A description of space weathering on asteroids that is
most consistent with the available evidence is as follows.
(1) Some lunarlike optical maturation occurs and its strength
is dependent on the composition of the target material; however,
it is not as effective on asteroids as it is on the Moon.
The main process is probably deposition of condensates
bearing SMFe on grain surfaces from vaporization of target
material by solar wind sputtering and micrometeorite bombardment.
(2) Some spectrally neutral darkening occurs and
is probably related to the processes of shock, solar wind gas
implantation, and contamination by carbonaceous material.
Space-weathering effects on asteroids strongly depend
on the composition of the target. Some asteroid types show
very little evidence of optical alteration (C types), while
other types show strong evidence of optical alteration (S
types). These trends indicate that objects composed of dark,
relatively opaque components exhibit minimal space-weathering
effects while objects composed of bright, relatively
transparent components exhibit maximal space-weathering
effects. Availability of Fe in target minerals influences the
abundance of SMFe. Experiments indicate that olivine is
more easily weathered than pyroxene, perhaps explaining
some of the variations in the degree of weathering observed
within an asteroid class.
A prediction of the model is that a pristine sample of
asteroid regolith in which space weathering has occurred
should possess a weak ESR ferromagnetic resonance.
9.2. Remaining Unresolved Issues and Problems
Soil samples from several different asteroid spectral
types are needed to verify the compositional dependencies
of space-weathering effects. There is no quantitative understanding
of the relative rates of the space-weathering processes
and their optical effects. In particular, it is not clear
why the color and albedo trends due to space weathering
on 951 Gaspra and 243 Ida differ from those on 433 Eros.
The values of the complex spectral refractive index of
Fe measured by various workers vary greatly (by more than
a factor of 2), probably because of surface oxidation effects.
Accurate values appropriate to the space environment are
badly needed for reliable modeling.
Thornhill: I predict that the crater on Vesta, when photographed more closely, will be circular also.
Near-Earth asteroid 2005 YU55 will pass within 0.85 lunar distances from the Earth on November 8, 2011. The upcoming close approach by this relatively large 400 meter-sized, C-type asteroid presents an excellent opportunity for synergistic ground-based observations including optical, near infrared and radar data.
Although classified as a potentially hazardous object, 2005 YU55 poses no threat of an Earth collision over at least the next 100 years. However, this will be the closest approach to date by an object this large that we know about in advance and an event of this type will not happen again until 2028 when asteroid (153814) 2001 WN5 will pass to within 0.6 lunar distances
-no threat of an Earth collision over at least the next 100 years-
http://www.nasa.gov/mission_pages/aster ... 11107.html
Asteroid 2005 YU55 Approaches Close Earth Flyby
This radar image of asteroid 2005 YU55 was obtained on Nov. 7, 2011, at 11:45 a.m. PST (2:45 p.m. EST/1945 UTC), when the space rock was at 3.6 lunar distances, which is about 860,000 miles, or 1.38 million kilometers, from Earth.
The giant asteroid Vesta possesses many features usually associated with rocky planets like Earth, according to data from a Nasa probe.
Vesta has been viewed as a massive asteroid, but after studying the surface in detail, scientists are describing it as "transitional".
The Dawn spacecraft has been orbiting Vesta - one of the Solar System's most primitive objects - since July 2011.
They have documented many unexpected features on its battered surface.
Mission scientists presented their latest results at the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas.
Dawn's principal investigator, Christopher T Russell, told the meeting that the science team found it hard not to refer to the object as a planet.
He said the rounded asteroid showed evidence of geological processes that characterise rocky worlds like Earth and the Moon.
Asteroids that have a 1:1 orbital resonance with a planet are also called co-orbital objects, because they follow the orbit of the planet. The most numerous known co-orbital asteroids are the so-called Trojans, which occupy the L4 and L5 Lagrangian points of the relevant planet. However, 2002 AA29 does not belong to these. Instead, it follows a so-called horseshoe orbit along the path of the Earth.
Asteroid 2002 AA29 is an asteroid that is in a 1:1 resonance with the Earth. Such asteroids are often refered to as co-orbital with the Earth. It spends much of its time in a horseshoe orbit. But every few hundred years, it leaves its horseshoe orbit and occupies the area normally avoided by its horseshoe orbit. During a period that lasts a few decades, asteroid 2002 AA29 never strays far from the Earth as it circles the Earth/Moon system in a quasi-orbit. Eventually, it will leave this configuration and exit the Earth system from the same direction it entered.
In this animation, we look down on the Solar System and rotate our view with the Earth as it goes around the Sun, much like a cameraman filming from the roof of a rotating merry-go-round. This makes the Earth appear to be fixed, allowing us to see just the motion of the asteroid relative to our planet. The animation begins in 1903, when 2002 AA29 was on the trailing side of Earth. The asteroid traces small loops because of variations in both its orbital speed and that of the Earth.
For the first few years after 1903, the asteroid loops towards our planet, but then it reverses due primarily to gravitational interaction with the Earth. It then begins a 95-year trek all the way around the Earth's orbit to our planet's leading side, where it will make a close approach on January 8, 2003, and reverse itself once again. Never passing through the gap near the Earth, the asteroid traces out a horseshoe pattern.
As it moves along the Earth's orbit, it winds in a spiral about it, in which each loop of the spiral takes one year. This spiral motion (in the Earth–Sun reference frame) arises from the slightly lower eccentricity and the tilt of the orbit: the inclination relative to the Earth's orbit is responsible for the vertical component of the spiral loop, and the difference in eccentricity for the horizontal component.
The loop results from AA29’s 10.7 degree inclination with respect to the ecliptic plane.
The asteroid traces small loops because of variations in both its orbital speed and that of the Earth.
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