http://www.jpl.nasa.gov/news/news.php?feature=4678
Tracking A Mysterious Group of Asteroid Outcasts
Jet Propulsion Laboratory
August 3, 2015
Fast Facts:
* A new NASA study has traced some members of the near-Earth asteroid
population back to their likely source.
* The source may be the Euphrosyne family of dark, asteroids on highly
inclined (or tilted) orbits in the outer asteroid belt.
* The study used data from NASA's NEOWISE space telescope, which has a
second life following its reactivation in 2013.
High above the plane of our solar system, near the asteroid-rich abyss
between Mars and Jupiter, scientists have found a unique family of space
rocks. These interplanetary oddballs are the Euphrosyne (pronounced
you-FROH-seh-nee)
asteroids, and by any measure they have been distant, dark and mysterious
-- until now.
Distributed at the outer edge of the asteroid belt, the Euphrosynes have
an unusual orbital path that juts well above the ecliptic, the equator
of the solar system. The asteroid after which they are named, Euphrosyne
-- for an ancient Greek goddess of mirth -- is about 156 miles (260 kilometers)
across and is one of the 10 largest asteroids in the main belt. Current-day
Euphrosyne is thought to be a remnant of a massive collision about 700
million years ago that formed the family of smaller asteroids bearing
its name. Scientists think this event was one of the last great collisions
in the solar system.
A new study conducted by scientists at NASA's Jet Propulsion Laboratory
in Pasadena, California, used the agency's orbiting Near-Earth Object
Wide-field Infrared Survey Explorer (NEOWISE) telescope to look at these
unusual asteroids to learn more about Near Earth Objects, or NEOs, and
their potential threat to Earth.
NEOs are bodies whose orbits around the sun approach the orbit of Earth;
this population is short-lived on astronomical timescales and is fed by
other reservoirs of bodies in our solar system. As they orbit the sun,
NEOs can occasionally have close approaches to Earth. For this reason
alone -- the safety of our home planet -- the study of such objects is
important.
As a result of their study, the JPL researchers believe the Euphrosynes
may be the source of some of the dark NEOs found to be on long, highly
inclined orbits. They found that, through gravitational interactions
with Saturn, Euphrosyne asteroids can evolve into NEOs over timescales
of millions of years.
NEOs can originate in either the asteroid belt or the more distant outer
reaches of the solar system. Those from the asteroid belt are thought
to evolve toward Earth's orbit through collisions and the gravitational
influence of the planets. Originating well above the ecliptic and near
the far edge of the asteroid belt, the forces that shape their trajectories
toward Earth are far more moderate.
The Euphrosynes have a gentle resonance with the orbit of Saturn that
slowly moves these objects, eventually turning some of them into NEOs,
said Joseph Masiero, JPL's lead scientist on the Euphrosynes study. This
particular gravitational resonance tends to push some of the larger fragments
of the Euphrosyne family into near-Earth space.
By studying the Euphrosyne family asteroids with NEOWISE, JPL scientists
have been able to measure their sizes and the amount of solar energy they
reflect. Since NEOWISE operates in the infrared portion of the spectrum,
it detects heat. Therefore, it can see dark objects far better than telescopes
operating at visible wavelengths, which sense reflected sunlight. Its
heat-sensing capability also allows it to measure sizes more accurately.
The 1,400 Euphrosyne asteroids studied by Masiero and his colleagues turned
out to be large and dark, with highly inclined and elliptical orbits.
These traits make them good candidates for the source of some of the dark
NEOs the NEOWISE telescope detects and discovers, particularly those that
also have highly inclined orbits.
NEOWISE was originally launched as an astrophysics mission in 2009 as
the Wide-field Infrared Survey Explorer, or WISE. It operated until 2011
and was then shut down. But the spacecraft, now dubbed NEOWISE, would
get a second life. NEOWISE is a great tool for searching for near-Earth
asteroids, particularly high-inclination, dark objects, Masiero said.
There are over 700,000 asteroidal bodies currently known in the main belt
that range in size from large boulders to about 60 percent of the diameter
of Earth's moon, with many yet to be discovered. This makes finding the
specific point of origin of most NEOs extremely difficult.
With the Euphrosynes it's different. Most near-Earth objects come from
a number of sources in the inner region of the main belt, and they are
quickly mixed around, Masiero said. But with objects coming from this
family, in such a unique region, we are able to draw a likely path for
some of the unusual, dark NEOs we find back to the collision in
