NASA is scheduled on Saturday to launch a probe toward clusters
of asteroids along Jupiter’s orbital path. They are known as the Trojan swarms,
and they represent the final unexplored regions of asteroids in the solar
system. The spacecraft, a deep-space robotic archaeologist named Lucy, will
seek to answer pressing questions about the origins of the solar system, how
the planets migrated to their current orbits and how life might have emerged on
Earth.
اضافة اعلان
“We have never gone this far to study asteroids,” said Bill
Nelson, the administrator of NASA. “In so doing, we’re going to be able to
better understand the formation of the solar system, and better understand
ourselves and our development.”
After a six-year cruise, Lucy will fly close to seven Trojan
asteroids through 2033, completing wild circuits of the sun that conjure the
outline of a Formula 1 racetrack in some graphic renderings.
The spacecraft will study the geology, composition, density and
structure of the Trojans, which are small bodies locked in stable points along
Jupiter’s orbit of the sun, fixed in their own orbits ahead or behind the
massive planet.
“It’s always interesting to go somewhere for the first time,”
said Cathy Olkin, the deputy principal investigator of the Lucy mission. “Every
time we’ve done that, we’ve learned more and more about our solar system and
the area in space that we live in.”
Humanity has explored a variety of small rocky bodies throughout
the solar system. The NEAR mission landed on Eros in the inner asteroid belt.
The Dawn mission orbited Ceres and Vesta, the two largest worlds in the belt
between Mars and Jupiter. Japan’s Hayabusa missions and NASA’s OSIRIS-REX
completed close encounters with near earth asteroids. And the New Horizons
mission visited Arrokoth, an object in the solar system’s distant Kuiper belt.
But the Trojans in Jupiter’s vicinity have yet to be
investigated. About 10,000 such objects have been discovered. When the first
was spotted more than a century ago, astronomers began naming them after heroes
of Homer’s Iliad. The result was the overall descriptor of “Trojan.”
The mission name “Lucy” is a nod to the 3.2 million-year-old
australopithecine skeleton discovered in 1974, which revealed secrets of human
evolution. The NASA team hopes that the robotic Lucy does the same for the
solar system’s evolution, and prehistory is a recurring theme among the
mission’s scientists.
The mission was born of necessity.
Thirty years ago, the concept of planetary formation was much
more orderly than it is today. A star formed in the center of a rotating disc
of protoplanetary material. Gradually, the material condensed and collected
into eight planets in simple orbits (as well as Pluto).
However, when Hal Levison, a planetary scientist, and fellow
theorists tried to simulate the formation of the solar system, they repeatedly
ran into a problem: It was virtually impossible to build Uranus and Neptune in
their present orbits. To account for those worlds, known as the ice giants,
Levison, now the principal investigator of the Lucy mission, and three other
researchers developed the Nice model of solar system evolution (named for the
city in France).
The model suggests that the giant planets formed much closer to
the sun than their current orbits, and that the increasingly eccentric orbits
of a young Jupiter and Saturn destabilized and rearranged the solar system. In
the process, as the giant planets moved, and Uranus and Neptune bounded outward,
they scattered the small bodies of the solar system. Some comets and asteroids
were flung to the deep outer solar system, and others were ejected entirely out
into the Milky Way.
A small minority of scattered asteroids were ensnared in two of
Jupiter’s permanent Lagrange points, which are regions of space where the
gravitational and orbital influences of the planet and the sun are balanced.
The regions both lead and follow Jupiter in its orbit. Those asteroids are the
Trojan swarms.
Today, the Nice model offers the predominant understanding of
how a disc of dust and gas about 4.6 billion years ago became a system of
planets circling a sun. Moreover, telescope observations of exoplanets prompted
a broader scientific reevaluation of how star systems, including our own, can
form. Some distant stars are orbited by giant planets that are closer to them
than Mercury is to our sun.
Levison came to believe that the planetary science community’s
ideas about planetary formation outpaced the data it had available. The best
way to constrain variables in the Nice model would be to account for the
origins of the Trojans.
“One of the surprising things about the Trojan population is
that they are physically very different from one another but occupy a really
small region of space,” he said. “That diversity in that small region is
telling us something important about the early evolution of the solar system.”
To understand the secrets locked in the Trojans’ orbits, Levison
needed to persuade NASA to build a spacecraft to study them and determine what
formed where. Lucy is the result. It was chosen for flight in 2014 through
NASA’s Discovery Program, in which scientists compete over smaller mission
proposals.
Studying camera imagery will be an important part of the Lucy
team’s scientific efforts. By counting the number of craters spotted on each
asteroid, an object’s surface age is revealed. (Older surfaces will have been
hit by more impactors and thus show more craters.) The scientists will also
analyze the returned images for the distribution of color across the asteroids’
surfaces, which can be an indicator of what the rocks are made of: Thermal
measurements will help identify the compositions and structures of the
asteroids. They will also use infrared spectra to measure the presence of
minerals, ices and organic molecules.
NASA is interested in finding primordial organic material on
asteroids because billions of years ago, they may have seeded Earth with the
chemical ingredients necessary for life.
Despite the Trojans sharing an orbit with Jupiter, Lucy will not
visit Jupiter. Before the spacecraft even launches from Earth, it will be
closer to Jupiter than when it visits the Trojans.
During its 12-year mission it will be powered by two giant solar
arrays that are stowed at launch and gradually expand outward like folding
fans. Lucy’s roller coaster-like trajectory will carry it farther than any
solar-powered spacecraft has ever flown. And it will be moving at about 6 miles
per second at its fastest clip.
“Its speed will be like running a 10K every second,” Olkin said.
The spacecraft will be in a sophisticated orbit of
loop-the-loops across the solar system, circling the sun, borrowing gravity
from Earth for free propulsion to Jupiter’s orbital path at a point known as
Lagrange 4. Gravity will take it back around the sun to the Earth, whose
gravity will again hurl it outward, this time to Lagrange 5, and back, the
process repeating. The trajectory is driven by the positions of planets and
gravity assists, meaning the spacecraft, if nothing stops it, will continue to
do this for hundreds of thousands — if not millions — of years.
Each encounter will be at an altitude of 600 miles or less from
the Trojan’s surface. After the final flyby, depending on Lucy’s health,
NASA can target future asteroids and other celestial objects for analyses.
“As we are flying past a Trojan asteroid, we are taking data,”
Olkin said.
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