Abstract
We present diameters and albedos computed for the near-Earth and main belt asteroids (MBAs) observed by the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft during ...the sixth and seventh years of its Reactivation mission. These diameters and albedos are calculated from fitting thermal models to NEOWISE observations of 199 near-Earth objects (NEOs) and 5851 MBAs detected during the sixth year of the survey and 175 NEOs and 5861 MBAs from the seventh year. Comparisons of the NEO diameters derived from Reactivation data with those derived from the WISE cryogenic mission data show a ∼30% relative uncertainty. This larger uncertainty compared to data from the cryogenic mission is due to the need to assume a beaming parameter for the fits to the shorter-wavelength data that the Reactivation mission is limited to. We also present an analysis of the orbital parameters of the MBAs that have been discovered by NEOWISE during Reactivation, finding that these objects tend to be on orbits that result in their perihelia being far from the ecliptic, and thus missed by other surveys. To date, the NEOWISE Reactivation survey has provided thermal fits of 1415 unique NEOs. Including the mission phases before spacecraft hibernation increases the count of unique NEOs characterized to 1845 from WISE’s launch to the present.
Impacts due to near-Earth objects (NEOs) are responsible for causing some of the great mass extinctions on Earth. While nearly all NEOs of diameter > 1 km, capable of causing a global climatic ...disaster, have been discovered and have negligible chance of impacting in the near future, we are far from completion in our effort to detect and characterize smaller objects. In an effort to test our preparedness to respond to a potential NEO impact threat, we conducted a community-led global planetary defense exercise with support from the NASA Planetary Defense Coordination Office. The target of our exercise was 2012 TC4, the ~10 m diameter asteroid that made a close pass by the Earth on 2017 October 12 at a distance of about 50,000 km. The goal of the TC4 observing campaign was to recover, track, and characterize 2012 TC4 as a hypothetical impactor in order to exercise the global planetary defense system involving observations, modeling, prediction, and communication. We made three attempts with the Very Large Telescope (VLT) on 2017 July 27, 31 and on 2017 August 5 and recovered 2012 TC4 within its ephemeris uncertainty at 2.2 arcmin from the nominal prediction. At visual magnitude V = 27, the recovery of 2012 TC4 is the faintest NEA detection thus far. If an impact during the 2017 close approach had been possible based on the 2012 astrometric data, these recovery observations would have been sufficient to confirm or rule out the impact. The first automatic detection by a survey (Pan-STARRS1) was on September 25, which is the earliest that 2012 TC4 would have been discovered in survey mode, if it had not been discovered in 2012. We characterized 2012 TC4 using photometry, spectroscopy and radar techniques. Based on photometric observations, we determined a rotation period of 12.2 min with an amplitude of 0.9 magnitudes. An additional lower amplitude period was detected, indicating that 2012 TC4 was in a state of non-principal axis rotation. The combined visible and near-infrared spectrum puts it in the taxonomic X-class. Radar images at 1.875 m resolution placed only a few range pixels on the asteroid, reveal an angular, asymmetric, and elongated shape, and establish that 2012 TC4 is less than 20 m on its long axis. We estimate a circular polarization ratio of 0.57 + -0.08 that is relatively high among NEAs observed to date by radar. We also performed a probabilistic impact risk assessment exercise for hypothetical impactors based on the 2012 TC4 observing campaign. This exercise was performed as part of ongoing efforts to advance effective impact risk models and assessment processes for planetary defense. The 2012 TC4 close approach provided a valuable opportunity to test the application of these methods using realistically evolving observational data to define the modeling inputs. To this end, risk assessments were calculated at several epochs before and during the close approach, incorporating new information about 2012 TC4 as it became available. Two size ranges were assessed—one smaller size range (H = 26.7) similar to the actual 2012 TC4, and one larger size range (H = 21.9) to produce a greater-damage scenario for risk assessment. Across the epochs, we found that only irons caused significant damage for smaller size. For the larger size case, however, hydrous stones caused the greatest damage, anhydrous stones caused the least damage, and irons caused moderate damage. We note that the extent of damage depends on composition in different size regimes and, after astrometry, size is the most important physical property to determine for an incoming object.
•We conducted a community-led global planetary defense exercise•The goal was to recover, track, and characterize 2012 TC4 as a hypothetical impactor•We found that after astrometric observations, size of the asteroid is the most important property for an impactor
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has been conducting a two-band thermal infrared survey to detect and characterize asteroids and comets since its ...reactivation in 2013 December. Using the observations collected during the fourth and fifth years of the survey, our automated pipeline detected candidate moving objects that were verified and reported to the Minor Planet Center. Using these detections, we perform thermal modeling of each object from the near-Earth object (NEO) and Main Belt asteroid (MBA) populations to constrain their sizes. We present thermal model fits of asteroid diameters for 189 NEOs and 5831 MBAs detected during the fourth year of the survey, and 185 NEOs and 5776 MBAs from the fifth year. To date, the NEOWISE Reactivation survey has provided thermal model characterization for 957 unique NEOs. Including all phases of the original Wide-field Infrared Survey Explorer survey brings the total to 1473 unique NEOs that have been characterized between 2010 and the present.
Abstract
Large potentially hazardous asteroids (PHAs) are capable of causing a global catastrophe in the event of a planetary collision. Thus, rapid assessment of such an object’s physical ...characteristics is crucial for determining its potential risk scale. We treated the near-Earth asteroid (99942) Apophis as a newly discovered object during its 2020–2021 close approach as part of a mock planetary defense exercise. The object was detected by the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), and data collected by the two active bands (3.4 and 4.6
μ
m) were analyzed using thermal and thermophysical modeling. Our results indicate that Apophis is an elongated object with an effective spherical diameter
D
eff
= 340 ± 70 m, a geometric visual albedo
p
V
=0.31 ± 0.09, and a thermal inertia Γ ∼ 150–2850 J m
−2
s
−
1
2
K
−1
with a best-fit value of 550 J m
−2
s
−
1
2
K
−1
. NEOWISE “discovery” observations reveal that (99942) Apophis is a PHA that would likely cause damage at a regional level and not a global one.
The 163 comets observed during the WISE/NEOWISE prime mission represent the largest infrared survey to date of comets, providing constraints on dust, nucleus size, and CO + CO sub(2) production. We ...present detailed analyses of the WISE/NEOWISE comet discoveries, and discuss observations of the active comets showing 4.6 mum band excess. We find a possible relation between dust and CO + CO sub(2) production, as well as possible differences in the sizes of long and short period comet nuclei.
The 163 comets observed during the WISE/NEOWISE prime mission represent the largest infrared survey to date of comets, providing constraints on dust, nucleus size, and CO + CO{sub 2} production. We ...present detailed analyses of the WISE/NEOWISE comet discoveries, and discuss observations of the active comets showing 4.6 μm band excess. We find a possible relation between dust and CO + CO{sub 2} production, as well as possible differences in the sizes of long and short period comet nuclei.
ABSTRACT The Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) mission continues to detect, track, and characterize minor planets. We present diameters and albedos calculated from ...observations taken during the second year since the spacecraft was reactivated in late 2013. These include 207 near-Earth asteroids (NEAs) and 8885 other asteroids. Of the NEAs, 84% NEAs did not have previously measured diameters and albedos by the NEOWISE mission. Comparison of sizes and albedos calculated from NEOWISE measurements with those measured by occultations, spacecraft, and radar-derived shapes shows accuracy consistent with previous NEOWISE publications. Diameters and albedos fall within ∼20% and ∼40%, 1-sigma, respectively, of those measured by these alternate techniques. NEOWISE continues to preferentially discover near-Earth objects which are large (>100 m), and have low albedos.
We present space-based thermal infrared observations of the presumably Geminid-associated asteroids: (3200)Phaethon, 2005 UD and 1999 YC using WISE/NEOWISE. The images were taken at the four ...wavelength bands 3.4\(\mu\)m(W1),4.6\(\mu\)m(W2),12\(\mu\)m(W3),and 22\(\mu\)m(W4). We find no evidence of lasting mass-loss in the asteroids over the decadal multi-epoch datasets. We set an upper limit to the mass-loss rate in dust of Q<2kg s\(^{-1}\) for Phaethon and <0.1kg s\(^{-1}\) for both 2005 UD and 1999 YC, respectively, with little dependency over the observed heliocentric distances of R=1.0\(-\)2.3au. For Phaethon, even if the maximum mass-loss was sustained over the 1000(s)yr dynamical age of the Geminid stream, it is more than two orders of magnitude too small to supply the reported stream mass (1e13\(-\)14kg). The Phaethon-associated dust trail (Geminid stream) is not detected at R=2.3au, corresponding an upper limit on the optical depth of \(\tau\)<7e-9. Additionally, no co-moving asteroids with radii r<650m were found. The DESTINY+ dust analyzer would be capable of detecting several of the 10\(\mu\)m-sized interplanetary dust particles when at far distances(>50,000km) from Phaethon. From 2005 UD, if the mass-loss rate lasted over the 10,000yr dynamical age of the Daytime Sextantid meteoroid stream, the mass of the stream would be ~1e10kg. The 1999 YC images showed neither the related dust trail (\(\tau\)<2e-8) nor co-moving objects with radii r<170m at R=1.6au. Estimated physical parameters from these limits do not explain the production mechanism of the Geminid meteoroid stream. Lastly, to explore the origin of the Geminids, we discuss the implications for our data in relation to the possibly sodium (Na)-driven perihelion activity of Phaethon.
PDF
