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THE STORY of the Trojan War, told by Homer, is a story of intrigue and heroes. And, as is the case with figures in Greek mythology, many of the names of those involved ended up being attached to objects in the sky.
At the start of the 20th century, when astronomical photography was getting good enough to see such things, it became clear that Jupiter, the largest planet in the solar system, was accompanied not only by its own system of moons, but also by two groups. more distant acolytes. . These were clusters of asteroids that precisely followed the orbit of the giant planet, but which led or followed Jupiter itself by about 60 ° of arc.
These clusters are centered on places called Lagrange points L4 and L5, according to the mathematician who predicted their existence. L4 and L5 are traps in space caused by the interaction of the gravitational fields of Jupiter and the sun. Once in one of them, an asteroid is usually stuck there. Given the division of these captured asteroids into two camps, he amused their first discoverers to call the bodies discovered at L4, which leads Jupiter, after the Greek heroes of the war, and those of L5, which follows this planet, after the Trojans (although each side also has a spy for the other in the middle).
The siege of Troy
Most of the known asteroids reside in a belt between the orbits of Mars and Jupiter. Spectroscopic analysis of the sunlight reflected from them and comparisons with pieces of space rock that fell to Earth as meteorites suggest that some are fragments of dwarf planets that then collided with each other ( although a few, like Ceres, remain intact), while the others are conglomerates of material from the early solar nebula, often including pebbles of once molten rock called chondrules. Many conglomerate-based asteroids are high in carbon, making them dark and vaguely reddish in color. In particular, their spectra resemble those of meteorites carrying chondrites called carbonaceous chondrites.
Some asteroids in the outer part of the main belt, however, are even redder – carbonaceous chondrites on steroids, so to speak. These types are also abundant in a second, more distant set of asteroids and dwarf planets, the Kuiper Belt, found beyond the orbit of Neptune, of which Pluto is the best-known member. The nuclei of active comets are often such objects. And they also constitute the bulk of the members of the Greek and Trojan camps.
The distant orbits of these asteroids mean they don’t appear to be contributing to the mix of meteorites falling to Earth, so there’s nothing to compare directly. Their color, however, is believed to be caused by complex organic molecules, similar to terrestrial kerogen, which accumulate on their surface. The exact chemistry of this material will reflect where in the solar system these asteroids formed, and thus shed light on how objects moved in the early days of the system. This makes Trojans of particular interest to those who study the formation of planetary systems. So on October 16, if all goes well, NASA, the US space agency, will launch a probe to observe them.
Lucy, as this planetary ancestor fact-finding mission is nicknamed, after a well-known specimen from Australopithecus, one of the first hominids, will take off from Cape Canaveral, Florida, and then follow one of the most complex paths around the solar system ever imagined by NASA’s orbital navigators. The diagram above (which displays Lucy’s orbit using Jupiter’s orbit as a fixed frame of reference) shows how the craft will first gain speed using two flyovers of Earth increasing in speed. It will then head to the Greek camp of L4, passing, for some observation training, passing by a practical main belt asteroid that mission scientists named Donaldjohanson, in honor of the discoverer of Lucy the Australopithecus. When he arrives at L4 in 2027, he will encounter five bodies: Eurybates and its tiny satellite Queta, Polymele, Leucus and Orus.
After examining each of them, he will leave the Greek camp in 2028 and cross, via another flyby over Earth at high speed, to the Trojan camp at L5. Her last scheduled meeting, when she reaches L5 in 2033, is with Patroclus and Menoetius.
It was thought to be a single object until 2001, when an examination by the Gemini North Telescope in Hawaii showed that it was in fact a pair of rocks, each of which ‘about 100 km in diameter, orbiting each other. Classics will recognize Patroclus, the name used when the couple were meant to be one, as the Greek spy in the Trojan camp. It is now the name given to the larger of the two. Menoetius, in a hurry to designate the smaller one, was Patroclus’ father. (For the curious, the Trojan spy in the Greek camp is Hektor, who is not on Lucythe route of.)
The most widely used asteroid classification, invented by David Tholen of the University of Hawaii, now recognizes 14 spectral types, each named by a letter of the alphabet. The asteroids in the main belt are mainly of type C (rich in carbon, like carbonaceous chondrites), M (considered metallic and derived from the nucleus of dwarf planets) and S (silicaceous or stony, and either other types of conglomerate or fragments of the outer layers of dwarf planets). Some Trojans are also type C. But the redder ones are rarer types called D and P, the difference between them being the exact shapes of the curves of their spectra.
To look at its targets in detail (Polymele, Patroclus and Menoetius are type P; Orus and Leucus are type D; and Eurybates and Donaldjohanson are type C), Lucy is equipped with three instruments: a high resolution camera, an optical and near infrared spectrometer, and a thermal spectrometer. Together, they will be able to map the different asteroids encountered by the probe and analyze the chemical compositions of their surfaces.
This, in turn, will test the ideas of Harold Levison, the mission’s principal scientific researcher. Dr Levison, who works at the Southwest Research Institute in Boulder, Colorado, is one of the creators of the Nice model of the early days of the solar system. This assumes that the four largest planets in the system, Jupiter, Saturn, Uranus, and Neptune, have moved significantly since their formation.
A heavenly odyssey
The gravitational effects of all this movement would have scattered the asteroids like leaves in a gust of wind, ejecting many of them completely from the solar system and displacing others far from their places of origin. Dr Levison suspects Trojan asteroids D and P to be such travelers, and that their surface chemistry will show that they formed much further from the sun than Jupiter’s current orbit. The Nice model suggests that such distant asteroids would have been dispersed, in particular, by the movement of Neptune, which began its life closer to the sun than Uranus but is now further from it, and that some would then have been trapped by the Lagrange gravitational traps. If types D and P Lucy meetings were indeed formed far from the sun, which would give weight to the Nice model.
Once Lucy visited Patroclus and Menoetius his formal mission will be completed. Nowadays, however, space probes often last longer than expected. The New Horizons mission to Pluto, for example, continued to observe a second Kuiper Belt object, Arrokoth, and Opportunity, an American Martian rover that had a nominal life of 90 Martian days, actually operated 57 times that period. In LucyIn this case, the final orbit of the craft will take it on an endless loop from L5 to L4 and vice versa. Each loop will take six years. So if the equipment holds up, there is enough fuel left for the maneuvers, and someone is willing to pay to keep the mission team together, other Trojans could still be visited.
Source: NASA
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