What Is A Near-Earth Object (NEO)?
What Is The Purpose Of The Near-Earth Object Program?
How Many Near-Earth Objects Have Been Discovered So Far?
What Are Asteroids And Comets?
What Are The Differences Between An Asteroid, Comet, Meteoroid, Meteor and Meteorite?
Why Study Asteroids?
Why Study Comets?
What Are Atens, Apollos and Amors?
What Is A Potentially Hazardous Asteroid (PHA)?
What Is A Great Comet?
What Spacecraft Are We Sending To Asteroids & Comets?
Also, see our FAQ on Impact Risk Assessment.
What Is A Near-Earth Object (NEO)?
Near-Earth Objects (NEOs) are comets and asteroids
that have been nudged by the
gravitational attraction of nearby planets into orbits that allow them to enter
the Earth's neighborhood. Composed mostly of water ice with embedded dust
particles, comets originally formed in the cold outer planetary system while
most of the rocky asteroids formed in the warmer inner solar system between the
orbits of Mars and Jupiter.
What Is The Purpose Of The Near-Earth Object Program?
The purpose of the Near-Earth Object Program is to coordinate
NASA-sponsored efforts to detect, track and characterize potentially
hazardous asteroids and comets that could approach the Earth.
The NEO Program will focus on the goal of
locating at least
90 percent of the estimated 1,000 asteroids and comets that approach the
Earth and are larger
than 1 kilometer (about 2/3-mile) in diameter, by the end of the
next decade.
In addition to managing the detection and cataloging of Near-Earth objects,
the NEO Program
office will be responsible for facilitating communications between the
astronomical community
and the public should any potentially hazardous objects be discovered.
How Many Near-Earth Objects Have Been Discovered So Far?
As of September 09, 2008, 5610 Near-Earth objects have been discovered.
752 of these NEOs are asteroids with a diameter of approximately 1 kilometer or larger.
Also, 971 of these NEOs have been classified as
Potentially Hazardous Asteroids (PHAs).
What Are Asteroids And Comets?
Asteroids and comets are believed to be ancient
remnants of the earliest years of the formation of
our solar system more than four billion years ago.
From the beginning of life on Earth to the recent
spectacular impact of Comet Shoemaker-Levy 9 with
Jupiter, these so-called "small bodies" play a key
role in many of the fundamental processes that have
shaped the planetary neighborhood in which we live.
Comets are bodies of ice, rock, and organic
compounds that can be several miles in diameter.
Comets are thought to originate from a region beyond
the orbits of the outermost planets. Scientists
believe that gravitational perturbations
periodically jar comets out of this population,
setting these "dirty snowballs" on orbital courses
that bring them closer to the Sun. Some, called
long-period comets, are in elliptical orbits of the
Sun that take them far out beyond the planets and
back. Others, called short-period comets, travel in
shorter orbits nearer the Sun.
When comets venture into the more intense sunlight
of the inner solar system, the ices in the comet
nucleus begin to vaporize and fall away. The evolved
gas forms a tenuous atmosphere around the nucleus
called a coma, while the dust previously in the
nucleus forms a tail that can be thousands of miles
long and sometimes can be seen from Earth. While
striking the early Earth billions of years ago,
comets are thought to have created major changes to
Earth's early oceans, atmosphere, and climate, and
may have delivered the first carbon-based molecules
to our planet, triggering the process of the origins
of life.
Most asteroids are made of rock, but some are
composed of metal, mostly nickel and iron. They
range in size from small boulders to objects that
are hundreds of miles in diameter. A small portion
of the asteroid population may be burned-out comets
whose ices have evaporated away and been blown off
into space. Almost all asteroids are part of the
Main Asteroid Belt, with orbits in the vast region
of space between Mars and Jupiter.
Some asteroids pass very close to Earth's orbit
around the Sun. Scientists have found evidence that
asteroids have hit our planet in the past. Usually,
asteroids and smaller debris called meteoroids are
too small to survive the passage through Earth's
atmosphere. When these burn up on their descent,
they leave a beautiful trail of light known as a
meteor or "shooting star." Larger asteroids
occasionally crash into Earth, however, and create
craters, such as Arizona's mile-wide Meteor Crater
near Flagstaff. Another impact site off the coast of
the Yucatan Peninsula in Mexico, which is buried by
ocean sediments today, is believed to be a record of
the event that led to the extinction of the
dinosaurs 65 million years ago. Fortunately for us,
these big asteroid impacts are rare. A smaller rocky
meteoroid or comet less than 100 yards in diameter
is believed to have entered the atmosphere over the
Tunguska region of Siberia in 1908. The resulting
shockwave knocked down trees for hundreds of square
miles.
What Are The Differences Between An Asteroid, Comet, Meteoroid, Meteor and Meteorite?
In space, a large rocky body in orbit about the Sun is referred to as an asteroid or minor
planet whereas much smaller particles in orbit about the Sun are referred to as meteoroids.
Once a meteoroid enters the Earth's atmosphere and vaporizes, it becomes a meteor (i.e.,
shooting star). If a small asteroid or large meteoroid survives its fiery passage through the
Earth's atmosphere and lands upon the Earth's surface, it is then called a meteorite.
Cometary debris is the source of most small meteoroid particles. Many comets generate
meteoroid streams when their icy cometary nuclei pass near the Sun and release the dust
particles that were once embedded in the cometary ices. These meteoroid particles then
follow in the wake of the parent comet. Collisions between asteroids in space create
smaller asteroidal fragments and these fragments are the sources of most meteorites that
have struck the Earth's surface.
Because they are readily available for study, many meteorites have already been subjected
to detailed chemical and physical analyses in laboratories. If particular asteroids can be
identified as the sources for some of the well-studied meteorites, a detailed knowledge of
the meteorite's composition and structure will provide important information on the
chemical mixture and conditions from which the parent asteroid formed 4.6 billion years
ago.
Summary Table
Asteroid
|
A relatively small, inactive, rocky body orbiting the Sun.
|
Comet
|
A relatively small, at times active, object whose ices can vaporize in sunlight
forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust
and/or gas.
|
Meteoroid
|
A small particle from a comet or asteroid orbiting the Sun.
|
Meteor
|
The light phenomena which results when
a meteoroid enters the Earth's atmosphere and vaporizes;
a shooting star.
|
Meteorite
|
A meteoroid that survives its passage through the Earth's atmosphere and
lands upon the Earth's surface.
|
Why Study Asteroids?
The scientific interest in asteroids is due largely to their status
as the remnant debris from the inner solar system formation process.
Because some of these objects can collide with the Earth, asteroids
are also important for having significantly modified the Earth's
biosphere in the past. They will continue to do so in the future.
In addition, asteroids offer a source of volatiles and an
extraordinarily rich supply of minerals that can be exploited for
the exploration and colonization of our solar system in the
twenty-first century.
Asteroids represent the bits and pieces left over from the process
that formed the inner planets, including Earth. Asteroids are also
the sources of most meteorites that have struck the Earth's surface
and many of these meteorites have already been subjected to detailed
chemical and physical analyses. If certain asteroids can be
identified as the sources for some of the well-studied meteorites,
the detailed knowledge of the meteorite's composition and structure will
provide important information on the chemical mixture, and conditions
from which the Earth formed 4.6 billion years ago. During the early
solar system, the carbon-based molecules and volatile materials that
served as the building blocks of life may have been brought to the
Earth via asteroid and comet impacts. Thus the study of asteroids is
not only important for studying the primordial chemical mixture from
which the Earth formed, these objects may hold the key as to how the
building blocks of life were delivered to the early Earth.
On a daily basis, the Earth is bombarded with tons of interplanetary
material. Many of the incoming particles are so small that they are
destroyed in the Earth's atmosphere before they reach the ground.
These particles are often seen as meteors or shooting stars. The vast
majority of all interplanetary material that reaches the Earth's
surface originates as the collision fragments of asteroids that have
run into one another some eons ago. With an average interval of about
100 years, rocky or iron asteroids larger than about 50 meters would be
expected to reach the Earth's surface and cause local disasters or
produce the tidal waves that can inundate low lying coastal areas.
On an average of every few hundred thousand years or so, asteroids
larger than a mile could cause global disasters. In this case, the
impact debris would spread throughout the Earth's atmosphere so that
plant life would suffer from acid rain, partial blocking of sunlight,
and from the firestorms resulting from heated impact debris raining
back down upon the Earth's surface. The probability of an asteroid
striking the Earth and causing serious damage is very remote but the
devastating consequences of such an impact suggests we should closely
study different types of asteroids to understand their compositions,
structures, sizes, and future trajectories.
The asteroids that are potentially the most hazardous because they
can closely approach the Earth are also the objects that could be
most easily exploited for raw materials. These raw materials could be
used in developing the space structures and in generating the rocket
fuel that will be required to explore and colonize our solar system
in the twenty-first century. By closely investigating the compositions
of asteroids, intelligent choices can be made as to which ones offer
the richest supplies of raw materials. It has been estimated that the
mineral wealth resident in the belt of asteroids between the orbits of
Mars and Jupiter would be equivalent to about 100 billion dollars for
every person on Earth today.
Why Study Comets?
Life on Earth began at the end of a period called the late
heavy bombardment, some 3.8 billion years ago. Before this
time, the influx of interplanetary debris that formed the
Earth was so strong that the proto-Earth was far too hot for
life to have formed. Under this heavy bombardment of asteroids
and comets, the early Earth's oceans vaporized and the fragile
carbon-based molecules, upon which life is based, could not
have survived. The earliest known fossils on Earth date from
3.5 billion years ago and there is evidence that biological
activity took place even earlier - just at the end of the
period of late heavy bombardment. So the window when life
began was very short. As soon as life could have formed on
our planet, it did. But if life formed so quickly on Earth
and there was little in the way of water and carbon-based
molecules on the Earth's surface, then how were these
building blocks of life delivered to the Earth's surface so
quickly? The answer may involve the collision of comets
with the Earth, since comets contain abundant supplies of
both water and carbon-based molecules.
As the primitive, leftover building blocks of the outer solar
system formation process, comets offer clues to the chemical
mixture from which the giant planets formed some 4.6 billion
years ago. If we wish to know the composition of the
primordial mixture from which the major planets formed,
then we must determine the chemical constituents of the
leftover debris from this formation process - the comets.
Comets are composed of significant fractions of water ice,
dust, and carbon-based compounds. Since their orbital paths
often cross that of the Earth, cometary collisions with the
Earth have occurred in the past and additional collisions are
forthcoming. It is not a question of whether a comet will
strike the Earth, it is a question of when the next one will
hit. It now seems likely that a comet or asteroid struck near the Yucatan
peninsula in Mexico some 65 million years ago and caused a
massive extinction of more than 75% of the Earth's living
organisms, including the dinosaurs.
Comets have this strange duality whereby they first brought the
building blocks of life to Earth some 3.8 billion years ago and
subsequent cometary collisions may have wiped out many of the
developing life forms, allowing only the most adaptable species
to evolve further. Indeed, we may owe our preeminence at the top
of Earth's food chain to cometary collisions. A catastrophic
cometary collision with the Earth is only likely to happen at
several million year intervals on average, so we need not be
overly concerned with a threat of this type. However, it is
prudent to mount efforts to discover and study these objects,
to characterize their sizes, compositions and structures and
to keep an eye upon their future trajectories.
As with asteroids, comets are both a potential threat and a
potential resource for the colonization of the solar system in
the twenty first century. Whereas asteroids are rich in the
mineral raw materials required to build structures in space,
the comets are rich resources for the water and carbon-based
molecules necessary to sustain life. In addition, an abundant
supply of cometary water ice can provide copious quantities of
liquid hydrogen and oxygen, the two primary ingredients in
rocket fuel. One day soon, comets may serve as fueling stations
for interplanetary spacecraft.
What Are Atens, Apollos and Amors?
Atens, Apollos and Amors are subgroups of Near-Earth asteroids, and are categorized
by their orbits.
In terms of orbital elements, NEOs are asteroids and comets with perihelion
distance q less than 1.3 AU.
The vast majority of NEOs are asteroids, referred to as Near-Earth Asteroids (NEAs).
NEAs are further divided into the following groups according to their perihelion distance (q), aphelion distance (Q) and
their semi-major axes (a):
Group |
Description |
Definition |
NEAs |
Near-Earth Asteroids |
q<1.3 AU |
Atens |
Earth-crossing NEAs with semi-major axes smaller than Earth's
(named after asteroid 2062 Aten). |
a<1.0 AU, Q>0.983 AU |
Apollos |
Earth-crossing NEAs with semi-major axes larger than Earth's
(named after asteroid 1862 Apollo). |
a>1.0 AU, q<1.017 AU |
Amors |
Earth-approaching NEAs with orbits exterior to Earth's but interior
to Mars' (named after asteroid 1221 Amor). |
a>1.0 AU, 1.017<q<1.3 AU |
What Is A Potentially Hazardous Asteroid (PHA)?
Potentially Hazardous Asteroids (PHAs) are currently defined
based on parameters that measure the asteroid's potential to make
threatening close approaches to the Earth.
Specifically, all asteroids with a minimum orbit intersection distance
(MOID) of
0.05 AU or less
and an absolute magnitude (H) of 22.0 or less
are considered PHAs.
In other words, asteroids that can't get any closer to the Earth
(i.e. MOID) than 0.05 AU
(roughly 7,480,000 km or 4,650,000 mi)
or are smaller than about 150 m (500 ft) in diameter
(i.e. H = 22.0 with assumed albedo of 13%)
are not considered PHAs.
This ``potential'' to make close Earth approaches does not mean
a
PHA will impact the Earth. It only means there is a possibility
for
such a threat. By monitoring these PHAs and updating their orbits as new
observations become available, we can better predict the close-approach
statistics and thus their Earth-impact threat.
What Spacecraft Are We Sending To Asteroids & Comets?
Comet And Asteroid Mission Summary Table
NEAR
|
Launch
|
February 17, 1996
|
Asteroid Mathilde Flyby
|
June 27, 1997
|
Asteroid Eros Initial Flyby
|
December 23, 1998
|
Asteroid Eros Rendezvous
|
February 14, 2000
|
The NEAR mission flew within 1200 km
of asteroid Mathilde and spent
nearly one year in orbit about asteroid Eros in 2001-2001.
|
|
DEEP SPACE 1
|
Launch
|
October 25, 1998
|
Asteroid 9969 Braille Flyby
|
July 28, 1999
|
Comet Borrelly Flyby
|
September 22, 2001
|
The primary Deep Space 1 mission objectives are
test space technologies.
The spacecraft flew within 2000 km of Comet Borrelly on September 22, 2001.
|
|
STARDUST
|
Launch
|
February 6, 1999
|
Comet Wild-2 Flyby
|
January 2, 2004
|
Earth Sample Return
|
January 15, 2006
|
The STARDUST spacecraft will image the nucleus of Comet Wild-2, collect dust
from both the comet's coma and from interplanetary space and bring these
dust samples back to Earth for study.
|
|
MUSES-C
|
Launch
|
December 2002
|
Asteroid 25143 Itokawa Rendezvous
|
September 2005
|
Earth Sample Return
|
June 2007
|
A cooperative mission between Japan and the U.S., the MUSES-C spacecraft will
rendezvous with a near-Earth asteroid and return asteroid surface samples to Earth for analysis.
|
|
ROSETTA
|
Launch
|
March 2, 2004
|
Comet Churyumov-Gerasimenko Rendezvous
|
May 2014
|
After three Earth gravity assists and a Mars gravity assit, the Rosetta
spacecraft will rendezvous with, land upon, the surface of a comet in an
effort to study its composition and structure.
|
|
DEEP IMPACT
|
Launch
|
December 30, 2004
|
Comet Tempel 1 Impact/Flyby
|
July 4, 2005
|
Deep Impact mission will impact the surface
of comet Tempel 1 thus creating a fresh crater larger than the
size of football field and deeper than a seven-story building.
The spacecraft will study the crater formation process and examine
the subsurface structure of one of the solar system's most primitive
objects, a remnant from the outer solar system formation process.
|
|
DAWN
|
Launch
|
May 2006
|
Asteroid Vesta Rendezvous
|
July 2010
|
Asteroid Ceres Rendezvous
|
August 2014
|
The Dawn mission will orbit two asteroids on
a single voyage.
Ceres and Vesta evolved under radically different circumstances in
different parts of the solar system more than 4.6 billion years ago.
By
observing both protoplanets with the same set of instruments, Dawn will provide new
insight into the formation and evolution of our solar system.
|
|
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