Trojan Asteroids

Image credit: https://theplanets.org/what-are-the-trojan-asteroids/

Trojan asteroids are very neat. Bound into a stable configuration by gravity, these are the remnants of the early solar system. What are the Trojan asteroids? Let’s begin by going back over four billion years, when the newly formed solar system consisted of trillions of microscopic rocky and icy objects. Many of these objects came together to form the planets. The majority of the others were scattered into the distant reaches of our solar system and beyond, but not all of them.

Some of these leftover space rocks are pristine asteroids that now orbit Jupiter in two huge swarms leading and trailing the planet. They’re known as the Trojan asteroids. They’re really mysterious, and we think that they come from the outer solar system. They’re also special in terms of understanding the evolution of the solar system because they’ve remained gravitationally stable for billions of years. There are currently over 4,800 known Trojan asteroids associated with Jupiter. About 65% of these belong to the leading group (L4) located 60^o in front of Jupiter in its orbit, while the other 35% cluster around the L5 Lagrangian point and trail 60^o behind Jupiter. Although their orbits are stabilized at the Lagrangian points by gravitational interactions with Jupiter and the sun, their actual distribution is elongated along the orbit. Perturbations by the other planets (primarily Saturn) cause the Trojans to oscillate around L4 and L5 by about 20° and at inclinations of up to 40° to the orbital plane. These oscillations generally take between 150 and 200 years to complete.

Planetary disturbances might be the reason there have been so few Trojans found around other planets. Basically, we assume that the Trojans formed at their present positions at the same time as Jupiter emerged from the solar nebula. If this is correct, we would also expect Saturn to be accompanied by families of Trojan asteroids. That no Trojans are found at the Lagrangian points of Saturn is most likely the result of Jupiter removing them from these stable orbits through gravitational perturbations.

The term ‘Trojan asteroid’ was coined when it was decided to name all asteroids discovered at the L4 and L5 points of Jupiter after warriors in the Trojan war, Greek and Trojan respectively. The exceptions are Hector (a Trojan spy in the Greek camp) and Patroclus (a Greek spy in the Trojan camp), the first two Trojan asteroids discovered and named before the two camps were established. In 1772 the French mathematician and astronomer Joseph- Louis Lagrange predicted the existence and location of two groups of small bodies located near a pair of gravitationally stable points along Jupiter’s orbit.

The presence of other planets, however—principally Saturn—perturbs the Sun-Jupiter-Trojan asteroid system enough to destabilize those points, and no asteroids have been found near them. In fact, because of that destabilization, most of Jupiter’s Trojan asteroids move in orbits inclined as much as 40° from Jupiter’s orbit and displaced as much as 70° from the leading and trailing positions of the true Lagrangian points.

In 1906 the first of the predicted objects, (588) Achilles, was discovered by German astronomer Max Wolf near L4. Within a year two more were found: (617) Patroclus, located near L5, and (624) Hektor, near L4. It was later decided to continue naming such asteroids after participants in the Trojan War as recounted in Homer’s epic work the lliad and, furthermore, to name those near the leading point after Greek warriors and those near the trailing point after Trojan warriors. Except for the two “misplaced” names already bestowed (Hektor, the lone Trojan in the Greek camp, and Patroclus, the lone Greek in the Trojan camp), that tradition has been maintained.

These strange objects are a remnant of the early universe. They’ve been alone for billions of years. But that’s about to change with the new LUCY mission. LUCY will go and visit eight of the hundreds of Trojan Asteroids. Lucy will be the first space mission to study the Trojans. The mission takes its name from the fossilized human ancestor (called “Lucy” by her discoverers) whose skeleton provided unique insight into humanity’s evolution. Likewise, the Lucy mission will revolutionize our knowledge of planetary origins and the formation of the solar system.

Lucy will launch in October 2021 and, with boosts from Earth’s gravity, will complete a 12-year journey to eight different asteroids — a Main Belt asteroid and seven Trojans, four of which are members of “two-for-the-price-of-one” binary systems. Lucy’s complex path will take it to both clusters of Trojans and give us our first close-up view of all three major types of bodies in the swarms (so-called C-, P- and D-types).

According to NASA, “No other space mission in history has been launched to as many destinations in independent orbits around our sun. Lucy will show us, for the first time, the diversity of the primordial bodies that built the planets.”

And, there you go! You’ve learned about Trojan asteroids! I hope you found this article informative, and let’s await the return of the LUCY mission in 12 years (*Yawn*) and see what we can learn!

Sources:

https://en.wikipedia.org/wiki/Jupiter_trojan

https://astronomy.swin.edu.au/cosmos/T/Trojan+Asteroids

https://www.seti.org/nasas-lucy-mission-trojan-asteroids