The discovery of 2024 YR4 presented the planetary defense community with the most significant impact threat in almost two decades, reaching level 3 on the Torino scale. The community, now mature and well-organized, responded with a global observational effort. Astrometric measurements, forming the basis for orbital refinement and impact prediction, were a central component of this response. In this paper, we present the astrometric data collected by the international community, from the time of discovery until the object became too faint for all existing observational assets, including JWST. We also discuss the coordination role played by the International Asteroid Warning Network, and the importance of publicly available image archives to enable precovery searches.

Compared to extensive studies on apparent magnitude and bi-directional reflectance functions (BRDFs) for Low Earth Orbit (LEO) satellites like Starlink, there has been a notable gap in the literature when it comes to spectral radiance characterization. Such characterization would be highly beneficial to the emerging Alternative Positioning, Navigation, and Timing (Alt-PNT) method of Terrestrial Optical Navigation, which involves collecting optical line-of-sight measurements to satellites in the presence of sky background, turbulence, and other atmospheric effects. Specifically, it could allow for careful design of optical filtering schemes to maximize signal-to-noise ratio (SNR) of the satellite measurements. To address the literature gap, the authors introduce a dataset resulting from a four-night observation campaign with the Magdalena Ridge Observatory’s 2.4-meter telescope. Spectra spanning 380–860 nm were collected for 50 satellites and orbital objects across LEO, Medium Earth Orbit (MEO), and Geostationary Orbit (GEO), with particular emphasis on all three generations of Starlink satellites. Analysis of the Starlink data reveals a local reflectance peak near 475 nm, a shape similar to the solar irradiance curve, and a distinct near-infrared spike for the v2 generation. Estimation of absolute spectral flux from relative flux density measurements is presented, as well as discussion on how near-infrared (NIR) or short wave infrared (SWIR) sensing could expand the operational envelope of terrestrial optical navigation. This dataset offers new insight into the feasibility of optical satellite tracking under challenging conditions, including daylight operations and onboard high-speed aerospace vehicles.

On 2022 September 26, the Double Asteroid Redirection Test (DART) mission was the first successful demonstration of a kinetic impactor for planetary defense. The DART mission utilized a novel autonomous data processing pipeline architecture to quickly produce and analyze the quality of raw and calibrated images from the camera mounted on board the spacecraft. Optimization of the data processing pipeline allowed the final 150 images prior to impact to be calibrated and delivered to the Investigation Team and the press within 15 minutes of acquisition. A data quality analysis pipeline allowed for rapid identification of detector misconfigurations, missing data, and other adverse events. DART data products, along with data from LICIACube and data from ground observatories, used common file formats to facilitate the development of analysis and archiving software. This architecture is described for future missions with large volumes of data and an emphasis on quick-turnaround applications such as planetary defense.

The world’s first planetary defense test mission was carried out in late 2022 by NASA’s Double Asteroid Redirection Test (DART) mission. The main DART spacecraft, which was accompanied by the ASI-provided LICIACube cubesat, intentionally impacted Dimorphos, the smaller secondary of the near-Earth object binary system (65803) Didymos, on 2022 September 26. The impact released a large amount of ejecta, which, combined with the spacecraft’s momentum, produced the observed 33 ± 1 minute period change that was subsequently observed from ground-based telescopes. The DART mission, in addition to having successfully changed the orbital period of Dimorphos, also activated the asteroid as a result of the impact but under known conditions, unlike other impacts on asteroids. We have conducted long-term monitoring over 5 months following the impact with the Las Cumbres Observatory Global Telescope (LCOGT) network and Magdalena Ridge Observatory (MRO). This was supplemented by almost 3 months of more sparsely sampled data, primarily from educational users of the LCOGT network during the period from 2022 July 5 to 2022 September 25, prior to the impact date of 2022 September 26. Here we report the observations of the Didymos system and DART impact ejecta with the telescopes of the LCOGT network from T+1.93 days to T+151.3 days after impact, and we study the evolving morphology of the ejecta cloud and evolving tail over the entire length of the data set. In addition, we combined these intensive data sets with the earlier sparse observations over the ∼90 days prior to impact to derive a new disk-integrated phase function model using the H , G _1 , G _2 parameterization.

On 26 September 2022, the Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos. This demonstrated the efficacy of a kinetic impactor for planetary defense by changing the orbital period of Dimorphos by 33 minutes (Thomas et al. 2023). Measuring the period change relied heavily on a coordinated campaign of lightcurve photometry designed to detect mutual events (occultations and eclipses) as a direct probe of the satellite's orbital period. A total of 28 telescopes contributed 224 individual lightcurves during the impact apparition from July 2022 to February 2023. We focus here on decomposable lightcurves, i.e. those from which mutual events could be extracted. We describe our process of lightcurve decomposition and use that to release the full data set for future analysis. We leverage these data to place constraints on the post-impact evolution of ejecta. The measured depths of mutual events relative to models showed that the ejecta became optically thin within the first ~1 day after impact, and then faded with a decay time of about 25 days. The bulk magnitude of the system showed that ejecta no longer contributed measurable brightness enhancement after about 20 days post-impact. This bulk photometric behavior was not well represented by an HG photometric model. An HG1G2 model did fit the data well across a wide range of phase angles. Lastly, we note the presence of an ejecta tail through at least March 2023. Its persistence implied ongoing escape of ejecta from the system many months after DART impact.

NASA's Double Asteroid Redirection Test (DART) mission was the first to demonstrate asteroid deflection, and the mission's Level 1 requirements guided its planetary defense investigations. Here, we summarize DART's achievement of those requirements. On 2022 September 26, the DART spacecraft impacted Dimorphos, the secondary member of the Didymos near-Earth asteroid binary system, demonstrating an autonomously navigated kinetic impact into an asteroid with limited prior knowledge for planetary defense. Months of subsequent Earth-based observations showed that the binary orbital period was changed by –33.24 minutes, with two independent analysis methods each reporting a 1 σ uncertainty of 1.4 s. Dynamical models determined that the momentum enhancement factor, β , resulting from DART's kinetic impact test is between 2.4 and 4.9, depending on the mass of Dimorphos, which remains the largest source of uncertainty. Over five dozen telescopes across the globe and in space, along with the Light Italian CubeSat for Imaging of Asteroids, have contributed to DART's investigations. These combined investigations have addressed topics related to the ejecta, dynamics, impact event, and properties of both asteroids in the binary system. A year following DART's successful impact into Dimorphos, the mission has achieved its planetary defense requirements, although work to further understand DART's kinetic impact test and the Didymos system will continue. In particular, ESA's Hera mission is planned to perform extensive measurements in 2027 during its rendezvous with the Didymos–Dimorphos system, building on DART to advance our knowledge and continue the ongoing international collaboration for planetary defense.

The impact of the Double Asteroid Redirection Test spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos’s orbit substantially, largely from the ejection of material. We present results from 12 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 mag, we find consistent dimming rates of 0.11–0.12 mag day ^−1 in the first week, and 0.08–0.09 mag day ^−1 over the entire study period. The system returned to its pre-impact brightness 24.3–25.3 days after impact though 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, though movement of the primary ejecta through the aperture likely played a role.

The Double Asteroid Redirection Test (DART) spacecraft successfully performed the first test of a kinetic impactor for asteroid deflection by impacting Dimorphos, the secondary of near-Earth binary asteroid (65803) Didymos, and changing the orbital period of Dimorphos. A change in orbital period of approximately 7 min was expected if the incident momentum from the DART spacecraft was directly transferred to the asteroid target in a perfectly inelastic collision 1 , but studies of the probable impact conditions and asteroid properties indicated that a considerable momentum enhancement ( β ) was possible 2 , 3 . In the years before impact, we used lightcurve observations to accurately determine the pre-impact orbit parameters of Dimorphos with respect to Didymos 4 – 6 . Here we report the change in the orbital period of Dimorphos as a result of the DART kinetic impact to be −33.0 ± 1.0 (3 σ ) min. Using new Earth-based lightcurve and radar observations, two independent approaches determined identical values for the change in the orbital period. This large orbit period change suggests that ejecta contributed a substantial amount of momentum to the asteroid beyond what the DART spacecraft carried.

Understanding and predicting the evolving spin states of defunct geosynchronous (GEO) satellites and rocket bodies is important for space situational awareness, active debris removal, satellite servicing, and anomaly resolution. There is clear evidence that many defunct GEO satellite spin states are predominantly driven by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. The YORP effect is spin state evolution due to solar radiation and thermal re-emission torques. Observations are crucial to understand how YORP drives spin states and to validate dynamical models. Unfortunately, GEO satellites are non-resolved from ground-based telescopes and extracting spin states (spin periods, rotational angular momentum vector, instantaneous attitude) from ubiquitous photometric light curve data is challenging. Even for well-known objects, light curve inversion often yields several or more well-fitting spin state solutions within the measurement noise and modeling uncertainty (i.e. detailed satellite geometry, reflective properties, etc.). Also, there is strong evidence that the YORP effect drives satellites from uniform rotation to non-principal axis tumbling. Such tumbling states further complicate the light curve inversion process because the motion is driven by two independent periods rather than one. To aid complete spin state analysis, particularly for the tumbling case, Doppler radar observations collected at NASA Goldstone Deep Space Communications Complex are incorporated. Observing the well-documented retired GOES 8–12 weather satellites, the radar data yielded unambiguous spin period estimates for all targets and greatly narrowed pole solutions, independent of light curve data. Significant changes in spin rates and pole directions were observed over a two month span. These findings are consistent with YORP-driven evolution.

Many defunct satellites in geosynchronous earth orbit (GEO) spin rapidly or have highly evolving spin states, with some transitioning between uniform and non-principal axis rotation (tumbling). It is hypothesized that the observed evolution of some defunct GEO satellites is caused by the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect and internal energy dissipation. YORP torques are generated by the absorption, reflection, and thermal re-emission of solar radiation and are known to cause secular changes in asteroid spin rates and obliquities. The defunct GOES 8 satellite is particularly notable. This satellite's uniform spin rate rapidly decreased in 2014 and it began tumbling. In this paper, newly analyzed light curve observations of the five nearly identical defunct GOES 8-12 satellites obtained between 2014 and 2018 are presented. The observations show large diversity in evolutionary time histories, with several satellites in consistent slow tumbling, GOES 10 in fast uniform rotation, and GOES 8 transitioning between both. To better understand this diversity, YORP dynamical models are investigated. They reveal that YORP driven spin state evolution is strongly dictated by end of life appendage orientations, which differ among the five satellites. The known end of life configurations are consistent with the observed evolution of GOES 8 and GOES 10. This provides a plausible explanation for the observed spin state diversity. Implications of the observed and simulated evolution for GEO debris mitigation are discussed.

On 15 February 2013, the asteroid 367943 Duende (2012 DA14) experienced a near-Earth encounter at an altitude of 27,700 km or 4.2 Earth radii. We present here the results of an extensive, multi-observatory campaign designed to probe for spectral and/or rotational changes to Duende due to gravitational interactions with the Earth during the flyby. Our spectral data reveal no changes within the systematic uncertainties of the data. Post-flyby lightcurve photometry places strong constraints on the rotation state of Duende, showing that it is in non-principal axis rotation with fundamental periods of P_1 = 8.71 +/- 0.03 and P_2 = 23.7 +/- 0.2 hours. Multiple lightcurve analysis techniques, coupled with theoretical considerations and delay-doppler radar imaging, allows us to assign these periods to specific rotational axes of the body. In particular we suggest that Duende is now in a non-principal, short axis mode rotation state with a precessional period equal to P_1 and oscillation about the symmetry axis at a rate equal to P_2. Temporal and signal-to-noise limitations inherent to the pre-flyby photometric dataset make it difficult to definitively diagnose whether these periods represent a change imparted due to gravitational torques during the flyby. However, based on multiple analysis techniques and a number of plausibility arguments, we suggest that Duende experienced a rotational change during the planetary encounter with an increase in its precessional rotation period. Our preferred interpretation of the available data is that the precession rate increased from 8.4 hours prior to the flyby to 8.7 hours afterwards. A companion paper by Benson et al. (2019) provides a more detailed dynamical analysis of this event and compares the data to synthetic lightcurves computed from a simple shape model of Duende. (abbreviated abstract)

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 similar to 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.

The Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is a proposed explanation for the observed rotation behavior of inactive satellites in Earth orbit. This paper further explores the YORP effect for highly asymmetric inactive satellites. Satellite models are developed to represent the GOES 8 and GOES 10 satellites, both of which are currently inactive in geosynchronous Earth orbit (GEO). A simple satellite model for the GOES 8 satellite is used to analyze the short period variations of the angular velocity and obliquity as a result of the YORP effect. A more complex model for the rotational dynamics of the GOES 8 and GOES 10 satellites are developed to probe their sensitivity and to match observed spin periods and states of these satellites. The simulated rotation periods are compared to observations for both satellites. The comparison between YORP theory and observed rotation rates for both satellites show that the YORP effect could be the cause for the observed rotational behavior. The YORP model also predicts a novel state for the GOES 8 satellite, namely that it could periodically fall into a tumbling rotation state. Recent observations of this satellite are consistent with this prediction.

Small near-Earth asteroids (NEAs) (< 20 m) are interesting, because they are progenitors for meteorites in our terrestrial collection. The physical characteristics of these small NEAs are crucial to our understanding of the effectiveness of our atmosphere in filtering low-strength impactors. In the past, the characterization of small NEAs has been a challenge, because of the difficulty in detecting them prior to close Earth flyby. In this study, we physically characterized the 2 m diameter NEA 2015 TC25 using ground-based optical, near-infrared and radar assets during a close flyby of the Earth (distance 128,000 km) in 2015 October 12. Our observations suggest that its surface composition is similar to aubrites, a rare class of high-albedo differentiated meteorites. Aubrites make up only 0.14% of all known meteorites in our terrestrial meteorite collection. 2015 TC25 is also a very fast rotator with a period of 133 +/- 6 s. We combined the spectral and dynamical properties of 2015 TC25 and found the best candidate source body in the inner main belt to be the 70 km diameter E-type asteroid (44) Nysa. We attribute the difference in spectral slope between the two objects to the lack of regolith on the surface of 2015 TC25. Using the albedo of E-type asteroids (50%-60%) we refine the diameter of 2015 TC25 to 2 m, making it one of the smallest NEAs ever to be characterized.

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is a proposed explanation for the observed rotation behavior of inactive satellites in Earth orbit. This paper propagates the rotational dynamics of the GOES 8 and GOES 10 satellites and compares the simulated rotation periods to the observed rotation periods. The observations of each satellite are taken over a few months, therefore, the short period terms are included in the propagation of the satellite's rotational dynamics. The comparison between YORP theory and the observed changes in rotation rate for both satellites show that the YORP effect could be the cause for the observed behavior.

Laboratory impact experiments on meter-scale targets were conducted to explore collisional outcomes pertinent to the problem of hazardous asteroid mitigation. In particular, the primary aim of the experiments was to improve our understanding of how the energy needed to fragment and disrupt an asteroid scales with size, such that mitigation strategies could better incorporate this parameter. Previous researchers found that this critical disruption energy decreased with increasing target size. A secondary goal of the research was to investigate post-impact other factors that might be affected by target size-scaling such as ejecta translational and rotational velocities. Preliminary results will be presented for these parameters such that comparisons to the asteroid population can be made in the future.

Asteroid surveys are the backbone of asteroid science, and with this in mind we begin with a broad review of the impact of asteroid surveys on our field. We then provide a brief history of asteroid discoveries so as to place contemporary and future surveys in perspective. Surveys in the United States have discovered the vast majority of the asteroids and this dominance has been consolidated since the publication of Asteroids III. Our descriptions of the asteroid surveys that have been operational since that time are focussed upon those that have contributed the vast majority of asteroid observations and discoveries. We also provide some insight into upcoming next-generation surveys that are sure to alter our understanding of the small bodies in the inner solar system and provide evidence to untangle their complicated dynamical and physical histories. The Minor Planet Center, the nerve center of the asteroid discovery effort, has improved its operations significantly in the past decade so that it can manage the increasing discovery rate, and ensure that it is well-placed to handle the data rates expected in the next decade. We also consider the difficulties associated with astrometric follow-up of newly identified objects. It seems clear that both of these efforts must operate in new modes in order to keep pace with expected discovery rates of next-generation ground- and space-based surveys.

Asteroid events such as the air blast above Chelyabinsk, Russia, in the spring of 2013 have highlighted the importance of understanding Earth's local asteroid population and the potential threat they pose. While much observational data exists for larger asteroids (those greater than 50 meters in diameter), comparatively little is known of the very small meteoroid population. This study utilized the non-illuminated part of the near side of the Moon as a detection screen upon which the faint flashes of small meteoroids hitting the lunar surface could be detected. Over 80 hours of observations were conducted between the fall of 2010 and the spring of 2013. The observed lunar area was determined nightly and used alongside the impact count to determine a meteoroid flux rate. Impact flashes were calibrated to extinction and color corrected reference stars to estimate the kinetic energies involved. Analysis indicated an average flux rate of 1.09×10−7km−2h−1, corresponding to 1 impactor larger than 3 centimeters in diameter observed every 30 minutes. Further, the size distribution dependence of small impactors was found to be consistent with the power law governing the larger-size meteoroid distribution. This data helps fix the low-end of the asteroid size distribution for solar system evolution models.

A superior mutual event of the Kuiper Belt binary system (79360) Sila–Nunam was observed over 15.47h on UT 7/8 February 2013 by a coordinated effort at four different telescope facilities; it started ∼1.5h earlier than anticipated, the duration was ∼9.5h (about 10% longer than predicted), and was slightly less deep than predicted. It is the first full event observed for a comparably sized binary Kuiper Belt object. We provide predictions for future events refined by this and other partial mutual event observations obtained since the mutual event season began.

: Many non-resolved techniques have been proposed and explored to infer space object's characteristics that may lend insight to object's identification and status. The latter attributes are hard to obtain in the absence of resolved imagery, as for objects not in Low Earth Orbit or for small-sized ones. Spectral unmixing is a non-resolved technique that derives an object's material composition from one or a series of spectra. While spectral unmixing techniques have been tested with space objects and spectrometric visible spectra, with spectral width smaller than 0.4 nanometers and over 100 spectral channels, its success against moderately resolved spectra has not been verified. An example of a moderate resolution sensor is that of a slit-less spectrograph which is desirable because of its simple implementation and arguably better temporal fidelity than systems with a slit. A moderate number of spectral bands are considered a challenge when the number of bands is much smaller than the number of material candidate spectra. We develop the Spectral Unmixing for Space Objects (SUSO) algorithm based on Sparse Recovery optimization techniques to deal with this challenge. Sparse recovery, a specialty area of Compressive Sensing, capitalizes on the knowledge that, while the number of candidate materials is expansive, the external surface of a satellite is effectively composed of a few representative materials. We discuss the technique and show the results of applying it on a number of simulated and measured spectra. Simulated signatures are in the Visible and reflective IR, and measured signatures were made with the Magdalena Ridge Observatory's slit-less spectrograph, which is based on a CCD and grating combination mounted on a 2.4 m telescope. We studied the preservation of temporal information when a time series of spectra is analyzed by SUSO and the prospect of augmenting typical shape recovery with material attribution.