Highlights:
› Asteroids, comets and other small objects in space hold clues to our origins, but may also pose hazards.

› Small worlds likely delivered the ingredients of life to Earth.
› Several NASA missions are either on their way to these small worlds, or are in development.

The entire
history of human existence is a tiny blip in our solar system’s 4.5-billion-year
history. No one was around to see planets forming and undergoing dramatic
changes before settling in their present configuration. In order to understand
what came before us — before life on Earth and before Earth itself — scientists
need to hunt for clues to that mysterious distant past.

Those clues
come in the form of asteroids, comets and other small objects. Like detectives sifting
through forensic evidence, scientists carefully examine these small bodies for insights about our
origins. They tell of a time when countless meteors and asteroids rained down
on the planets, burned up in the Sun, shot out beyond the orbit of Neptune or
collided with one another and shattered into smaller bodies. From distant, icy
comets to the asteroid that ended the reign of the dinosaurs, each space rock
contains clues to epic events that shaped the solar system as we know it today —
including life on Earth.

NASA’s missions
to study these “non-planets” help us understand how planets including
Earth formed, locate hazards from incoming objects and think about the future
of exploration. They have played key roles in our solar system’s history, and
reflect how it continues to change today.

“They
might not have giant volcanoes, global oceans or dust storms, but small worlds
could answer big questions we have about the origins of our solar system,”
said Lori Glaze, acting director for the Planetary Science Division at NASA Headquarters
in Washington.

NASA
has a long history of exploring small bodies, beginning with Galileo’s 1991
flyby of asteroid Gaspra. The first spacecraft to orbit an asteroid, Near
Earth Asteroid Rendezvous (NEAR) Shoemaker, also successfully landed on
asteroid Eros in 2000 and took measurements that originally hadn’t been planned.
The Deep
Impact mission drove a probe into Comet Tempel 1 in 2005 and prompted scientists
to rethink where comets formed. More recent efforts have built on those
successes and will continue to teach us more about our solar system. Here’s an
overview of what we can learn:

This representation of Ceres’ Occator Crater in false colors shows differences in the dwarf planet’s surface composition. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
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Building Blocks of Planets

Our solar
system as we know it today formed from grains of dust — tiny particles of
rock, metal and ice — swirling in a disk around our infant Sun. Most of the
material from this disk fell into the newborn star, but some bits avoided that
fate and stuck together, growing into asteroids, comets and even planets. Lots
of leftovers from that process have survived to this day. The growth of planets
from smaller objects is one piece of our history that asteroids and comets can
help us investigate.

“Asteroids,
comets and other small bodies hold material from the solar system’s birth. If
we want to know where we come from, we must study these objects,” Glaze
said.

Two ancient fossils
providing clues to this story are Vesta and Ceres, the largest bodies in the asteroid
belt between Mars and Jupiter. NASA’s Dawn spacecraft, which recently
ended its mission, orbited both of them and showed definitively that they are
not part of the regular “asteroid club.” While many asteroids are
loose collections of rubble, the interiors of Vesta and Ceres are layered, with
the densest material at their cores. (In scientific terms, their interiors are
said to be “differentiated.”) This indicates both of these bodies
were on their way to becoming planets, but their growth was stunted — they
never had enough material to get as big as the major planets.

But while Vesta
is largely dry, Ceres is wet. It may have as much as 25 percent water, mostly
bound up in minerals or ice, with the possibility of underground liquid. The
presence of ammonia at Ceres is also interesting, because it typically requires
cooler temperatures than Ceres’ current location. This indicates the dwarf
planet could have formed beyond Jupiter and migrated in, or at least
incorporated materials that originated farther from the Sun. The mystery of Ceres’
origins shows how complex planetary formation can be, and it underscores the
complicated history of our solar system.

This artist’s concept depicts the spacecraft of NASA’s Psyche mission near the mission’s target, the metal asteroid Psyche. Image Credit: NASA/JPL-Caltech/Arizona State Univ./Space Systems Loral/Peter Rubin
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Although
we can indirectly study the deep interiors of the planets for clues about their
origins, as NASA’s
InSight mission will do on Mars, it’s impossible to drill down into
the core of any sizeable object in space, including Earth. Nevertheless, a rare
object called Psyche may offer the opportunity to explore a planet-like body’s
core without any digging. Asteroid Psyche appears to be the exposed iron-nickel
core of a protoplanet — a small world that formed early in our solar system’s
history but never reached planetary size. Like Vesta and Ceres, Psyche saw its path
to planethood disrupted. NASA’s Psyche mission, launching in
2022, will help tell the story of planet formation by studying this metal object
in detail.

Artist’s impression of NASA’s New Horizons spacecraft encountering 2014 MU69, a Kuiper Belt object that orbits the Sun 1 billion miles (1.6 billion kilometers) beyond Pluto, on Jan. 1, 2019. Image Credit: NASA/JHUAPL/SwRI
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Farther
afield, NASA’s New
Horizons spacecraft is currently on its way to a distant object
called 2014 MU69, nicknamed “Ultima Thule” by the mission. One
billion miles farther from the Sun than Pluto, MU69 is a resident of the Kuiper
Belt, a region of ice-rich objects beyond the orbit of Neptune. Objects like
MU69 may represent the most primitive, or unaltered, material that remains in the
solar system. While the planets orbit in ellipses around the Sun, MU69 and many
other Kuiper Belt objects have very circular orbits, suggesting they have not
moved from their original paths in 4.5 billion years. These objects may
represent the building blocks of Pluto and other distant icy worlds like it. New
Horizons will make its closestapproach to MU69 on Jan. 1, 2019– the farthest planetary flyby in
history.

“Ultima Thule is incredibly
scientifically valuable for understanding the origin of our solar system and
its planets,”said Alan
Stern, principal investigator of New Horizons, based at Southwest Research
Institute in Boulder, Colorado. “It’s ancient and pristine, andnotlike
anything we’ve seen before.”

Delivery of the Elements of Life

Small worlds
are also likely responsible for seeding Earth with the ingredients for life. Studying
how much water they have is evidence for how they helped seed life on Earth.

“Small bodies are the game changers. They participate in the
slow and steady evolution of our solar system over time, and influence
planetary atmospheres and opportunities for life. Earth is part of that story,”
said NASA’s chief scientist Jim Green.

This “super-resolution” view of asteroid Bennu was created using eight images obtained by NASA’s OSIRIS-REx spacecraft on Oct. 29, 2018, from a distance of about 205 miles (330 kilometers). Image credit: NASA/Goddard/University of Arizona
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One example of
an asteroid containing the building blocks of life is Bennu, the target of NASA’s OSIRIS-REx
(Origins, Spectral Interpretation, Resource
Identification, Security-Regolith Explorer) mission. Bennu
may be loaded with molecules of carbon and water, both of which are necessary
for life as we know it. As Earth formed, and afterward, objects like Bennu
rained down and delivered these materials to our planet. These objects did not
have oceans themselves, but rather water molecules bound up in minerals. Up to
80 percent of Earth’s water is thought to have come from small bodies like
Bennu. By studying Bennu, we can better understand the kinds of objects that
allowed a barren young Earth to blossom with life.

Bennu likely
originated in the main asteroid belt between Mars and Jupiter, and it’s thought
to have survived a catastrophic collision that happened between 800 million and 2 billion years ago. Scientists think a big,
carbon-rich asteroid shattered into thousands of pieces, and Bennu is one of
the remnants. Rather than a solid object, Bennu is thought to be a “rubble
pile” asteroid — a loose collection of rocks stuck together through
gravity and another force scientists call “cohesion.” OSIRIS-REx,
which will arrive at Bennu in early December 2018, after a 1.2-billion-mile (2-billion-kilometer)
journey, and will bring back a sample of this intriguing object to Earth in a
sample-return capsule in 2023.

The Japanese
Hayabusa-2 mission is also looking at an asteroid from the same
family of bodies thought to have delivered ingredients for life to Earth. Currently
in orbit at asteroid Ryugu, with small hopping rovers on the surface, the mission
will collect samples and return them in a capsule to Earth for analysis by the
end of 2020. We will learn a lot comparing Bennu and Ryugu, and understanding
the similarities and differences between their samples.

Tracers of Solar System Evolution

Most of the
material that formed our solar system, including Earth, didn’t live to tell the
tale. It fell into the Sun or was ejected beyond the reaches of our most
powerful telescopes; only a small fraction formed the planets. But there are
some renegade remnants of the early days when the stuff of planets swirled with
an uncertain fate around the Sun.

A particularly
catastrophic time for the solar system was between 50 and 500 million years
after the Sun formed. Jupiter and Saturn, our system’s most massive giants,
reorganized the objects around them as their gravity interacted with smaller
worlds such as asteroids. Uranus and Neptune may have originated closer to the
Sun and been kicked outward as Jupiter and Saturn moved around. Saturn, in
fact, may have prevented Jupiter from “eating” some of the
terrestrial planets, including Earth, as its gravity counteracted Jupiter’s
further movement toward the Sun.

Conceptual image of the Lucy mission to the Trojan asteroids. Image credit: NASA/SwRI
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Swarms of
asteroids called the Trojans could help sort out the details of that turbulent
period. The Trojans comprise two clusters of small bodies that share Jupiter’s orbit
around the Sun, with one group ahead of Jupiter and one trailing behind. But some
Trojans seem to be made of different materials than others, as indicated by
their varying colors. Some are much redder than others and may have originated
beyond the orbit of Neptune, while the grayer ones may have formed much closer
to the Sun. The leading theory is that as Jupiter moved around long ago, these
objects were corralled into Lagrange points — places where the gravity of
Jupiter and the Sun create holding areas where asteroids can be captured. The Trojans’
diversity, scientists say, reflects Jupiter’s journey to its present location. “They’re
the remnants of what was going on the last time Jupiter moved,” said Hal
Levison, researcher at Southwest Research Institute.

NASA’s Lucy mission, launching in October 2021, will send a
spacecraft to the Trojans for the first time, thoroughly investigating six
Trojans (three asteroids in each swarm). For Levison, the mission’s principal
investigator, the spacecraft will test ideas he and colleagues have been
working on for decades about Jupiter’s reshaping of the solar system. “What
would really be interesting is what we don’t expect,” he said.

Processes in an Evolving Solar System

After sundown,
under the right conditions, you may notice scattered sunlight in the ecliptic
plane, the region of the sky where the planets orbit. This is because sunlight
is being scattered by dust left over from the collisions of small bodies such
as comets and asteroids. Scientists call this phenomenon “zodiacal light,”
and it’s an indication that our solar system is still active. Zodiacal dust
around other stars indicates that they, too, may harbor active planetary
systems.

Dust from small
bodies has had an important role in our planet in particular. About 100 tons of
meteoritic material and dust material fall on Earth every day. Some of it comes
from comets, whose activity has direct implications for Earth’s evolution. As
comets approach the Sun and experience its heat, gases inside the comet bubble
up and carry away dusty material from the comet — including ingredients for
life. NASA’s Stardust spacecraft flew by Comet 81P/Wild and found that cometary
dust contains amino acids, which are building blocks of life.

This view shows Comet 67P/Churyumov-Gerasimenko as seen by the OSIRIS wide-angle camera on ESA’s Rosetta spacecraft on September 29, 2016, when Rosetta was at an altitude of 14 miles (23 kilometers). Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
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Occasional
outbursts of gas and dust observed in comets indicate activity on or near their
surfaces, such as landslides. The European Space Agency’s Rosetta mission,
which completed its exploration of Comet 67P/Churyumov-Gerasimenko in 2016,
delivered unprecedented insights about cometary activity. Among the changes in
the comet, the spacecraft observed a massive cliff collapse, a large crack get
bigger and a boulder move. “We discovered that boulders the size of a
large truck could be moved across the comet’s surface a distance as long as
one-and-a-half football fields,” Ramy El-Maarry, a member of the U.S.
Rosetta science team from the University of Colorado, Boulder, said
in 2017.

Comets also influence
planetary motion today. As Jupiter continues to fling comets outward, it moves
inward ever so slightly because of the gravitational dance with the icy bodies.
Neptune, meanwhile, throws comets inward and in turn gets a tiny outward push.
Uranus and Saturn are also moving outward very slowly in this process.

“Right now
we’re talking about teeny amounts of motions because there’s not a lot of mass left,”
Levison said.

Fun fact: The
spacecraft that has seen the most comets is NASA’s Solar & Heliospheric
Observatory (SOHO), most famous for its study of the Sun. SOHO has seen the Sun
“eat” thousands of comets, which means that these small worlds were
spraying material in the inner part of the solar system on their journey to
become the Sun’s dinner.

This animation portrays a comet as it approaches the inner solar system. Light from the Sun warms the comet’s core, or nucleus, an object so small it cannot be seen at this scale. Image credit: NASA/JPL-Caltech
See animation

Hazards to Earth

Asteroids can
still pose an impact hazard to the planets, including our own.

While the Trojans are stuck being Jupiter groupies,
Bennu, the target of the OSIRIS-REx mission, is one of the most potentially
hazardous asteroids to Earth that is currently known, even though its odds of
colliding with Earth are still relatively small; scientists estimate Bennu has
a 1?in?2,700 chance of impacting our planet during
one of its close approaches to Earth in the late 22nd century. Right now,
scientists can predict Bennu’s path quite precisely through the year 2135, when
the asteroid will make one of its close passes by Earth. Close observations by
OSIRIS-REx will get an even tighter handle on Bennu’s journey, and help
scientists working on safeguarding our planet against hazardous asteroids to
better understand what it would take to deflect one on an impact trajectory.

“We’re
developing a lot of technologies for operating with precision around these
kinds of bodies, and targeting locations on their surfaces, as well as
characterizing their overall physical and chemical properties. You would need
this information if you wanted to design an asteroid deflection mission,”
said Dante Lauretta, principal investigator for the OSIRIS-REx mission, based
at the University of Arizona in Tucson.

This animation shows how NASA’s Double Asteroid Redirection Test (DART) would target and strike the smaller (left) element of the binary asteroid Didymos to demonstrate how a kinetic impact could potentially redirect an asteroid as part of the agency’s planetary defense program.

Another upcoming mission that will test a technique for
defending the planet from naturally occurring impact hazards is NASA’s Double Asteroid Redirection Test (DART)
mission, which will attempt to change a small asteroid’s motion. How? Kinetic
impact — in other words, collide something with it, but in a more precise and
controlled way than nature does it.

DART’s target is Didymos, a binary asteroid composed of
two objects orbiting each other. The larger body is about half a mile (800
meters) across, with a small moonlet that is less than one-tenth of a mile (150
meters) wide. An asteroid this size could result in widespread regional damage
if one were to impact Earth. DART will deliberately crash itself into the
moonlet to slightly change the small object’s orbital speed. Telescopes on
Earth will then measure this change in speed by observing the new period of
time it takes the moonlet to complete an orbit around the main body, which is
expected to be a change of less than a fraction of one percent. But even that
small of change could be enough to make a predicted impactor miss Earth in some
future impact scenario. The spacecraft, being built by the Johns Hopkins University
Applied Physics Laboratory, is scheduled for launch in spring-summer 2021.

Didymos and Bennu are just two of the almost 19,000 known
near-Earth asteroids. There are over 8,300 known near-Earth asteroids the size
of the moonlet of Didymos and larger, but scientists estimate that about 25,000
asteroids in that size range exist in near-Earth space. The space telescope
helping scientists discover and understand these kinds of objects, including
potential hazards, is called NEOWISE (which
stands for Near-Earth Object Wide-field Infrared Survey Explorer).

“For most asteroids, we know little about them except for
their orbit and how bright they look. With NEOWISE, we can use the heat emitted
from the objects to give us a better assessment of their sizes,” said Amy
Mainzer, principal investigator of NEOWISE, based at NASA’s Jet Propulsion
Laboratory. “That’s important because asteroid impacts can pack quite a
punch, and the amount of energy depends strongly on the size of the object.”

This artist’s concept shows the Wide-field Infrared Survey Explorer, or WISE, spacecraft, in its orbit around Earth. In its NEOWISE mission it finds and characterizes asteroids. Image credit: NASA/JPL-Caltech
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Small Worlds as Pit Stops, Resources for
Future Exploration

There are no
gas stations in space yet, but scientists and engineers are already starting to
think about how asteroids could one day serve as refueling stations for
spacecraft on the way to farther-flung destinations. These small worlds might
also help astronauts restock their supplies. For example, Bennu likely has water
bound in clay minerals, which could perhaps one day be harvested for hydrating
thirsty space travelers.

“In
addition to science, the future will indeed be mining,” Green said. “The
materials in space will be used in space for further exploration.”

How did metals
get on asteroids? As they formed, asteroids and other small worlds collected
heavy elements forged billions of years ago. Iron and nickel found in asteroids
were produced by previous generations of stars and incorporated in the
formation of our solar system.

These small
bodies also contain heavier metals forged in stellar explosions called
supernovae. The violent death of a star, which can lead to the creation of a
black hole, spreads elements heavier than hydrogen and helium throughout the
universe. These include metals like gold, silver and platinum, as well as
oxygen, carbon and other elements we need for survival. Another kind of
cataclysm — the collision of supernova remnants called neutron stars — can
also create and spread heavy metals. In this way small bodies are also forensic
evidence of the explosions or collisions of long-dead stars.

Because of big
things, we now have a lot of very small things. And from small things, we get
big clues about our past — and possibly resources for our future. Exploring
these objects is important, even if they aren’t planets.

They are small worlds, after all. News Media ContactBy Elizabeth Landau

2018-259

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