NearEarth Object NEO Surveyor will detect asteroids and comets using midinfrared thermal emission however groundbased followup resources will require knowledge of the expected visible light brightness in order to plan characterization observations. Here we describe the range of visualtoinfrared colors that the NEOs detected by Surveyor will span and demonstrate that for objects that have no previously reported Visual band observations estimates of the Johnson Visualband brightness based on infrared flux alone will have significant uncertainty. Incidental or targeted photometric followup of objects discovered by Surveyor enables predictions of the fraction of reflected light visible and nearinfrared wavelengths supporting additional detailed characterization.
['Asteroids', 'Albedo', 'Near-Earth objects', 'Surveys', '72', '2321', '1092', '1671', 'Astrophysics - Earth and Planetary Astrophysics', 'Astrophysics - Instrumentation and Methods for Astrophysics']
Visualband Brightnesses of NearEarth Objects that will be Discovered in the Infrared by NEO Surveyor
{'Visual-band brightnesses of Near Earth Objects that will be discovered in the infrared by NEO Surveyor': 'Joseph R. Masiero 1 , Tyler Linder 2 , Amy Mainzer 3 , Dar W. Dahlen 1 , Yuna G. Kwon 1', 'ABSTRACT': 'NEO Surveyor will detect asteroids and comets using mid-infrared thermal emission, however ground-based followup resources will require knowledge of the expected visible light brightness in order to plan characterization observations. Here we describe the range of visual-to-infrared colors that the NEOs detected by Surveyor will span, and demonstrate that for objects that have no previously reported Visual band observations, estimates of the Johnson Visual-band brightness based on infrared flux alone will have significant uncertainty. Incidental or targeted photometric followup of objects discovered by Surveyor enables predictions of the fraction of reflected light visible and near-infrared wavelengths, supporting additional detailed characterization.', '1. Introduction': "The Near-Earth Object (NEO) Surveyor mission will survey the Solar system at two thermal infrared wavelengths (4 -5 . 2 µ m and 6 -10 µ m) with the singular goal of detecting the majority of objects that pose a regional hazard to Earth. Over the course of the 5-year required survey period, the mission is expected to detect ∼ 100 , 000 -200 , 000 near-earth asteroids larger than 25 m in size, and will report these observations to the MPC within 3 days of data acquisition. Mainzer et al. (2023) provide an overview of the mission and the design of survey simulation tools that will be used for survey completeness validation.  \nThe survey strategy employed by the NEO Surveyor mission is designed to provide sufficient self-followup to establish the orbit and effective spherical diameter of the NEOs observed. Diameters can be determined from NEO Surveyor data even if visible light flux is not available (Mainzer et al. 2015), as was done for the objects discovered by the NEOWISE survey that did not receive ground based followup (e.g. Mainzer et al. 2011; Masiero et al. 2011). This is possible because the total thermal emission is only weakly dependent on albedo, and so the added uncertainty due to the unknown albedo is only a small component of the total error.  \nThe mission also has a Targeted Followup Observation (TFO) mode for high-priority objects that can be used for improving orbital knowledge, obtaining high-cadence light curve observations (individual Exposures are taken every 30 s), and improving thermal modeling constraints. The TFO mode, however, takes time away from normal survey operations and so will only be used sparingly for the most critical objects. Additional characterization such as spectroscopy would need to employ ground- or space-based followup telescopes, the vast majority of which observe at visible or near-infrared wavelengths. In order to support asteroid and comet followup observations by the community, the NEO Surveyor mission will provide estimated Johnson Visual magnitude brightnesses for discovered objects to enable these investigations.  \nThe recent International Asteroid Warning Network (IAWN 1 ) observing campaigns present a model for the types of followup that could be obtained for a potential impactor. IAWN is a global collection of amateur and professional astronomers dedicated to studying and monitoring the nearEarth asteroid population. IAWN to date has held six asteroid campaigns, two of which were dedicated to the observational timing assessments of fast-moving asteroids and four were intended to assess the ability of the community to rapidly recover and characterize NEOs. Those latter four campaigns in particular demonstrated how additional characterization data would be used to help assess the hazard posed by an impactor, and how this knowledge would be disseminated among the various stakeholders. The types of characterization data that could possibly be obtained and the timing for when it would be available can have significant impacts on the responses that are available. Therefore, it is important to understand the resources that would be able to access new NEO discoveries.  \nThe most recent campaign was a rapid response exercise following the discovery of NEO 2023 DZ2, whose impact probability quickly rose with follow-up observations before measurements from precovery data retired the impact risk (Reddy et al. 2024). This object was discovered at a V = 20 . 3 mag ten days prior to close approach, and rose to a peak brightness of V ∼ 13 mag, allowing a wide range of characterization tools to be used. Visible and near-infrared photometry, spectroscopy, and polarimetry all provided an immediate estimate of the albedo of this NEO that was fed into the hazard model.  \nPrevious characterization campaigns focused on recovery and tracking of 2012 TC4 (Reddy et al. 2019), spectral characterization of (66391) Moshup (Reddy et al. 2022a), and simulated discovery and hazard assessment of (99942) Apophis (Reddy et al. 2022b). The characterization campaign for 2012 TC4 was able to obtain light curve properties while the object was at V ∼ 22 . 5 mag, however these rotational observations were at the practical limit for the 4 -5 m-class telescopes used. In each case, ground-based followup allowed the impact hazard model to be refined and improved.  \nTo assess the potential future capacity for followup of NEO Surveyor discoveries, we take the  \nsynthetic population of near-Earth objects that were created to verify the mission's Level 1 survey requirements and extend this population down to diameters of D = 25 m. This population is assigned both orbital and physical properties. Using the current best knowledge of the survey rules and system sensitivities for NEO Surveyor, we determine the Visual and mid-infrared fluxes of each NEO at the time when it would first be discovered by the survey. We present the results of this analysis and related discussion below.", '2. Population Model and Survey Simulation': "Weuse the survey simulation tools recently developed for the NEO Surveyor mission to generate a synthetic population of near-Earth objects down to sizes below the current completeness limit. We simulate objects with diameters of D ≥ 25 m following the size-frequency distribution described in Mainzer et al. (2023), with orbital and albedo distributions based on the nearly-complete regime of the currently known NEO population (see Stokes et al. 2017, for more details about the NEO size distribution). Our survey simulator calculates the position and expected flux of each object for every time it would be in the field of view of a survey pointing. Dynamical and flux calculation routines were validated against standard software tools as well as the NEOWISE data, as described in Masiero et al. (2023).  \nPredicted fluxes for NEOs are compared to the current best estimate of the system sensitivity and the predicted zodiacal light background flux for each observation to determine if the synthetic object would be detected. A fraction of these potential detections are then discarded based on a signal-to-noise-dependent factor to account for incompleteness in the data extraction due to e.g. confusion, bad pixels, etc. The remaining detections are then assembled into tracklets following the same rules that will be used when processing the flight data. This produces a set of data analogous to what the NEO Surveyor will be submitting to the Minor Planet Center once the survey commences.  \nBecause physical properties are generated for each object, it is possible to determine the Visual band flux from reflected light as well as the thermal infrared flux from emission at the time each object would be detected. While thermal emission is determined using the Near-Earth Asteroid Thermal Model (NEATM Harris et al. 1998), Visual band flux is determined using the H-G asteroid photometric system (Bowell et al. 1989) and the orbital geometry at the time of observation. Using these two fluxes for each detection, we can determine the apparent magnitude difference from V band to the NC2 bandpass centered at 8 µ m (the band where Surveyor will be most sensitive to NEOs). In this implementation, this difference doesn't strictly indicate a spectral reflectance gradient, however we will still refer to it here as the V -NC 2 color.  \nApproximately 40% of the NEOs larger than 140 m in diameter are expected to be known at the time of launch based on current survey capabilities (Grav et al. 2023) and thus will already have visible light measurements available to use for estimating visible brightnesses. The Vera Rubin  \ntelescope is scheduled to begin survey prior to the launch of NEO Surveyor, and so will improve the catalog of reflected light measurements with observations of a large number of Solar system objects. Wagg et al. (2024) show that Rubin will typically observe NEOs at V ∼ 22 -23 (see their Figure 7), but will observe few NEOs with V > 24 mag. The remaining NEOs detected by Surveyor will have unknown visible light properties (in particular an unknown Visual band albedo) and as such would be expected to be drawn randomly from the population below. Quantification of this uncertainty is needed to properly plan for any followup observations.", '3. Survey Simulation Results': "We show in Figure 1 the distribution of Johnson V-band magnitudes for all NEOs at the first time that they would be detected during the survey. We use the time of the initial detection, rather than the peak brightness of each object over the 5-year survey period, as it is more representative of the brightness at time when immediate characterization followup would be most desirable for an object with non-zero impact probability, and when the predicted on-sky position will be best known prior to followup. We mark the location of V = 22 mag in this plot to show the approximate limits for the most common NEO survey and followup assets in use today (e.g. Larson et al. 2003; Denneau et al. 2013; Bellm et al. 2019; Devog'ele et al. 2019). As demonstrated in this plot, the vast majority of NEOs detected by Surveyor will be magnitudes fainter than what is currently submitted to the MPC and published for followup on the NEO Confirmation Page 2 . This is consistent with the previous findings of Mainzer et al. (2015).  \nFigure 2 shows the spatial histogram of NEO V magnitudes compared to the elongation (onsky angular distance from the Sun) at the time of first detection by NEO Surveyor. The magnitude distribution is consistent across most elongations, with a ∼ 1 mag brightening at the lowest elongations. However, low elongations are the hardest for ground-based telescopes to access as they are only above 2 airmasses in the twilight sky or for a short period of time outside twilight. Incidental visible light photometry will likely be available for objects at larger elongations from ground-based surveys such as the Vera C. Rubin telescope (Jones et al. 2018). The trend of increasing numbers of objects at larger elongations is due primarily to the large population of amor-class NEOs, which spend all their orbit outside the Earth's, and thus are most accessible at high elongations. Amors tend to have higher minimum orbital intersection distances (MOID) with the Earth's orbit, and thus are less likely to be categorized as potentially hazardous objects which are defined as objects with MOID < 0 . 05 AU.  \nThe planning of any desired characterization followup of NEO Surveyor discoveries beyond mission-obtained TFO characterization will be difficult for objects that do not receive any incidental visible light observations, and thus have unknown brightnesses at reflected light wavelengths.  \nFig. 1.- V-band magnitude distribution of all simulated NEOs at the time they would be first detected by NEO Surveyor. The vertical dotted line indicates V= 22 mag, the approximate limiting magnitude of current NEO surveys and followup.  \n<!-- image -->  \nBecause of the wide range of albedos that have been observed for NEOs (Mainzer et al. 2011; Wright et al. 2016) and the albedo distribution that is expected for the population observed by NEO Surveyor (Mainzer et al. 2023), the predicted reflected light brightness for an object of a given infrared flux will be drawn from a large potential range. Figure 3 shows the distribution of V-NC2 colors for all NEOs in our simulation at the time of first detection. While the distribution is peaked at V -NC 2 ∼ 10, the full-width at half-maximum of this distribution spans ∼ 3 mags. The figure also shows the median V -NC 2 color (10.2) as well as the 2 . 5 th and 97 . 5 th percentile ranges ( -2 . 2 and +2 . 3, respectively.) which span the 2-sigma region of the distribution.  \nOther detection parameters available at the time of observation, such as elongation or signalto-noise in the NEO Surveyor bandpasses, do not provide constraints on the expected V -NC 2 color. As shown in Figures 4, the color distribution does not show significant structure with respect to brightness at the NC2 wavelengths. Parameters that can be derived from NEO Surveyor data after self-followup and orbit fitting such as diameter also show large spreads in V -NC 2 color, as shown in Figure 5. Larger objects do tend to show bluer colors, as they are more likely to be detected at larger heliocentric distances where their temperatures are lower, however these are also the most likely to be already recorded in the current catalog.  \nFig. 2.- V-band magnitude as a function of elongation for all simulated NEOs at the time they would be first detected by NEO Surveyor. The horizontal dotted line indicates V= 22 mag, the approximate limiting magnitude of current NEO surveys and followup. Spatial bins in this plot represent a linear count of objects.  \n<!-- image -->", '4. Discussion': "Our simulations show that there is a wide dispersion in possible V band magnitudes for infrared-detected NEOs. It is not possible to make a precise prediction of these objects' expected Visual magnitudes using only the data obtained by NEO Surveyor. This means that targeted followup of individual objects of interest will need to account for a wide range of possible brightnesses, where only part of the possible range may be detectable with the followup instrumentation available, e.g. spectroscopy.  \nA complete analysis of the capabilities of follow-up assets currently available is challenging due to the large number of variables. The location of the telescope, observing time availability, and specific instrument parameters such as image scale and readout time make a direct general comparison difficult. Instead, we discuss a few specific cases commonly needed for astrometric followup.  \nThe most critical type of astrometric observation will be if NEO Surveyor discovers a virtual impactor. Selecting the correct ground-based assets is crucial to successful recovery. For example, the first IAWN campaign recovered 2012 TC4 using VLT at V ∼ 27 mag which is the practical  \nFig. 3.- V-NC2 color distribution of all simulated NEOs at the time they would be first detected by NEO Surveyor. The cross indicates the median value of the distribution, while the whiskers show the range spanned by the 2 . 5 th and 97 . 5 th percentiles.  \n<!-- image -->  \nupper limit of recovery using an 8 meter ground-based telescope. The NEO community will likely be limited by observing time constraints to a handful of recoveries at these brightnesses per telescope per year. Considering all the 8 meter telescope time that could be even potentially be available for recovery, less than 50 asteroids per year could receive astrometric follow-up fainter than V = 26 mag.  \nThe typical limiting magnitude of 2-4m class telescopes is V ∼ 25 mag. A 2 meter-class telescope optimized for moving object astrometric recovery can outperform a 4 meter telescopes with a more generalized setup and instrument set. However, the availability of facilities for asteroid recovery between V ∼ 23 -25 mag will be limited to a theoretical maximum of hundreds of asteroids per night if all potential facilities were used in a dedicated fashion. Objects brighter than V ∼ 23 mag are in the range of the current NEO survey and followup programs, and so could be expected to receive followup based on the current operational program, however the number of potential targets will likely saturate the available time for the currently operational programs.  \nPhotometric and spectroscopic characterization observations are limited to V ∼ 22 mag currently, which means that it will be difficult to obtain further analysis for many members of the population discovered by NEO Surveyor. This is especially true for objects where the possible Visual band brightness is predicted to straddle this sensitivity threshold. One method to address this challenge is for ground-based telescopes to report negative detections and the limiting magnitude  \nFig. 4.- V-NC2 color vs signal-to-noise (SNR) of the NC2 detection for simulated NEOs at the time of first detection. Spatial bins in this plot represent a logarithmic count of objects; the vast majority of detections are at SNR ∼ 5. Due to the expected photometric systematic uncertainties, sources have an effective upper limit to their SNR causing the apparent increase in source density at SNR ∼ 34 for all objects brighter than this limit.  \n<!-- image -->  \nthat was searched. This would allow for setting a limit on the possible H V magnitude and rule out the possibility of characterization for some objects.  \nIt is important for the NEO Surveyor project to report the minor planet observations in a way that provides as much utility as possible for the followup observer community. Experience from NEOWISE, where the team reported estimated V magnitudes for most objects to aid in followup, found that predicted V magnitudes could often be 2 mags fainter than the recovery magnitude, as NEOWISE preferentially discovered low-albedo NEOs (Mainzer et al. 2011). Some NEOWISE-discovered objects had no reported estimated visible brightness at all, resulting in objects in the orbit catalog without H V magnitudes, which significantly hampered followup planning. The NEOWISE reported V magnitudes were estimates based on the infrared flux, not visible light measurements, but this fact is not explicitly indicated as such in the MPC observation archive. Users of the data would need to refer to the NEOWISE documentation (e.g. Cutri et al. 2012, 2015) to learn the source of these values.  \nTo resolve this situation, we propose to use the new reporting fields enabled by the Astrom-  \nFig. 5.- V-NC2 color at the time of first detection vs the size of the simulated NEOs. Spatial bins in this plot represent a logarithmic count of objects. Larger objects are skewed bluer as they are more likely to be detected further from the Sun, but they still span a significant range of colors.  \n<!-- image -->  \netry Data Exchange Standard (ADES 3 ) format, the preferred format for submitting data to the Minor Planet Center. Specifically, the mission can report estimated V magnitudes as was done for NEOWISE, but now can include an uncertainty on that value that will indicate it should only be considered a weak constraint on the actual expected V mag. Based on the distribution shown in Fig 3, and the range spanned by the 2-sigma region, the value to report would be  \nV estimate ≈ NC 2 + 10 ± 2 mag  \nwhich would cover the majority of the expected V mag distribution.  \nThe benefit of a large uncertainty value is that any real photometric measurement that follows the NEO Surveyor observations would quickly dominate the H V calculation carried out during orbit fitting (assuming magnitude uncertainties are also reported for those measurements). Expansion of the MPC's NEO Confirmation Page to include the uncertainty on both the predicted V and H V values will enable observers to understand the provenance of these predicted values.  \nBeyond NEO Surveyor, estimated V magnitudes may be useful for observation reports from other infrared observatories such as the James Webb Space Telescope (JWST). As described by  \nMuller et al. (2023), during calibration observations the JWST Mid-Infrared Instrument (MIRI) detected a previously unknown asteroid, and the authors estimate that every MIRI observation near the ecliptic will likely contain a few unknown asteroids. These MIRI data alone were sufficient to constrain the size of this unknown object, despite the lack of orbital arc coverage needed to fit an orbit. While astrometric recovery and followup of these short-arc objects is nearly impossible with current resources, the ability to report visual magnitude estimates for infrared detections will potentially help future efforts to link these detections to later observations.", '5. Conclusions': "We use the survey simulation tools developed for NEO Surveyor to investigate the expected brightnesses of near-Earth objects that the mission will discover. We find that the vast majority of objects will be significantly fainter than the current depth reachable with most reflected light followup assets. Ground-based targeted astrometric followup will be difficult to achieve on a large scale, although the mission's survey is designed to provide astrometric self-followup. In order to obtain optical photometry and spectral characterization of any virtual impactors discovered by NEO Surveyor it will be necessary to invest in more sensitive followup resources and exercise these systems.  \nEstimating the Visual brightness of NEO Surveyor discoveries is complicated by the wide range of possible V -NC 2 colors that detected objects are predicted to have. Reports of these estimates will use the photometric uncertainty field to identify to followup observers the large range of brightnesses that are possible. Propagation of these uncertainties to the predicted apparent and absolute magnitudes provided on the MPC's NEO Confirmation Page will allow followup observers to plan for a range of activities based on the recovered brightness. This will fulfill the needs of the followup community for observation planning purposes, as well as avoid empty entries in the orbit catalogs, all while making it clear to data users that this value is only an estimate of the actual brightness of the object.", 'Acknowledgments': "We thank the referees for their helpful comments that improved this manuscript. This publication makes use of software and data products from the NEO Surveyor, which is a joint project of the University of California, Los Angeles and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This research has made use of data and services provided by the International Astronomical Union's Minor Planet Center. This research has made use of NASA's Astrophysics Data System Bibliographic Services. This research has made use of the neospy , numpy , scipy , astropy , and matplotlib Python packages.", 'REFERENCES': "Stokes, G.H., Barbee, B.W., Bottke Jr., W.F. et al. , 2017, NASA Report of the Near-Earth Object Science Definition Team, 'Update to Determine the Feasibility of Enhancing the Search and Characterization of NEOs', https://cneos.jpl.nasa.gov/doc/2017 neo sdt final e-version.pdf  \nWagg, T., Juric, M., Yoachim, P., et al. 2024, arXiv:2408.12517. doi:10.48550/arXiv.2408.12517  \nWright, E. L., Mainzer, A., Masiero, J., et al. , 2016, AJ, 152, 79. doi:10.3847/0004-6256/152/4/79"}