Edge-on View of Near-Earth Asteroids
New
results from NASA's NEOWISE survey find that more potentially hazardous
asteroids, or PHAs, are closely aligned with the plane of our solar
system than previous models suggested. PHAs are the subset of near-Earth
asteroids (NEAs) with the closest orbits to Earth's orbit, coming
within 5 million miles (about 8 million kilometers). They are also
defined as being large enough to survive passage through Earth's
atmosphere and cause damage on a regional, or greater, scale.
This diagram shows an edge-on view of our solar system. The dots
represent a snapshot of the population of NEAs and PHAs that scientists
think are likely to exist based on the NEOWISE survey. Positions of a
simulated population of PHAs on a typical day are shown in bright
orange, and the simulated NEAs are blue. Earth's orbit is green.
The diagram shows that the orbits of the PHAs tend to be more closely
aligned with the plane of our solar system, or less tilted above and
below the plane, than the NEAs. This characteristic of PHAs was known
before the NEOWISE survey. Now, NEOWISE has found the PHAs to be about
twice as likely to have these "lower-inclination" orbits than previously
thought.
To make these estimates, the NEOWISE project observed
a small portion of the total PHA and NEA populations. The survey not
only looked at the objects' orbits but also their total numbers and
physical properties such as size. The latest results provide the best
count yet of the total PHA population, finding about 4,700 plus or minus
1,500, with diameters larger than 330 feet (about 100 meters). These
numbers are in loose agreement with prior, rougher predictions. The
NEOWISE team estimates that about 20 to 30 percent of the PHAs thought
to exist have actually been discovered to date.
In an earlier
study, NEOWISE estimated that there are about 20,500 near-Earth
asteroids larger than 330 feet, fewer than previous estimates.
NEOWISE is the asteroid-hunting portion of NASA's Wide-field Infrared
Survey Explorer, or WISE, mission, which scanned the entire sky twice in
infrared light before entering hibernation mode in 2011.
Image credit: NASA/JPL-Caltech
The Hustle and Bustle of our Solar System
This
diagram illustrates the differences between orbits of a typical
near-Earth asteroid (blue) and a potentially hazardous asteroid, or PHA
(orange). PHAs are a subset of the near-Earth asteroids (NEAs). They
have the closest orbits to Earth's orbit, coming within 5 million miles
(about 8 million kilometers), and they are large enough to survive
passage through Earth's atmosphere and cause damage on a regional, or
greater, scale.
› Unlabelled version
Our yellow sun sits at the center of the crowd, while the orbits of the
planets Mercury, Venus and Mars are shown in grey. Earth's orbit stands
out in green between Venus and Mars. As the diagram indicates, the PHAs
tend to have more Earth-like orbits than the rest of the NEAs. The
asteroid orbits are simulations of what a typical object's path around
the sun might look like.
The dots in the background are based
on data from NASA's NEOWISE, the asteroid-hunting portion of the
Wide-field Infrared Survey Explorer (WISE) mission, which scanned the
whole sky twice in infrared light before entering hibernation mode in
2011. The blue and orange dots represent a simulation of the population
of near-Earth asteroids and the PHAs, respectively, which are larger
than 330 feet (100 meters).
NEOWISE has provided the best
overall look at the PHA population yet, refining estimates of their
numbers, sizes, types of orbits and potential hazards. The NEOWISE team
estimates that about 20 to 30 percent of the PHAs thought to exist have
actually been discovered to date.
Image credit: NASA/JPL-Caltech
PASADENA, Calif. -- Observations from NASA's Wide-field Infrared Survey
Explorer (WISE) have led to the best assessment yet of our solar
system's population of potentially hazardous asteroids. The results
reveal new information about their total numbers, origins and the
possible dangers they may pose.
Potentially hazardous asteroids, or PHAs, are a subset of the larger
group of near-Earth asteroids. The PHAs have the closest orbits to
Earth's, coming within five million miles (about eight million
kilometers), and they are big enough to survive passing through Earth's
atmosphere and cause damage on a regional, or greater, scale.
The new results come from the asteroid-hunting portion of the WISE
mission, called NEOWISE. The project sampled 107 PHAs to make
predictions about the entire population as a whole. Findings indicate
there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger
than 330 feet (about 100 meters). So far, an estimated 20 to 30 percent
of these objects have been found.
While previous estimates of PHAs predicted similar numbers, they were
rough approximations. NEOWISE has generated a more credible estimate of
the objects' total numbers and sizes.
"The NEOWISE analysis shows us we've made a good start at finding those
objects that truly represent an impact hazard to Earth," said Lindley
Johnson, program executive for the Near-Earth Object Observation Program
at NASA Headquarters in Washington. "But we've many more to find, and
it will take a concerted effort during the next couple of decades to
find all of them that could do serious damage or be a mission
destination in the future."
The new analysis also suggests that about twice as many PHAs as
previously thought are likely to reside in "lower-inclination" orbits,
which are more aligned with the plane of Earth's orbit. In addition,
these lower-inclination objects appear to be somewhat brighter and
smaller than the other near-Earth asteroids that spend more time far
away from Earth. A possible explanation is that many of the PHAs may
have originated from a collision between two asteroids in the main belt
lying between Mars and Jupiter. A larger body with a low-inclination
orbit may have broken up in the main belt, causing some of the fragments
to drift into orbits closer to Earth and eventually become PHAs.
Asteroids with lower-inclination orbits would be more likely to
encounter Earth and would be easier to reach. The results therefore
suggest more near-Earth objects might be available for future robotic or
human missions.
"NASA's NEOWISE project, which wasn't originally planned as part of
WISE, has turned out to be a huge bonus," said Amy Mainzer, NEOWISE
principal investigator, at NASA's Jet Propulsion Laboratory in Pasadena,
Calif. "Everything we can learn about these objects helps us understand
their origins and fate. Our team was surprised to find the
overabundance of low-inclination PHAs. Because they will tend to make
more close approaches to Earth, these targets can provide the best
opportunities for the next generation of human and robotic exploration."
The discovery that many PHAs tend to be bright says something about
their composition; they are more likely to be either stony, like
granite, or metallic. This type of information is important in assessing
the space rocks' potential hazards to Earth. The composition of the
bodies would affect how quickly they might burn up in our atmosphere if
an encounter were to take place.
The NEOWISE results have been accepted for publication in the Astrophysical Journal.
The WISE spacecraft scanned the sky twice in infrared light before
entering hibernation mode in early 2011. It catalogued hundreds of
millions of objects, including super-luminous galaxies, stellar
nurseries and closer-to-home asteroids. The NEOWISE project snapped
images of about 600 near-Earth asteroids, about 135 of which were new
discoveries. Because the telescope detected the infrared light, or heat,
of asteroids, it was able to pick up both light and dark objects,
resulting in a more representative look at the entire population. The
infrared data allowed astronomers to make good measurements of the
asteroids' diameters and, when combined with visible light observations,
how much sunlight they reflect.
JPL manages, and operates the Wide-field Infrared Survey Explorer for
NASA's Science Mission Directorate, Washington. The principal
investigator, Edward Wright, is at UCLA. The mission was competitively
selected under NASA's Explorers Program managed by the Goddard Space
Flight Center, Greenbelt, Md. The science instrument was built by the
Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by
Ball Aerospace & Technologies Corp., Boulder, Colo. Science
operations and data processing and archiving take place at the Infrared
Processing and Analysis Center at the California Institute of Technology
in Pasadena. Caltech manages JPL for NASA.
More information is online
NASA
http://www.nasa.gov/mission_pages/WISE/news/wise20120516.html
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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