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What Are Trojan Asteroids?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.


When it comes to orbiting objects in space, one of the most interesting consequences happen to be Lagrange points, or places in space where an object can orbit and feel a net gravitational force of zero courtesy of geometry involving the planet and the Sun. Five of them exist for a given planet, with the first three (L1, L2, L3) on the orbital line and the other two (L4 and L5) at opposite sides of the planet, making an equilateral triangle with the Sun at the opposite vertex. Earth has these points just like all the other planets and we can put satellites and observatories there to keep them fixed relative to us. Sometimes space debris can get caught at these points, and this is especially so with Jupiter. At the L4 and L5 points, we have Trojan asteroids located roughly at 5.2 AU from the planet. Ironically, the interactions between the asteroids cause acceleration and deceleration via gravity, so the regions they exist in are not clustered tightly but spread out over a 26 degree spread, totaling in a region of 2.6 AU in length and 0.6 AU in width. The total inclination of this with respect to the ecliptic can vary also, but only by a few degrees (Davis 30, Holler).

The Trojans, or green objects, around Jupiter. The magenta are other gravitationally bound asteroids distinct from the Trojans.

The Trojans, or green objects, around Jupiter. The magenta are other gravitationally bound asteroids distinct from the Trojans.


The first Trojan asteroid found was on February 22, 1906 by Max Wolf. More followed and by 1961 about 20 were known. Today, more than 6,500 have been found. As far as labeling them goes, the naming convention for them were characters who participated in the Trojan War as depicted by Greek mythology. Asteroids found at the L4 point are people from the Greek camp while the L5 has the Trojan camp. It should be noted that while not a group of Trojan asteroids, the Hilda family of asteroids around Jupiter sometimes can cross into the different camps but remains unique to their group (which orbits in a triangular fashion around the Sun using the two noted Lagrange points and a location directly opposite Jupiter!) (Davis 31, Holler).

As far as ranges go for their properties, we can look at extreme cases to give boundaries. The largest asteroid found is 624 Hektor at 140 miles wide while the smallest is 2002 CO208 at 4 miles wide. Hektor has some other interesting properties also, including that it’s likely a contact binary and has a moon named Skamandrios (which is 7.5 miles wide). The only other known Trojan with a moon is 617 Patrioclus with moon Menoetius. As far as classifications within the Trojans go, we have C-, P-, and D-types. The latter two have many properties in common with Kuiper Belt Objects, namely their icy and tholin nature (but the levels of both are different, meaning they are not the same population). C-types have more in common with Main-Belt asteroids, namely carbon levels (hence the C) (Davis 32, Holler, Crockett).

Lingering Mysteries

Are the two camps the same? No, and the differences are important. For starters, the Greek camp (which precedes Jupiter in its orbit) has double to triple the asteroids of the Trojan camp (40-100% more). Computer simulations of the early solar system that such a grouping would occur if Jupiter migrated inward, but from a crazy 18 AU to its current 5.2 AU over 700,000 years. That is crazy fast on a universal scale, and seems unlikely. But if Jupiter did this, then the gravity ahead of it is stabilized better than behind it, essentially allowing ti to collect more asteroids ahead of it than behind it. If this is correct, then it implies Trojans are more in line with the formation of Jupiter, being pulled along for the ride while the Greeks are a mismatch collection over a wide span of space (Parks).

Visiting the Trojan Asteroids

Will we ever explore these places? Lucy will, hopefully. A discovery-level mission led by Hal Levison (SwRI) and built by Lockheed Martin, it will explore both camps in an intricate orbit. The current plan is

  • October 2021 launch,
  • April 2025 visit to Donaldjohnson (a main-belt asteroid)
  • August 2027 visit to Eurybates (L4 Trojan)
  • September 2027 visit to Polymele (L4 Trojan)
  • April 2028 visit to Laucus (L4 Trojan)
  • November 2028 visit to Orus (L4 Trojan)
  • March 2033 visit to Patrioclus with moon Menoetius (L5 Trojan)

Yes, this will set a record for most objects visited by a single mission. The probe itself will be based off the New Horizons model that visited Pluto and Ultima Thule but will look different, more like a Mars Orbiter. Measuring in at 11.5 feet by 44 feet, it will have 2 circular solar arrays and will utilize oxidizer/hydrazine for its rocket burns. It will study the masses, surface composition and layout as well as interior features of each object (Davis 33, Jones).

What is the purpose of such an elaborate mission? Simply put, to find out the origination of the asteroids and how it pertains to the evolution of the solar system. We think they are leftovers from that formation which Jupiter captured but a full analysis will be required to confirm or deny this. That’s the reason behind the name: Lucy was the skeleton of primitive man that gave us evidence for the evolution of our species from apes. Perhaps the Lucy space probe will perform a similar function for the world of astronomy (Jones).

The potential flight plan for Lucy.

The potential flight plan for Lucy.

Works Cited

Crockett, Christopher. “Trojan Asteroids Are in a Class of Their Own.” Skyandtelescope.com. Sky & Telescope, 26 Oct. 2018. Web. 08 Mar. 2019.

Davis, Joel. “Exploring Jupiter’s Trojan Asteroids.” Astronomy. June 2018. Print. 30-3.

Holler, M. Wade. “Trojan Asteroids Around Jupiter Explained.” Exploremars.org. Explore Mars Inc., 29 Jun. 2013. Web. 08 Mar. 2019.

Jones, Nancy Neal. “Lucy: The First Mission to Jupiter’s Trojans.” Nasa.gov. National Aeronautics and Space Administration. Web. 08 Mar. 2019.

Parks, Jake. "Trojan asteroids reveal Jupiter's great migration." astronomy.com. Kalmbach Publishing Co., 27 Mar. 2019. Web. 17 Aug. 2020.

© 2020 Leonard Kelley

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