near-Earth Research Articles

Size, albedo, and rotational period of the Hayabusa2# target (98943) 2001 CC21

This study aims to determine the size, albedo, and rotational period of (98943) 2001 CC21, a target of the Hayabusa2 extended mission, using thermal data from the Spitzer Space telescope and ground-based observations. The Spitzer data were acquired with the Infrared Spectrograph in the 6-38 mu m range, reduced using the Spitzer pipeline, and modeled with the near-Earth asteroid thermal model to determine the asteroid size and albedo. The absolute magnitude and rotational period were determined thanks to new observations carried out at the 3.5m New Technology Telescope, the 1.2m Observatoire de Haute Provence, and the 0.7m Abastumani telescope. Three complete light curves were obtained in 2023 and 2024 at the last-mentioned telescope. We determine an absolute magnitude of H=18.94pm 0.05 and a rotational period of 5.02124pm 0.00001 hours, with a large light curve amplitude of sim 0.8 mag. at a phase angle of 22$^o$, indicating a very elongated shape with an estimated a/b semiaxis ratio geq 1.7, or a close-contact binary body. The emissivity of 2001 CC21 is consistent with that of silicates, and its albedo is 21.6pm 1.6 <!PCT!>. Finally, the spherical-equivalent diameter of 2001 CC21 is 465pm 15 m. The albedo value and emissivity determined here, coupled with results from polarimetry and spectroscopy from the literature, confirm that 2001 CC21 is an S-complex asteroid, and not an L-type one as was previously suggested. The size of 2001 CC21 is less than 500 m, which is smaller than its first size estimation (sim 700 m). These results are relevant in preparation of the observing strategy for 2001 CC21 of the Hayabusa2 extended mission.

Diffraction modelling of a 2023 March 5 stellar occultation by subkilometer-sized asteroid (98943) 2001 CC21

Abstract We present an analysis of a stellar occultation event caused by a near-Earth asteroid (98943) 2001 CC21, an upcoming flyby target in the Hayabusa2 extended mission, on 2023 March 5. To determine the asteroid’s shape from diffraction-affected light curves accurately, we developed a novel data-reduction technique named the Diffracted Occultation’s United Simulator for Highly Informative Transient Explorations (DOUSHITE). Using DOUSHITE-generated synthetic models, we derived constraints on (98943) 2001 CC21’s shadow shape from the single-chord occultation data. Our results suggest a significant elongation of the shadow with an axis ratio of $b/a = 0.37\pm 0.09$. This shape could be crucial for planning Hayabusa2’s high-speed flyby to optimise the limited imaging opportunities.

ASTERIA — Thermal inertia evaluation of asteroid Didymos

Asteroid Didymos, recently targeted by the NASA DART mission, is also planned to be visited by the ESA Hera mission. The main goal of the DART mission was to impact Dimorphos, the small satellite of Didymos, which was accomplished in September 2022. This collision altered the Didymos–Dimorphos system, generating a notable quantity of ejecta that turned Dimorphos into an active asteroid, with some ejecta potentially settling on the surfaces of both components. This prompts the investigation into the extent of post-impact surface alterations on these bodies, compared to their original states. The purpose of this study is to independently evaluate the pre-impact thermal inertia of Didymos. We employed ASTERIA, an alternative to conventional thermophysical modeling, to estimate the surface thermal inertia of Didymos. The approach is based on a model-to-measurement comparison of the Yarkovsky effect-induced drift on the orbital semi-major axis. These results, alongside existing literature, enable an evaluation of the impact-induced alterations in Didymos’s thermal inertia. Our nominal estimate with a constant thermal inertia model stands at Γ=211−55+81 J m−2 K−1 s−1/2, while assuming it varies with the heliocentric distance with an exponent of −0.75 thermal inertia of Didymos is found to be 258−63+94 J m−2 K−1 s−1/2. Subsequent verification confirmed that this result is robust against variations in unknown physical parameters. The thermal inertia estimates for Didymos align statistically with values reported in the literature, derived from both pre- and post-impact data. The forthcoming Hera mission will provide an opportunity to further corroborate these findings. Additionally, our results support the hypothesis that the thermal inertia of near-Earth asteroids is generally lower than previously expected.

Origin of asteroid (101955) Bennu and its connection to the New Polana family

The asteroid (142) Polana is classified as a B-type asteroid located in the inner Main Belt. This asteroid is the parent of the New Polana family, which has been proposed to be the likely source of primitive near-Earth asteroids such as the B-type asteroid (101955) Bennu. To investigate the compositional correlation between Polana and Bennu at the 3 µm band and their aqueous alteration histories, we analyzed the spectra of Polana in the ~ 2.0–4.0 µm spectral range using the NASA Infrared Telescope Facility in Hawai’i. Our findings indicate that Polana does not exhibit discernable 3 µm hydrated mineral absorption (within 2σ), which is in contrast to asteroid Bennu. Bennu displayed a significant 3 µm absorption feature similar to CM- and CI-type carbonaceous chondrites. This suggests two possibilities: either Bennu did not originate from the New Polana family parented by asteroid Polana or the interior of Bennu’s parent body was not homogenous, with diverse levels of aqueous alteration. Several explanations support the latter possibility, including heating due to shock waves and pressure, which could have caused the current dehydrated state of Bennu’s parent body.

The dynamical origins of the dark comets and a proposed evolutionary track

So-called ‘dark comets’ are small, morphologically inactive near-Earth objects (NEOs) that exhibit nongravitational accelerations inconsistent with radiative effects. These objects exhibit short rotational periods (minutes to hours), where measured. We find that the strengths required to prevent catastrophic disintegration are consistent with those measured in cometary nuclei and expected in rubble pile objects. We hypothesize that these dark comets are the end result of a rotational fragmentation cascade, which is consistent with their measured physical properties. We calculate the predicted size-frequency distribution for objects evolving under this model. Using dynamical simulations, we further demonstrate that the majority of these bodies originated from the ν6 resonance, implying the existence of volatiles in the current inner main belt. Moreover, one of the dark comets, (523599) 2003 RM, likely originated from the outer main belt, although a JFC origin is also plausible. These results provide strong evidence that volatiles from a reservoir in the inner main belt are present in the near-Earth environment.

Numerical simulations suggest asteroids (101955) Bennu and (162173) Ryugu are likely second or later generation rubble piles

Rubble pile asteroids are widely understood to be composed of reaccumulated debris following a catastrophic collision between asteroids in the main asteroid belt, where each disruption can make a family of new asteroids. Near-Earth asteroids Ryugu and Bennu have been linked to collisional families in the main asteroid belt, but surface age analyses of each asteroid suggest these bodies are substantially younger than their putative families. Here we show, through a coupled collisional and dynamical evolution of members of these families, that neither asteroid was likely to have been created at the same time as the original family breakups, but rather are likely remnants of later disruptions of original family members, making them second, or later, generation remnants. Our model finds about 80% and 60% of asteroids currently being delivered to near-Earth orbits from the respective families of New Polana and Eulalia are second or later generation. These asteroids delivered today in the 0.5-1 km size range have median ages since their last disruption that are substantially younger than the family age, reconciling their measured crater retention ages with membership in these families.

Trajectory Design of Potentially Hazardous Asteroid Exploration with Reusable Probes from Cislunar Space

This article presents a trajectory design problem concerning the exploration of potentially hazardous near-Earth asteroids (PHAs) with reusable probes from cislunar space. A total of 20 probes, making round trips departing from and returning to a service space station in a lunar distant retrograde orbit, are expected to explore as many PHAs as possible by means of close flyby within a 10-year time window. The trajectory design problem was released in the 12th edition of China’s Trajectory Optimization Competition on 20 August 2022, and a total of 10 sets of trajectory solutions were submitted. As the authors who proposed the competition problem, we present in this article the problem descriptions, trajectory analysis, and design, as well as an impressive trajectory solution in which a total of 105 PHAs are explored. It is concluded that taking advantage of reusable probes from cislunar space is a promising option to efficiently explore large numbers of PHAs.

BYORP and Dissipation in Binary Asteroids: Lessons from DART

The near-Earth binary asteroid Didymos was the target of the planetary defense demonstration mission DART in 2022 September. The smaller binary component, Dimorphos, was impacted by the spacecraft in order to measure momentum transfer in kinetic impacts into rubble piles. DART and associated Earth-based observation campaigns have provided a wealth of scientific data on the Didymos–Dimorphos binary. DART revealed the largely oblate and ellipsoidal shape of Dimorphos before the impact, while the postimpact observations suggest that Dimorphos now has a prolate shape. Here we add those data points to the known properties of small binary asteroids and propose new paradigms of the radiative binary Yarkovsky–O’Keefe–Radzievskii–Paddack (BYORP) effect as well as tidal dissipation in small binaries. We find that relatively spheroidal bodies like Dimorphos made of small debris may experience a weaker and more size-dependent BYORP effect than previously thought. This could explain the observed values of period drift in several well-characterized binaries. We also propose that energy dissipation in small binaries is dominated by relatively brief episodes of large-scale movement of (likely surface) materials, rather than long-term steady-state tidal dissipation. We propose that one such episode was triggered on Dimorphos by the DART impact. Depending on the longevity of this high-dissipation regime, it is possible that Dimorphos will be more dynamically relaxed in time for the Hera mission than it was in the weeks following the impact.

Warm start for optimal transfer between close circular orbits with first generation E-sail

The Electric Solar Wind Sail (E-sail) is a propellantless propulsion system for deep space navigation that exploits the dynamic pressure of the solar wind to generate thrust using a web of long, conducting tethers. The heliocentric trajectory of an E-sail-based spacecraft in a classic transfer between two Keplerian orbits is usually analyzed, in an optimal framework, by minimizing the total flight time. This paper discusses an analytical, approximate procedure for solving the two-point boundary value problem associated with the trajectory optimization process in a heliocentric, two-dimensional scenario involving a first-generation E-sail. The procedure is applied in a typical transfer between two coplanar circular orbits whose (assigned) radii are sufficiently close to each other. Paralleling the approach proposed in the recent literature, the mathematical model presented in this paper discusses a set of analytical equations that give an accurate approximation of both the minimum flight time and the (unknowns) initial adjoint variables. The paper also analyzes a set of mission applications, which involve interplanetary transfers to some near-Earth asteroids whose orbits have a very small value of both eccentricity and inclination.

Stochastic Mission Exploration Tool for Asteroid Tours

Asteroid tours facilitate asteroid visitation in great volume but are notoriously difficult to design. There exists a substantial research effort toward designing optimal asteroid tours; however, there is minimal research toward exploring all tour solutions including those that exist far from local or global optima within the asteroid tour design space. In this paper, we introduce the Orbital Tours Through Exploratory Random Search (OTTERS) tool. The OTTERS tool uses a multiple-filtering method to preprune the asteroid candidate pool and a tree algorithm to stochastically construct feasible asteroid tours. The OTTERS tool emphasizes exploration and can construct a wide range of viable asteroid tour options to be considered for asteroid missions or mission extension projects. In this paper, we use the OTTERS tool to find 1,243,558 tour solutions ranging from 2 to 10 targets within the near-Earth asteroid population for a spacecraft given two launch dates that are one year apart.

Determination of the Dynamic and Physical Characteristics of Near-Earth Asteroids Based on the Results of Observations in 2022–2023

Determination of the Dynamic and Physical Characteristics of Near-Earth Asteroids Based on the Results of Observations in 2022–2023

Astrometric Observations of NEA 1998 HH49 Using the Daocheng 50 cm Telescope

This study details an astrometric observation campaign of the Near-Earth Asteroid 1998 HH49, conducted with the aim of refining our understanding of its physical characteristics. Utilizing the 50 cm telescope located at the Wumingshan Mountain in Daocheng, Sichuan, images were obtained over four nights, from 2023 October 19 to October 22. These observations were processed using Astrometrica software, facilitating the precise determination of the asteroid’s position. The observational results were compared with the ephemerides from three distinct sources to verify accuracy: the Jet Propulsion Laboratory (JPL) Horizons System, the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) Miriade, and the Near-Earth Objects Dynamic Site (NEODyS-2). When compared with the JPL ephemeris, a mean observed-minus-calculated (O-C) result of 0.″07 in the R.A. direction and −0.″35 in the decl. direction was yielded. Furthermore, the comparison with the IMCCE ephemeris yielded mean O-C results of 0.″08 in the R.A. direction and −0.″06 in the decl. direction. The comparison with the NEODyS-2 ephemeris yielded the mean O-C results of 0.″06 in R.A. and −0.″49 in decl. direction. The study’s findings demonstrate a general consistency between the observed data and the ephemeris predictions, with minor discrepancies observed across the data sets. Notably, both the JPL and NEODyS-2 ephemerides show that the residuals in the decl. direction exceed those in the R.A. direction. The disparities may result from atmospheric differential color refraction, ephemeris discrepancies, observational errors, and other factors. Additionally, it is worth noting that further investigation is required due to the potential influence of additional factors. Overall, the Daocheng 50 cm Telescope exhibits the ability to conduct high-precision positional measurements.

A Digital Backend with Pulse Detection for Radar Astronomy

In radar astronomy, the digital backend and data recording system process and store echo signals in real-time, facilitating the monitoring of near-earth objects such as space debris, satellites, and asteroids. In this paper, a radar astronomy digital backend (RADB), designed for radar astronomy observation and data recording, is proposed. With a sampling rate of 1.12 GSps, the RADB accommodates various analog intermediate frequency (IF) front-ends. In addition to continuously recording echo signals, the RADB has three pulse storage modes and employs an architecture that combines a two-stage decimation (TSD) unit and a decimated pulse detection (DPD) unit. The TSD unit reduces the sampling frequency based on the bandwidth of the signal, after shifting the IF signal to the baseband. Compared to a single-stage decimation, the proposed TSD structure effectively reduces FIR resource consumption without compromising performance. Meanwhile, the DPD unit identifies pulse echo signals and selectively enables the backend to store data only when pulses are detected. This process further reduces the burden on data transmission and storage. Furthermore, the matched filtering pulse detection method in the DPD unit enhances triggering performance, particularly under weak signal conditions. Preliminary performance evaluations in a laboratory demonstrate that the TSD unit reduces data volume by 56 times, while the DPD unit achieves a further reduction of 20 times. Concurrently, a Moon reflection experiment is also conducted at the Yunnan Kunming Electromagnetic Environment Observation and Research Station by using a 29 m antenna. Analysis and processing of stored data validate the effectiveness of the proposed design.

NELIOTA: New results and updated statistics after 6.5 years of lunar impact flashes monitoring

We present results of the Near-Earth objects Lunar Impacts and Optical TrAnsients (NELIOTA) campaign for lunar impact flashes observed with the 1.2 m Kryoneri telescope. From August 2019 to August 2023, we report 113 validated and 70 suspected flashes. For the validated flashes, we calculate the physical parameters (masses, radii) of the corresponding projectiles, the temperatures developed during the impacts, and the expected crater sizes. For the multiframe flashes, we present light curves and thermal evolution plots. Using the whole sample of NELIOTA that encompasses 192 validated flashes in total from 2017, the statistics of the physical parameters of the meteoroids, the peak temperatures of the impacts, and the expected crater sizes has been updated. Using this large sample, empirical relations correlating the luminous energies per photometric band were derived and used to roughly estimate the parameters of 92 suspected flashes of the NELIOTA archive. For a typical value of the luminous efficiency, we found that the majority (>75%) of the impacting meteoroids have masses between 1 and 200 g, radii between 0.5 and 3 cm and produced craters up to 3.5 m. 85% of the peak temperatures of the impacts range between 2000 and 4500 K. Statistics regarding the magnitude decline and the cooling rates of the multiframe flashes are also presented. The recalculation of the appearance frequency of meteoroids (lying within the aforementioned ranges of physical parameters) on the Moon yields that the total lunar surface is bombarded with 7.4 sporadic meteoroids per hour and up to 12.6 meteoroids per hour when the Earth-Moon system passes through a strong meteoroid stream. By extrapolating these rates on Earth, the respective rates for various distances from its surface are calculated and used to estimate the probability of an impact of a meteoroid with a hypothetical infrastructure on the Moon, or with a satellite orbiting Earth for various impact surfaces and duration times of the missions.

The 18 May 2024 Iberian superbolide from a sunskirting orbit: USG space sensors and ground-based independent observations

ABSTRACT On 18 May 2024, a superbolide traversed the western part of the Iberian Peninsula, culminating its flight over the Atlantic Ocean and generating significant media attention. This event was caused by a weak carbonaceous meteoroid of 1 m, entering the atmosphere at 40.4 km s$^{-1}$ with an average slope of 8.5$^\circ$. The luminous phase started at 133 km and ended at an altitude of 54 km. The meteoroid’s heliocentric orbit had an inclination of 16.4$^\circ$, a high eccentricity of 0.952, a semimajor axis of 2.4 au, and a short perihelion distance of 0.12 au. The superbolide was recorded by multiple ground-based stations of the Spanish Fireball and Meteorite Network and the European Space Agency, as well as by the U.S. Government sensors from space. Due to the absence of observable deceleration, we successfully reconciled satellite radiometric data with a purely dynamic atmospheric flight model, constraining the meteoroid’s mass and coherently fitting its velocity profile. Our analysis shows a good agreement with the radiant and velocity data reported by the Center for Near-Earth Object Studies, with a deviation of 0.56$^\circ$ and 0.1 km s$^{-1}$, respectively. The presence of detached fragments in the lower part of the luminous trajectory suggests that the meteoroid was a polymict carbonaceous chondrite, containing higher-strength macroscopic particles in its interior due to collisional gardening, or a thermally processed C-type asteroid. The orbital elements indicate that the most likely source is the Jupiter-Family Comet region, aligning with the Solar and Heliospheric Observatory comet family, as its sunskirting orbit is decoupled from Jupiter. This event provides important information to characterize the disruption mechanism of near-Sun objects.

The Population of Small Near-Earth Objects: Composition, Source Regions, and Rotational Properties

The study of small (<300 m) near-Earth objects (NEOs) is important because they are more closely related than larger objects to the precursors of meteorites that fall on Earth. Collisions of these bodies with Earth are also more frequent. Although such collisions cannot produce massive extinction events, they can still produce significant local damage. Here we present the results of a photometric and spectroscopic survey of small NEOs that include near-infrared spectra of 84 objects with a mean diameter of 126 m and photometric data of 59 objects with a mean diameter of 87 m. We found that S-complex asteroids are the most abundant among the NEOs, comprising ∼66% of the sample. Most asteroids in the S-complex were found to have compositions consistent with LL-chondrites. Our study revealed the existence of NEOs with spectral characteristics similar to those in the S-complex but that could be hidden within the C- or X-complex due to their weak absorption bands. We suggest that the presence of metal or shock darkening could be responsible for the attenuation of the absorption bands. These objects have been grouped into a new subclass within the S-complex called Sx-types. The dynamical modeling showed that 83% of the NEOs escaped from the ν 6 resonance, 16% from the 3:1, and just 1% from the 5:2 resonance. Lightcurves and rotational periods were derived from the photometric data. No clear trend between the axis ratio and the absolute magnitude or rotational period of the NEOs was found.

Laboratory Spectral Characterization of Ribbeck Aubrite: Meteorite Sample of Earth-impacting Near-Earth Asteroid 2024 BX1

Characterization of near-Earth objects (NEOs) is critical for Earth-impact hazard assessment. Particularly crucial to our physical understanding of NEOs are laboratory spectral measurements of meteorites as they are the best and most widely available analog materials, barring sample return missions. However, most meteorites do not have direct orbital links to specific asteroids, making it challenging to identify their source body in the NEO or main-belt asteroid populations. Near-Earth asteroid (NEA) 2024 BX1 was discovered on 2024 January 20 at 21:48 UTC from MPC code K88, impacting the Earth (west of Berlin, Germany) 165 minutes later. The incoming bolide was observed by multiple meteor cameras, which enabled successful reconstruction of its exo-atmospheric orbit and quick recovery. We present results from laboratory spectral characterization of the Ribbeck meteorite in the UV–mid-infrared wavelengths (0.2–14.2 μm) over seven grain size bins (<45 μm–slab). Our results suggest that Ribbeck has spectral properties consistent with enstatite achondrite (aubrite) meteorites. Our grain-size spectral analysis shows that albedo and spectral slope decrease as grain size increases. In addition, increasing grain size also shifts the taxonomic type in the Bus–DeMeo system from Xn to B types, suggesting the limitations of taxonomy in classifying small, regolith-free NEAs. We also present results of our comparison between Ribbeck data and spectra of E types in the main-belt and NEA populations. Principal component analysis of our Ribbeck samples shows variations parallel to the α line, which can be confused with space weathering in PC space.

2023 DZ2 Planetary Defense Campaign

We present the results of a fourth planetary defense exercise, focused this time on the small near-Earth asteroid (NEA) 2023 DZ2 and conducted during its close approach to the Earth in 2023 March. The International Asteroid Warning Network (IAWN), with support from NASA's Planetary Defense Coordination Office (PDCO), has been coordinating planetary defense observational campaigns since 2017 to test the operational readiness of the global planetary defense capabilities. The last campaign focused on the NEA Apophis, and an outcome of that exercise was the need for a short burst campaign to replicate a real-life near-Earth object impact hazard scenario. The goal of the 2023 DZ2 campaign was to characterize the small NEA as a potential impactor and exercise the planetary defense system including observations, hypothetical risk assessment and risk prediction, and hazard communication with a short notice of just 24 hr. The entire campaign lasted about 10 days. The campaign team was divided into several working groups based on the characterization method: photometry, spectroscopy, thermal IR photometry and optical polarimetry, radar, and risk assessment. Science results from the campaign show that 2023 DZ2 has a rotation period of 6.2745 ± 0.0030 minutes; visible wavelength color photometry/spectroscopy/polarimetry and near-IR spectroscopy all point to an E-type taxonomic classification with surface composition analogous to aubrite meteorites; and radar observations show that the object has a diameter of 30 ± 10 m, consistent with the high albedo (0.49) derived from polarimetric and thermal IR observations.

Dimorphos’s Orbit Period Change and Attitude Perturbation due to Its Reshaping after the DART Impact

On 2022 September 26 (UTC), NASA's Double Asteroid Redirection Test (DART) mission achieved a successful impact on Dimorphos, the secondary component of the near-Earth binary asteroid system (65803) Didymos. Subsequent ground-based observations suggest a significant reshaping of Dimorphos, with its equatorial axis ratio changing from 1.06 to ∼1.3. Here we report the effects of this reshaping event on Dimorphos's orbit and attitude. Given the reported reshaping magnitude, our mutual dynamics simulations show that approximately 125 s of the observed 33 minute orbit period change after the DART impact may have resulted from reshaping. This value, however, is sensitive to the precise values of Dimorphos's post-impact axis ratios and may vary by up to 2 times that amount, reaching approximately 250 s within the current uncertainty range. While the rotational state of the body is stable at the currently estimated axis ratios, even minor changes in these ratios or the introduction of shape asymmetry can render its attitude unstable. The perturbation to Dimorphos’s orbital and rotational state delivered by the impact directly, combined with any reshaping, leads to a strong possibility for a tumbling rotation state. To accurately determine the momentum enhancement factor (β) through measurements by the European Space Agency’s Hera spacecraft and to evaluate the effectiveness of the kinetic deflection technique for future planetary defense initiatives, the effects of reshaping should not be overlooked.

Asteroid Impact Hazard Warning from the Near-Earth Object Surveyor Mission

NASA’s Near-Earth Object Surveyor mission, scheduled for launch in 2027 September, is designed to detect and characterize at least two-thirds of the potentially hazardous asteroids with diameters larger than 140 m in a nominal 5 yr mission. We describe a model to estimate the survey performance using a faster approach than the time domain survey simulator described in Mainzer et al. (2023). This model is applied to explain how the completeness for 5 and 10 yr surveys varies with orbit type and asteroid size and to identify orbits with notably high or low likelihoods of detection. Size alone is an incomplete proxy for impact hazard, so for each asteroid orbit, we also calculate the associated hazard based on the impact velocity and the relative likelihood of impact. We then estimate how effective the mission will be at anticipating impacts as a function of impact energy, finding that a 5 yr mission will identify 87% of potential impacts larger than 100 Mt (Torino-9, “Regional Devastation”). For a 10 yr mission, this increases to 94%. We also show how the distribution of warning time varies with impact energy.