NASA's complex Lucy mission will open up new vistas of understanding in solar system's creation and working.
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- The story: In the early 20th century, astronomical photography showed that Jupiter, the solar system’s largest planet, was accompanied not only by its moons, but also by more distant bodies. These were clusters of asteroids that followed the giant planet’s orbit precisely, but either led or trailed Jupiter itself by about 60° of arc.
- Lagrange points: Those clusters are centred on places called Lagrange points l4 and l5, after 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 generally stuck there.
- How they were created: Most known asteroids reside in a belt between the orbits of Mars and Jupiter. Spectroscopic analysis of sunlight reflected from these, and comparisons with chunks of space rock that have fallen to Earth as meteorites, suggest that some are fragments of dwarf planets which subsequently collided with each other (a few such dwarfs, for example Ceres, remain intact), while the rest are conglomerations of material from the early solar nebula, often including pebbles of once-molten rock called chondrules. Many of the conglomerate-based asteroids are rich in carbon, which makes them dark and vaguely reddish in colour. In particular, their spectra resemble those of chondrule-bearing meteorites called carbonaceous chondrites.
- Certain asteroids in the outer part of the main belt are, however, redder still. 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 objects of this sort. And they constitute, too, the bulk of the members the Greek and Trojan camps.
- The distant orbits of these asteroids mean that they do not seem to contribute to the mixture of meteorites that fall to Earth, so there is nothing to compare them with directly.
- Their colour, though, is suspected to be caused by complex organic molecules similar to terrestrial kerogen collecting on their surfaces.
- This material’s exact chemistry will reflect where in the solar system these asteroids formed, and thus cast light on how objects moved around in the system’s early days. That makes the Trojans (as the two groups are known collectively) of special interest to those who study the formation of planetary systems. So, on October 16th nasa, America’s space agency, launched a probe to have a look at them.
- Lucy to the Trojans: Lucy, as the planetary-ancestor-investigating mission is named, after a well-known specimen of Australopithecus, an early hominid, blasted off from Cape Canaveral, Florida, and will follow one of the most complex paths around the solar system yet devised by NASA. Lucy’s journey plan shows how it will first pick up speed using two velocity-boosting fly-bys of Earth. Its final planned encounter, when it reaches l5 in 2033, is with Patroclus and Menoetius.
- These were thought to be a single object until 2001, but were seen to be actually a pair of rocks, each about 100km across, in orbit around one another.
- The most widely used classification of asteroids, invented by David Tholen of the University of Hawaii, now recognises 14 spectral types, each called by a letter of the alphabet.
- Main-belt asteroids are mostly types c (carbon-rich, as with carbonaceous chondrites), m (believed to be metallic, and derived from the cores 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 deep-red ones are of rarer types known as 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, these will be able to map the various asteroids the probe encounters, and analyse the chemical compositions of their surfaces.
- Background: The gravitational effects of all this movement would have scattered asteroids like leaves in a gale, ejecting many of them from the solar system altogether and moving others far from their places of origin. Experts suspect the Trojan d and p asteroids to be such travellers, and that their surface chemistry will show they formed much farther from the sun than Jupiter’s current orbit.
- Finally: Once Lucy has visited Patroclus and Menoetius its formal mission will be over. These days, however, space probes often have an afterlife beyond the period intended. The New Horizons mission to Pluto has gone on to observe a second Kuiper-belt object, Arrokoth, and Opportunity, an American Mars rover that had a design life of 90 Martian days, actually operated for 57 times that period. In Lucy’s case, the craft’s final orbit will take it on an endless loop from l5 to l4 and back again. Each loop will take six years. So, if the hardware doesn not break down, there is sufficient fuel left for manoeuvring.
- EXAM QUESTIONS: (1) Explain the purpose of the NASA Lucy mission. (2) What was the story of creation of the solar system? Explain. (3) Differentiate between asteroids, meteorites, meteoroids and meteors.
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