Credits: NASA

L’Ralph is two instruments in one. A beam splitter is used to separate light into two components: the visible one will go to the MVIC color visible imager, while the infrared one will go to LEISA infrared spectrometer. Their goal, however, is the same: to use spectrometry to determine the chemical composition of the asteroids. They will in particular look for organic compounds, ice, and hydrated minerals.

L’TES is a thermal infrared spectrometer, meaning it will be used to measure the temperature of a point on the surface. While other instruments are more similar to cameras, this is closer to a remote thermometer. Measuring the temperature and its changes will not only allow the characterization of the thermal properties of the materials. It will also yield insight into the consistency of surface material, since small particles, like dust or sand, change temperature more quickly than bigger rocks.

An artist's concept ot Lucy flying by one of its targets. Credits: NASA — *An artist’s concept of Lucy flying by one of its targets. The Instrument Pointing Platform is visible on the left. Credits: NASA*

While being primarily a navigation device, T2CAM will also be used to take wide-field images of the asteroids, capturing their shape. Another device with a “secondary” scientific use is the high-gain antenna. By measuring the Doppler shift of radio signals, scientists will be able to deduce the precise changes in the velocity of the spacecraft, and thus the mass of the asteroids. Combining this data to calculate density will further increase our knowledge of these celestial bodies.

The next upcoming milestone for the mission is the flyby of the main belt asteroid 152830 Dinkinesh on November 1, 2023. The small asteroid carries the Ethiopian name of the Lucy fossil, meaning “you are marvelous”. After an Earth flyby in 2024, Lucy will also visit an asteroid named after the discoverer of the fossil, 52246 Donaldjohanson, on April 20, 2025. For the first flyby of a Trojan asteroid, we’ll have to wait until August 2027. But it will surely be worth it.