653 research outputs found
Detection of Semi-Major Axis Drifts in 54 Near-Earth Asteroids: New Measurements of the Yarkovsky Effect
We have identified and quantified semi-major axis drifts in Near-Earth
Asteroids (NEAs) by performing orbital fits to optical and radar astrometry of
all numbered NEAs. We focus on a subset of 54 NEAs that exhibit some of the
most reliable and strongest drift rates. Our selection criteria include a
Yarkovsky sensitivity metric that quantifies the detectability of semi-major
axis drift in any given data set, a signal-to-noise metric, and orbital
coverage requirements. In 42 cases, the observed drifts (~10^-3 AU/Myr) agree
well with numerical estimates of Yarkovsky drifts. This agreement suggests that
the Yarkovsky effect is the dominant non-gravitational process affecting these
orbits, and allows us to derive constraints on asteroid physical properties. In
12 cases, the drifts exceed nominal Yarkovsky predictions, which could be due
to inaccuracies in our knowledge of physical properties, faulty astrometry, or
modeling errors. If these high rates cannot be ruled out by further
observations or improvements in modeling, they would be indicative of the
presence of an additional non-gravitational force, such as that resulting from
a loss of mass of order a kilogram per second. We define the Yarkovsky
efficiency f_Y as the ratio of the change in orbital energy to incident solar
radiation energy, and we find that typical Yarkovsky efficiencies are ~10^-5.Comment: Accepted for publication by The Astronomical Journal. 42 pages, 8
figure
The Increasing Rotation Period of Comet 10P/Tempel 2
We imaged comet 10P/Tempel 2 on 32 nights from 1999 April through 2000 March.
R-band lightcurves were obtained on 11 of these nights from 1999 April through
1999 June, prior to both the onset of significant coma activity and perihelion.
Phasing of the data yields a double-peaked lightcurve and indicates a nucleus
rotational period of 8.941 +/- 0.002 hr with a peak-to-peak amplitude of ~0.75
mag. Our data are sufficient to rule out all other possible double-peaked
solutions as well as the single- and triple- peaked solutions. This rotation
period agrees with one of five possible solutions found in post-perihelion data
from 1994 by Mueller and Ferrin (1996, Icarus, 123, 463-477), and unambiguously
eliminates their remaining four solutions. We applied our same techniques to
published lightcurves from 1988 which were obtained at an equivalent orbital
position and viewing geometry as in 1999. We found a rotation period of 8.932
+/- 0.001 hr in 1988, consistent with the findings of previous authors and
incompatible with our 1999 solution. This reveals that Tempel 2 spun-down by
~32 s between 1988 and 1999 (two intervening perihelion passages). If the
spin-down is due to a systematic torque, then the rotation period prior to
perihelion during the 2010 apparition is expected to be an additional 32 s
longer than in 1999.Comment: Accepted by The Astronomical Journal; 22 pages of text, 3 tables, 6
figure
Constraining the Physical Properties of Near-Earth Object 2009 BD
We report on Spitzer Space Telescope IRAC observations of near-Earth object
(NEO) 2009 BD that were carried out in support of the NASA Asteroid Robotic
Retrieval Mission (ARRM) concept. We did not detect 2009 BD in 25 hrs of
integration at 4.5 micron. Based on an upper-limit flux density determination
from our data, we present a probabilistic derivation of the physical properties
of this object. The analysis is based on the combination of a thermophysical
model with an orbital model accounting for the non-gravitational forces acting
upon the body. We find two physically possible solutions. The first solution
shows 2009 BD as a 2.9+/-0.3 m diameter rocky body (rho = 2.9+/-0.5 g cm-3)
with an extremely high albedo of 0.85(+0.20/-0.10) that is covered with
regolith-like material, causing it to exhibit a low thermal inertia (Gamma =
30(+20/-10) SI units). The second solution suggests 2009 BD to be a 4+/-1 m
diameter asteroid with pV = 0.45(+0.35/-0.15) that consists of a collection of
individual bare rock slabs (Gamma = 2000+/-1000 SI units, rho = 1.7(+0.7/-0.4)
g cm-3). We are unable to rule out either solution based on physical reasoning.
2009 BD is the smallest asteroid for which physical properties have been
constrained, in this case using an indirect method and based on a detection
limit, providing unique information on the physical properties of objects in
the size range smaller than 10 m.Comment: 28 pages, 8 figures, accepted for publication in Ap
Physical Properties of Near-Earth Asteroid 2011 MD
We report on observations of near-Earth asteroid 2011 MD with the Spitzer
Space Telescope. We have spent 19.9 h of observing time with channel 2 (4.5
{\mu}m) of the Infrared Array Camera and detected the target within the
2{\sigma} positional uncertainty ellipse. Using an asteroid thermophysical
model and a model of nongravitational forces acting upon the object we
constrain the physical properties of 2011 MD, based on the measured flux
density and available astrometry data. We estimate 2011 MD to be 6 (+4/-2) m in
diameter with a geometric albedo of 0.3 (+0.4/-0.2) (uncertainties are
1{\sigma}). We find the asteroid's most probable bulk density to be 1.1
(+0.7/-0.5) g cm^{-3}, which implies a total mass of (50-350) t and a
macroporosity of >=65%, assuming a material bulk density typical of
non-primitive meteorite materials. A high degree of macroporosity suggests 2011
MD to be a rubble-pile asteroid, the rotation of which is more likely to be
retrograde than prograde.Comment: 20 pages, 4 figure
Infrared Lightcurves of Near Earth Objects
We present lightcurves and derive periods and amplitudes for a subset of 38
near earth objects (NEOs) observed at 4.5 microns with the IRAC camera on the
the Spitzer Space Telescope, many of them having no previously reported
rotation periods. This subset was chosen from about 1800 IRAC NEO observations
as having obvious periodicity and significant amplitude. For objects where the
period observed did not sample the full rotational period, we derived lower
limits to these parameters based on sinusoidal fits. Lightcurve durations
ranged from 42 to 544 minutes, with derived periods from 16 to 400 minutes. We
discuss the effects of lightcurve variations on the thermal modeling used to
derive diameters and albedos from Spitzer photometry. We find that both
diameters and albedos derived from the lightcurve maxima and minima agree with
our previously published results, even for extreme objects, showing the
conservative nature of the thermal model uncertainties. We also evaluate the
NEO rotation rates, sizes, and their cohesive strengths.Comment: 16 pages, 4 figures, 3 tables, to appear in the Astrophysical Journal
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