Orbit Quality Projections for New Discoveries by the Near-Earth Object Surveyor

The Near-Earth Object Surveyor (NEO Surveyor; Mainzer et al., 2023) is a NASA mission designed to advance planetary defense by discovering near-Earth asteroids. Operating from a halo orbit at the Sun-Earth L1 Lagrange point, NEO Surveyor will conduct a wide-field, mid-infrared survey with an emphasis on detecting potentially hazardous asteroids (PHAs) — objects larger than 140m with minimum orbital intersection distances (MOIDs) with Earth of less than 0.05au. NEO Surveyor’s mid-infrared bandpasses will directly measure asteroids’ thermal emission, providing a strong constraint on size which is necessary for hazard assessment. 

In this work, we analyze the expected fitted-orbit quality and time-evolution of the corresponding on-sky ephemeris uncertainties for newly-discovered PHAs and smaller asteroids from NEO Surveyor. This assessment is important for the mission itself and for understanding the ground-based follow-up regime that NEO Surveyor will create.

To discover previously unknown near-Earth asteroids, NEO Surveyor’s images will be processed through the NEO Surveyor Survey Data System (NSDS). The NSDS will construct difference-images that will be processed by the Moving Object Detection Pipeline (MODP) to identify “tracklets” from Solar system objects — groups of four or more detections within several hours of each other. These tracklets will be submitted to the Minor Planet Center (MPC) within three days of detection, which will associate the tracklet with a known object, link the tracklet to previously-submitted tracklets, or retain the “isolated” tracklet for possible future linking. Previously unknown objects will require at least two linked NEO Surveyor tracklets to be cataloged as a new discovery. Previously known objects (not the focus of this study) will benefit from linkage to archival observations, yielding longer arcs and well-constrained orbits.

We simulate NEO Surveyor’s discoveries using the NEO Surveyor Survey Simulator (NSS; Masiero et al., 2023), an injection-and-recovery framework that compares the expected flux of simulated objects to the observatory’s projected sensitivity at that point on the sky. Our simulations begin with a reference population of near-Earth asteroids, to which we apply the NEO Surveyor Known Object Model (Grav et al., 2023). This method tags each object as “known” or “unknown” at the beginning of the mission, allowing us to specifically estimate the orbit quality of new objects despite using an injected population of only synthetic asteroids.

After constructing tracklets with NSS, we process those tracklets in find_orb (Gray, 2022) for initial orbit determination. To assess orbit quality, we examine formal uncertainties in orbital elements, the MPC’s orbital uncertainty “U” parameter, and projected ephemeris uncertainties at future epochs. For planetary defense applications, we also focus on the precision in MOID determination — this metric provides a key assessment of impact risk. We validate our methodology by running known objects through the survey simulation and orbit-fitting pipeline, then comparing the orbital solutions and ephemeris predictions with JPL Horizons.

NEO Surveyor’s observational strategy has typical revisit times on the order of two weeks, meaning that a two-tracklet observational arc spans at least this time; this attribute of the cadence places the discovery tracklets in a fundamentally different regime than contemporary ground-based surveys that usually construct shorter discovery arcs. NEO Surveyor will observe across a range of Solar elongations (as low as ~45 degrees), including areas challenging or impossible for ground-based telescopes to observe. Thus, ground-based follow-up will often need to wait until NEO Surveyor discoveries reach an observable Solar elongation (if ever). For objects that do eventually become observable from Earth, understanding the evolution of on-sky uncertainties after NEO Surveyor’s discovery is paramount.

We analyze the expected orbit quality from NEO Surveyor across orbital classes (Apollos, Amors, Atens, and Atiras) and physical sizes. Specifically, we correlate arc length and other detection statistics with orbit uncertainties from find_orb. Most PHAs and a large fraction of smaller objects achieve discovery arcs beyond the minimum two-tracklet track. The median track for an observed PHA apparition contains four tracklets; find_orb returns excellent orbital solutions for nearly all these median cases and for many shorter arcs. For objects with the shortest allowable discovery track — the most challenging orbit-fitting scenario with two minimally-spaced tracklets — we characterize how on-sky uncertainties change over time to inform ground-based follow-up. Many PHAs with two-tracklet arcs, even those that aren’t immediately ground-observable, should be recoverable with modern follow-up systems.

Finally, we project the evolution of orbit quality at the catalog-level throughout the mission, showing the expected state after each year of operations. NEO Surveyor’s cadence will naturally follow-up many of its own discoveries during operations and produce high-quality orbits. For those objects, ground-based data in visible wavelengths can produce complementary measurements of albedo, color, rotation rates, and lightcurve amplitudes. In summary, our analysis helps maximize NEO Surveyor’s contributions to planetary defense and planetary science by ensuring that new discoveries can be effectively monitored and characterized.