Jupiter’s Trojan asteroids provide surprises even earlier than NASA’s Lucy mission has an opportunity to go to them.

A new study released this month suggests that Jupiter’s Trojan asteroids may be stranger than previously thought. The Trojan asteroids are rocky objects that orbit the sun just in front of and just behind the gas giant in gravitational sweet spots known as Lagrangian points. The swarm in front of Jupiter, known as the L4 group (Greek), is slightly larger than the L5 swarm (Trojans) behind it, but until now astronomers believed that there was little other than that between the two swarms. The paper released this month seems to change that.

The research team, using data from the Hawaii-based asteroid Terrestrial-Impact Last Alert System (ATLAS), discovered unexpected differences in the shape of the Trojans. This new study suggests that objects in the L4 population are on average longer than objects in the L5 population.

In this image, Jupiter’s Trojan asteroids are shown in green. The L4 group is referred to as “Greeks” as these objects are usually named after Greek heroes of the Trojan War, while the L5 asteroids are referred to as “Trojans” as they are named for Trojan heroes. Photo credit: Wikipedia Commons

Why is that important? Well, the difference “may imply a different collision development within each cloud,” the paper suggests. The larger population of the L4 swarm means that objects within it had more opportunities to collide with one another. When one Trojan hits another, larger objects are worn away or broken into smaller pieces. In billions of years of impact, more L4 objects have been knocked into eccentric shapes than L5.

This discovery is a lesson from the evolutionary history of the solar system, and the Jupiter Trojans may have much more to offer scientists in the near future. To get a closer look at these original remnants of the early solar system, NASA will launch a robotic spacecraft to visit the Trojans later this year. The mission is called Lucy, after the fossilized remains of an early human ancestor found in Ethiopia in 1974. Lucy taught paleontologists about human evolution, and similarly, the spaceship Lucy can teach astronomers the early history of the solar system.

Artist’s impression of Lucy. Photo credit: Southwest Research Institute

One of Lucy’s primary goals is to understand the makeup and diversity of Trojans. It is believed that these asteroids represent the remains of planet formation. So by learning about their structure, age, and constituents, we can understand the ingredients that made the planets we see today, including possibly the organic materials found making their way to Earth in their infancy.

Lucy will leverage lessons from previous missions and carry instruments similar to those flown on NASA’s New Horizons that flew past Pluto in 2015 and OSIRIS-Rex, which is currently bringing a sample of asteroid Bennu back to Earth. With a number of clever gravity aids, Lucy can visit more destinations on a mission than any other probe in the solar system. It flies past at least eight different asteroids over twelve years, starting with one in the main asteroid belt, and then bouncing back and forth between objects in swarms L4 and L5.

Lucy’s railroad. Photo credit: Southwest Research Institute

Careful planning and a bit of astronomical luck mean Lucy even has the option of visiting two targets in the L5 swarm that move in high-incline orbits, which usually makes them very difficult to get to. The double pair Patroclus and Menoetius will be within reach of Lucy in 2033 and thus represent a spectacular finale for Lucy’s main mission.

Lucy will launch an Atlas V rocket from Cape Canaveral, Florida in October. The full mission timeline is as follows:

• October 16, 2021: The three-week start window opens.
• April 20, 2025: Main object of the asteroid belt (52246) Donaldjohanson.
• August 12, 2027: L4 object (3548) Eurybates and its Queta satellite.
• September 15, 2027: L4 object (15094) Polymele.
• April 18, 2028: Object L4 (11351) Leucus.
• November 11, 2028: Object L4 (21900) Orus.
• March 2, 2033: L5 object (617) Patroclus and his partner Menoetius.

Whatever else Lucy may discover, it is clear that a combination of ground-based astronomy and spacecraft flyby will open a new chapter in our understanding of planet formation, and the Trojans are likely to hold more surprises in store for us in the years to come.

Since this video was recorded, the asteroid Queta has been added to the mission profile following its discovery in 2020.]

Reference. A. McNeill et al. “Comparison of the physical properties of the Trojan asteroids L4 and L5 using ATLAS data.” arXiv preprint arXiv: 2101.04602 (2021)

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