23 research outputs found
Nuclear and Orbital Characterization of the Transition Object (4015) 107P/Wilson-Harrington
Comet 107P/Wilson-Harrington, cross-listed as asteroid 4015, is one of the
original transition objects whose properties do not neatly fit into a cometary
or asteroidal origin. Discovered in a period of apparently gas-dominated
activity in 1949, it was subsequently lost and recovered as the inactive
asteroid 1979 VA. We obtained new and re-analyzed archival observations of the
object, compared to meteorites, and conducted new orbital integrations in order
to understand the nature of this object and to understand where it falls on the
asteroid-comet continuum. Wilson-Harrington's reflectance spectrum is
approximately neutral from visible to near-infrared wavelengths, but has a
reflectance maximum near 0.8-0.9 microns. The object's spectrum is well matched
by laboratory spectra of carbonaceous chondrite meteorites like the CM
Murchison or the CI Ivuna. The object's phase curve is compatible with either
an asteroidal or cometary origin, and its recent orbital history has no periods
with high enough temperatures to have altered its surface. While it is possible
that some unknown process has acted to change the surface from an originally
cometary one, we instead prefer a fundamentally asteroidal origin for
Wilson-Harrington which can explain its surface and orbital properties.
However, this would require a way to maintain significant (hyper-)volatile
supplies on the near-Earth objects beyond what is currently expected.
Wilson-Harrington's similar meteorite affinity and possible orbital link to
sample return targets (162173) Ryugu and (101955) Bennu suggest that the
returned samples from the Hayabusa-2 and OSIRIS-REx missions might hold the key
to understanding this object.Comment: 22 pages, 5 figures, accepted for publication in the AAS's Planetary
Science Journal (PSJ
Near-Infrared Spectroscopy Of The Nucleus Of Low-Activity Comet P/2016 BA During Its 2016 Close Approach
The Near-Earth Comet P/2016 BA (PanSTARRS) is a slow-rotatating
nearly-dormant object, a likely dynamical twin of 252P/LINEAR, and was recently
shown to have a mid-infrared spectrum very dissimilar to other comets.
BA also recently selected one of the back-up targets for the ESA's
\textit{Comet Interceptor}, so a clearer understanding of BA's modern
properties would not just improve our understanding of how comets go dormant,
but could also aid planning for a potential spacecraft visit. We present
observations of BA taken on two dates during its 2016 Earth close
approach with the NASA Infrared Telescope Facility, both of which are
consistent with direct observations of its nucleus. The reflectance spectrum of
BA is similar to 67P/Churyumov-Gerasimenko, albeit highly
phase-reddened. Thermal emission contaminates the reflectance spectrum at
longer wavelengths, which we correct with a new Markov Chain Monte Carlo
thermal modeling code. The models suggest 's visible geometric albedo
is , consistent with radar observations, its beaming parameter
is typical for NEOs observed in its geometry, and its reflectrance spectrum is
red and linear throughout H and K band. It appears very much like a "normal"
comet nucleus, despite its mid-infrared oddities. A slow loss of fine grains as
the object's activity diminished might help to reconcile some of the lines of
evidence, and we discuss other possibilities. A spacecraft flyby past BA
could get closer to the nucleus than with a more active target, and we
highlight some science questions that could be addressed with a visit to a
(nearly-)dormant comet.Comment: 18 pages, 3 figures, accepted for publication in the Planetary
Science Journal on April 1, 202
Near-infrared observations of active asteroid (3200) Phaethon reveal no evidence for hydration
Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body
of the Geminid Meteor Shower. Because of its small perihelion distance,
Phaethon's surface reaches temperatures sufficient to destabilize hydrated
materials. We conducted rotationally resolved spectroscopic observations of
this asteroid, mostly covering the northern hemisphere and the equatorial
region, beyond 2.5-micron to search for evidence of hydration on its surface.
Here we show that the observed part of Phaethon does not exhibit the 3-micron
hydrated mineral absorption (within 2-sigma). These observations suggest that
Phaethon's modern activity is not due to volatile sublimation or
devolatilization of phyllosilicates on its surface. It is possible that the
observed part of Phaethon was originally hydrated and has since lost volatiles
from its surface via dehydration, supporting its connection to the Pallas
family, or it was formed from anhydrous material
Physical Characterization of the December 2017 Outburst of the Centaur 174P/Echeclus
The Centaurs are the small solar system bodies intermediate between the
active inner solar system Jupiter Family Comets and their inactive progenitors
in the trans-Neptunian region. Among the fraction of Centaurs which show
comet-like activity, 174P/Echeclus is best known for its massive 2005 outburst
in which a large apparently active fragment was ejected above the escape
velocity from the primary nucleus. We present visible imaging and near-infrared
spectroscopy of Echeclus during the first week after its December 2017 outburst
taken at the Faulkes North & South Telescopes and the NASA IRTF, the largest
outburst since 2005. The coma was seen to be highly asymmetric. A secondary
peak was seen in the near-infrared 2D spectra, which is strongly hinted at in
the visible images, moving hyperbolically with respect to the nucleus. The
retrieved reflectance spectrum of Echelcus is consistent with the unobscured
nucleus but becomes bluer when a wider extraction aperture is used. We find
that Echeclus's coma is best explained as dominated by large blue dust grains,
which agrees with previous work. We also conducted a high-resolution orbital
integration of Echeclus's recent evolution and found no large orbital changes
that could drive its modern evolution. We interpret the second peak in the
visible and near-infrared datasets as a large cloud of larger-than-dust debris
ejected at the time of outburst. If Echeclus is typical of the Centaurs, there
may be several debris ejection or fragmentation events per year on other
Centaurs that are going unnoticed.Comment: Accepted for publication in the Astronomical Journal, 18 pages, 4
figures, 4 table
Physical Characterization of Active Asteroid (6478) Gault
Main belt asteroid (6478) Gault has been dynamically linked with two
overlapping asteroid families: Phocaea, dominated by S-type asteroids, and
Tamara, dominated by low-albedo C-types. This object has recently become an
interesting case for study, after images obtained in late 2018 revealed that it
was active and displaying a comet-like tail. Previous authors have proposed
that the most likely scenarios to explain the observed activity on Gault were
rotational excitation or merger of near-contact binaries. Here we use new
photometric and spectroscopic data of Gault to determine its physical and
compositional properties. Lightcurves derived from the photometric data showed
little variation over three nights of observations, which prevented us from
determining the rotation period of the asteroid. Using WISE observations of
Gault and the near-Earth Asteroid Thermal Model (NEATM) we determined that this
asteroid has a diameter 6 km. NIR spectroscopic data obtained with the
Infrared Telescope Facility (IRTF) showed a spectrum similar to that of
S-complex asteroids, and a surface composition consistent with H chondrite
meteorites. These results favor a compositional affinity between Gault and
asteroid (25) Phocaea, and rules out a compositional link with the Tamara
family. From the spectroscopic data we found no evidence of fresh material that
could have been exposed during the outburst episodes.Comment: 9 pages, 4 figures, accepted for publication in ApJ
Photometry of Particles Ejected From Active Asteroid (101955) Bennu
AbstractNearâEarth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeterâ to centimeterâscale particles. The close proximity of the Origins, Spectral Interpretations, Resource Identification, and SecurityâRegolith Explorer spacecraft enabled monitoring of particles for a 10âmonth period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the bestâobserved particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Groundâbased data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor
Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks
The impact of the DART spacecraft into Dimorphos, moon of the asteroid
Didymos, changed Dimorphos' orbit substantially, largely from the ejection of
material. We present results from twelve Earth-based facilities involved in a
world-wide campaign to monitor the brightness and morphology of the ejecta in
the first 35 days after impact. After an initial brightening of ~1.4
magnitudes, we find consistent dimming rates of 0.11-0.12 magnitudes/day in the
first week, and 0.08-0.09 magnitudes/day over the entire study period. The
system returned to its pre-impact brightness 24.3-25.3 days after impact
through the primary ejecta tail remained. The dimming paused briefly eight days
after impact, near in time to the appearance of the second tail. This was
likely due to a secondary release of material after re-impact of a boulder
released in the initial impact, through movement of the primary ejecta through
the aperture likely played a role.Comment: 16 pages, 5 Figures, accepted in the Astrophysical Journal Letters
(ApJL) on October 16, 202