A whole host of leftovers from planet formation litter the solar system. Some of them are in the asteroid belt, others in the Kuiper belt, and a lot more in the distant Oort Cloud. They are of all sizes and compositions, orbiting the Sun in their unique orbits. But did you know that some of the small bodies share the orbit of other bigger bodies? These minor planets are called trojan asteroids. Let us know more about these curious objects and the exciting secrets they will reveal about the origin of our solar system.
What are the Trojan Asteroids?
Trojan asteroids, or simply “trojans,” are celestial bodies that share the same orbit with a larger body, like a planet. These co-orbitals are generally found in the orbit of the planet Jupiter. However, other planets have trojans too.
Just like other small bodies in the solar system, trojans are believed to be remnants of the solar nebula. Trojans remain in a stable orbit with the planet they are sharing their orbit with. There are two asteroid groups in the orbit of Jupiter, the leading group and the trailing group. These groups are both 60 degrees ahead and behind Jupiter respectively. We can get a clearer picture of the positions of these bodies by referring to the Lagrangian points.
There are five Lagrangian points labeled L1, L2, L3, L4, and L5. The trojan asteroids gather around L4 and L5. They form a 60° angle in relation to Jupiter and the Sun. The “Lagrangian points” was named after Joseph-Louis Lagrange. He was an 18th-century mathematician and astronomer who had many great scientific contributions.
The term “trojan” is a reference to Greek mythology’s Trojan War. We can think of the jovian asteroids near L4 and L5 as the two warring camps in that story. Those that are near L4 belong to the Greek camp while those near L5 are members of the Trojan camp. The names of these asteroids stay true to the camp they belong in. It is also one way for people to identify whether an asteroid is among the leading or trailing asteroids of Jupiter.
The only two exceptions for this convention are the trojans 624 Hektor (Greek) and 617 Patroclus (Trojan) which were named before the naming system was finalized.
Aside from that of Jupiter, there are also trojans in the orbits of other planets in the solar system. So far there four known Martian trojans and 28 Neptunian trojans. Those numbers are only expected to increase as more and more of them are discovered and accepted by the official body Minor Planet Center.
Interestingly, the only known trojan moons are in the system of Saturn. Some are called temporary trojans like in Venus. The first confirmed trojan of our planet, Earth, was affirmed in 2011. It was given the designation 2010 TK7.
Location Of The Trojan Asteroids
So where are the trojan asteroids? These asteroids are located in the stable Lagrangian points of a planet’s orbit. These asteroids stay at a 60-degree distance from the planet, either ahead or behind it. The leading cluster is near L4 and the trailing group is near L5. They are mostly found in the orbit of Jupiter but are also found in the orbits of Venus, Earth, Mars, Uranus, and Neptune.
It is believed that the trojans of Jupiter were captured in the orbit of this planet during its birth. It can also be that they were trapped as the giant planets migrated.
The discovery of the trojan asteroids is attributed to Max Wolf. He was born in June 1863. The German astronomer started young in his pursuits of discovering celestial bodies, particularly asteroids.
Being a pioneer in astrophotography, he became the first to use wide-field photography and the stereo comparator in his quest of discovering more asteroids. His findings will, later on, become one of the most important pieces that will lead future studies in the origin of the solar system.
It was on February 22, 1906, that Wolf discovered a particular asteroid in the orbit of Jupiter. He observed that it stayed ahead of Jupiter as the planet was moving around the Sun. This made another astronomer, Carl Charlier, believe that Wolf’s discovery must be related to the Lagrange points.
Wolf’s photographs confirmed Lagrange’s calculations which were previously purely theoretical. According to Lagrange, there are two stable points in a planet’s orbit. These are L4 and L5. Small bodies near these points would remain stable due to the Sun and planet’s gravitational forces.
Months after Wolf made a discovery in L4, August Kopff discovered yet another one. But this time, it was in the L5 point or the trailing group. A third trojan asteroid was detected months after the second one was discovered.
Other Trojan Asteroids
Trojan asteroids are not only found in the orbit of Jupiter. Venus, Earth, Mars, Uranus, and Neptune also have these small bodies near their Lagrangian points. No trojan asteroids are discovered yet in the orbits of Mercury and Saturn.
The first known trojan of Venus was a temporary trojan. It was discovered in 2013 and was given the designation 2013 ND15.
The Earth has two trojans discovered so far. Both are in the leading group or the L4 point. The 300-meter diameter 2010 TK7 was discovered in 2010. The status of this Earth trojan is already confirmed. The other one is a suspected Earth trojan asteroid called 2020 XL5.
There are about 14 trojan asteroids in the orbit of Mars. However, only some of them are confirmed by the Minor Planet Center. These small bodies are (121514) 1999 UJ7, 5261 Eureka (1990 MB), (101429) 1998 VF31, and (311999) 2007 NS2. Only 1999 UJ7 lies in L4.
The first trojan of Uranus to be discovered was 2011 QF99. It is located in L4 together with the other one 2014 YX49.
Many Neptunian trojans are already discovered. Astronomers assume that the known number of trojans in the orbit of this planet will soon surpass Jupiter’s trojan asteroids. The first known Neptune trojan is 2001 QR322. It is in the L4 region. Other trojans in the orbit of Neptune include 2005 TN53, 2008 LC18, 2011 HM102, among others.
NASA’s Lucy Mission
Lucy Mission will be the first space probe to venture into space and collect information about trojan asteroids. It aims to target seven asteroids namely Donaldjohanson, Eurybates, Polymele, Leucus, Orus, Patroclus, and Menoetius. The first one is in the asteroid belt while the six others are Jupiter trojans.
The launch of the Lucy probe is in 2021. The launch vehicle is Atlas V 401 of Cape Canaveral Space Force Station (CCSFS) in Florida. The Lucy Team is directed by Harold Levison of the Southwest Research Institute in Boulder, Colorado.
The mission is expected to last for 12 years. Data such as the asteroids’ colors, size, and compositions will be obtained from this mission through flyby encounters. The Jupiter Trojans are reddish in color with low albedos. Because of that, they look similar to centaurs, some outer main-belt asteroids, and even Kuiper Belt objects. This is about to change as information gathered by the Lucy Mission will be useful in distinguishing trojans from other small bodies.
Behind the Name
Since the Lucy Mission will be the first of its kind, it was also named after an important discovery— the Lucy fossil.
Lucy was a fossilized bone of the Australopithecus afarensis species. It was discovered in Ethiopia in the year 1974. The fossil gave light to a better understanding of the hominids. NASA is also positive that the Lucy Mission will do the same in our understanding of the evolution of the solar system.
The path that the Lucy Mission will take is illustrated by the Southwest Research Institute, as shown above.
Before going to the orbit of Jupiter, Lucy will visit a main-belt asteroid first called Donald Johanson. This small body was named after the discoverer of the Lucy fossil. This will serve as an exercise before the probe goes to Jupiter’s trojan asteroids.
After visiting the asteroid belt, the Lucy mission will head to the jovian trojans. The first cluster that it will visit is the leading group or those in the L4 of Jupiter’s orbit. Four trojans will be targeted there. These are Eurybates, Polymele, Orus, and Leucus.
The Lucy spacecraft will fly past Earth after visiting the trojans in L4 (the Greek camp). After that, it will head to Jupiter’s L5 swarm (the Trojan camp). In this group, the probe will examine Patroclus and Menoetius. These two trojans are an exciting subject because they form a binary. These asteroids have a common center of mass.
Even the team behind the Lucy Mission is excited about the flyby to Patroclus, to which Levison said:
“This object has an orbital inclination of more than 20°, and it just so happens that it will be crossing the plane of the solar system just as Lucy goes by. It was pure luck. I’ve been studying celestial mechanics for 30 years, and the celestial mechanics gods are paying me back!”
More Facts About the Trojan Asteroids
- Unlike planets which have undergone a lot of changes and internal processing, asteroids have not changed that much since their formation. This is why experts think that they hold clues about the early stages of the solar system and its planets.
- The term “trojans” generally refers to the asteroids in Jupiter’s Lagrangian points. Since trojans in the orbit of other planets are getting discovered, we can distinguish them from each other by mentioning the planet first. Like the Martian trojans, Earth trojans, and the likes.
- At the time of the discovery of the trojans, asteroids were named after characters from Greek or Roman mythology. It was the Austrian astronomer Johann Palisa who suggested that the newly discovered asteroids be named differently since they have unique orbits.
- Jovian trojans are named after the characters of Homer’s epic The Iliad. The first three named trojan asteroids are Achilles, Patroclus, and Hektor.
- The largest trojan of Jupiter is Hektor. This trojan is smaller than the 15 biggest asteroids in the main belt. It is just about 140 miles in diameter. Its brightness is also comparable to that of Pluto.
- Only a few trojans are known to be binary. These are Patroclus, Hektor, Iphthime, and Eurybates.
- The first binary to be discovered is Patroclus. The companion was discovered using the Gemini North telescope in Hawaii. William Merline and his team were behind this discovery.
- The Jovian trojans are estimated to be as many as the main-belt asteroids. However, identifying the dynamical families of the trojans in Jupiter is more challenging because their possible positions are narrower.
- Most of the Jupiter trojans are D-type asteroids, having a reddish spectrum and very low albedo. The others are P-type asteroids while there are also C-type asteroids.
None For Saturn?
- No trojan asteroids are found in the orbit of Saturn so far. Astronomers believe that it might be because they were affected by the gravitational perturbations of its bigger neighbor, Jupiter. This may have influenced and removed them from their stable points.
- There are estimated to be more than a million Jupiter trojans larger than one kilometer in diameter. Over 7,000 have been catalogued so far.
- The distribution of trojans in Jupiter’s orbit is not the same. Roughly two-thirds of them belong to the L4 cluster while the rest are near L5.
- The elongated distribution of Jupiter trojans is because of perturbations by the other planets, especially Saturn. The semi-major axis is 5.2 AU.