INFORMATION SHEET
BINARY NEAR-EARTH ASTEROID (66391) 1999 KW4:
EXOTIC PHYSICAL PROPERTIES REVEALED BY RADAR IMAGING AND SUPERCOMPUTER MODELING
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This sheet accompanies papers published in Science Express on Oct. 12, 2006:
Radar Imaging of Near-Earth Binary Asteroid (66391) 1999 KW4 
        (by Steve Ostro and 15 colleagues)
and
Dynamical Configuration of Binary Near-Earth Asteroid 66391 
        (by Dan Scheeres and 15 colleagues)
______________
These papers report the most detailed information yet about the nature
of a near-Earth binary asteroid, presenting radar images with
resolution as fine as about 15 meters, 3D computer models of the
system's two components, and a description of the system's
extraordinary physical and dynamical properties.
The radar images were obtained in May 2001, when asteroid (66391) 1999
KW4 passed 0.032 AU from Earth, its closest approach until 2036.  The
observations used the Goldstone 70-m antenna in California and the
Arecibo 305-m antenna in Puerto Rico.  Goldstone was able to track the
asteroid for up to eight hours daily for a week, and then close-up
images of each component were obtained using the Arecibo telescope in
Puerto Rico, which is not as fully steerable but is much more
powerful.  The radar images provide a combination of high fidelity,
fine spatial resolution, and thorough rotational and orbital coverage
that is unprecedented for astronomical observations of a binary
system.
The dynamics of this system are unique in several ways.  The orbit and
rotation of the two bodies continually undergo clearly detectable
oscillations, making this system the most excited solar system orbital
system found to date.  The spin rate of the primary is so fast that
its surface is only meters or less from drifting off the surface into
orbit.  Relative to the Earth it is as if geo-synchronous satellites
had an altitude less than 60 kilometers.  This fast spin also makes
the equator of the primary the lowest point on the body, even though
it is the furthest point from the center of the body.  The extreme
state of this system presents a challenge for theories of how binary
asteroids form, and provides clues as to their properties and
evolution.  The most likely physical causes of its current state can
be attributed either to the effect of gravitational forces during
close planetary flybys or to small but persistent thermal effects on
asteroids.
KW4's orbit gets closer to the Sun than Mercury and also gets very
close to Earth's orbit.  Every six months the binary asteroid has a
close pass by the sun, during which its orbit is excited and shifted
in space.
KW4's larger component, called Alpha, is 1.5 km in diameter, has a
nearly circular outline when viewed from above its pole, and rotates
with a period of about 2.8 hours.  The smaller component, Beta, is
about one-third as large as Alpha, and orbits around Alpha once every
17.4 hours in a circular path about 2.5 km from Alpha.  Due to the
system's excitation, the system's orbit and rotational motion has
oscillations that would be dramatically visible to an observer in the
system.  For example, Beta's average orientation has one of its ends
facing Alpha, but oscillations, called librations, would make Beta
appear to wobble about that orientation.  The detailed shape and size
of the binary system's orbit also oscillates.
Thanks to the orbital mechanics of a binary system, information about
the system's orbit could be converted into determinations of the
components' masses and, since the radar imaging yielded the
components' shapes and volumes, their densities as well.  Alpha's
density, combined with information from optical observations about the
composition, reveals that the object is very porous. Alpha's shape is
dominated by a prominent equatorial bulge whose several-hundred-meter
vertical extent is defined in the north by a continuous, very abrupt
ridge and in the south by more subtle, discontinuous gradations.
Alpha's size, fast rotation, and high porosity reveal it to be a
rubble pile bound primarily by gravity, rather than by tensile
strength, rotating very close to the maximum rate it can withstand
without flying apart.  Because of its rapid spin, a particle placed on
the surface of Alpha would naturally seek out the equator as the
lowest point on the body.  When it reached the equator, the particle
would be in an almost weightless environment, very close to being in
orbit about the body.  If Alpha spun only a small amount faster these
particles would be in orbit, but would be trapped close to Alpha by
the presence of Beta.  Thus, the particle would eventually fall back
onto Alpha and preferentially travel towards the equator again.
Alpha's characteristics suggest that KW4's origin involved spin-up and
disruptive mass shedding of a loosely bound precursor object, probably
within the past million years, and perhaps much more recently.  The
disruption may have been caused by tidal effects of a close encounter
with a planet or by torques due to anisotropic thermal radiation of
absorbed sunlight (the "YORP" effect).  The near-circularity of
Alpha's pole-on profile further suggests that the disruption may have
produced a quasi-circular disc of particles rather than merely a
prolate elongated body.
Beta appears slightly denser than Alpha, possibly because its
librational motion subjects it to persistent gentle shaking that might
drive material toward a relatively compact configuration.
The KW4 system is a complex dynamical laboratory for studying
solid-body effects and a coupling between rotational and orbital
motion that can be more pronounced and can have different time scales
than with the other binaries that nature has provided (binary stars,
the Earth-Moon and Pluto-Charon systems, and much larger binary
asteroids like the Ida-Dactyl system).  Previous studies of binary
system dynamics have not had to wrestle with interactions of
components whose shapes are irregular and asymmetrical and whose
interiors are nonrigid, porous assemblages of granular materials.  The
new research establishes the techniques needed to investigate binary
NEAs and discloses phenomena critical to understanding how these
asteroids originated and evolved.  It also opens new, very challenging
domains of dynamics (e.g., trajectories of a third particle such as a
spacecraft, a piece of cratering ejecta, or an astronaut).
The KW4 results have profound consequences for ideas about mitigation
of the asteroid collision hazard, not just because KW4 is now the best
characterized kilometer-scale "Potentially Hazardous Asteroid" but
also because the physical nature of the primary component would make
landing very difficult and the system's complex dynamics would make
even simple maneuvering of spacecraft close to the bodies harrowing.
The extraordinary nature of the KW4 system and the components' motions
are unique among other comparably well characterized astronomical
entities.  However, some of KW4's overall characteristics apparently
are shared by many of the other NEA binaries, so KW4's exotic physical
attributes may be common.  About 1/6 of NEAs as large as 200 meters
are binary systems.
1999 KW4 is classified a Potentially Hazardous Asteroid (PHA) because
eventually its path through space could eventually intersect the
Earth.  However, the radar measurements indicate there is no
significant chance of 1999 KW4 colliding with Earth for at least 1000
years.
If a PHA as large as KW4 were to collide with Earth, the global
effects of the impact would be so severe that agriculture would become
impossible and perhaps a quarter of the world's population would
starve to death, triggering what would likely be a collapse of
civilization.  Thanks to the radar investigation, we know much more
about the physical nature of KW4 than any of the more than 800 known
PHAs as large as a kilometer.
People are destined to go to NEAs someday, either for defense against
one of them, to exploit mineral resources, to satisfy our curiosity
about what they're like close-up, or simply for the adventure of
exploring a diminutive nearby world.  Because of their proximity and
low gravity, NEAs include the cheapest destinations beyond the
Earth-Moon system, and missions to NEAs would be logical steps toward
the eventual human exploration of Mars.  A mission to a binary NEA
would give us two objects for the price of one, and a system
exhibiting unique sorts of dynamical phenomena.
Imagine the sky as seen from KW4's components.  The Moon in our sky
subtends about half a degree, but Beta would subtend about 9 deg as
seen from Alpha's pole and 13 deg as seen from Alpha's equator.
Beta's libration would be evident as a slow wobbling.  For an observer
on Beta, Alpha would subtend about 33 deg and would be an amazing
sight.  The KW4 system undergoes counterparts of our lunar and solar
eclipses, with Beta passing between Alpha and the Sun, and Alpha
passing between Beta and the Sun, once every 17.4 hours.
The KW4 results are published in Science magazine in a pair of papers
led by Steven Ostro of the NASA/Caltech Jet Propulsion Laboratory and
Daniel Scheeres of the University of Michigan.  Other members of the
team include Jean-Luc Margot of Cornell University; Christopher Magri
of the University of Maine; Michael Nolan of the Arecibo Observatory;
Petr Pravec and Petr Scheirich of the Academy of Sciences of the Czech
Republic; Eugene Fahnestock, Stephen Broschart, and Julie Bellerose of
the University of Michigan; and Lance Benner, Jon Giorgini, Randy
Rose, Raymond Jurgens, Eric de Jong, and Shigeru Suzuki of JPL.