Constraints on fifth forces and ultralight dark matter from OSIRIS-REx target asteroid Bennu

The study tests for hypothetical long-range fifth forces mediated by ultralight bosons predicted in many extensions of the Standard Model (e.g., gauged U(1) symmetries, dark photons, baryon-coupled scalars). Historical anomalies in celestial mechanics have revealed new physics, and modern, high-precision tracking of near-Earth asteroids provides a novel laboratory to probe deviations from Newtonian gravity. The OSIRIS-REx mission to asteroid (101955) Bennu delivered meter-level tracking and improved ephemerides, enabling sensitive tests of Yukawa-type modifications to the gravitational potential. The research question is whether Bennu’s precisely tracked trajectory shows evidence for a Yukawa fifth force and, if not, what upper limits can be set on its strength and range, and how these translate into constraints on ultralight mediators such as dark photons and baryon-coupled scalars.

A broad set of probes constrain fifth forces and ultralight particles: laboratory and space tests, planetary ephemerides (e.g., INPOP) constraining Yukawa suppressions and graviton mass; Lunar Laser Ranging tests of Newton’s constant variability and the strong equivalence principle; and analyses using asteroid/planetary perihelion precessions. Prior work also derived limits on local dark matter density from solar-system dynamics and proof-of-principle asteroid-based fifth-force sensitivity estimates. Additional constraints include EP tests (e.g., MICROSCOPE), LLR anomalous lunar precession, and black hole superradiance bounds for vector mediators, though the latter carry astrophysical uncertainties. Some recent limits leveraging LISA Pathfinder assume the mediator constitutes all dark matter. The present study improves over earlier qualitative asteroid-based sensitivities by fitting high-fidelity force models directly to high-precision OSIRIS-REx and ground-based tracking data for Bennu.

- Theory and modeling: A fifth force is modeled as a Yukawa-type modification to the Newtonian potential, parameterized by a dimensionless strength α and range λ = ħ/(mc). In the limit m → 0 (λ → ∞), the effect reduces to a rescaling G → G(1+α). The central potential leads to perihelion precession, enabling constraints via precise orbit determination. While specific microphysical realizations (e.g., gauged U(1)B or U(1)B−L, dark photon) can map α to a gauge coupling depending on composition, this work remains agnostic and constrains α and λ directly. - Data: Same dataset as the Bennu ephemeris and hazard assessment (Farnocchia et al. 2021). It includes: (i) 489 ground-based optical astrometric (RA/Dec) observations from 1999-09-11 to 2018-05-15; (ii) ground-based radar astrometry from three close encounters (Sept 1999, Sept 2005, Sept 2011): 7 Doppler and 22 delay measurements; (iii) 36 geocentric pseudo-range points for Bennu’s barycenter between 2019-01-03 and 2020-10-03, derived from OSIRIS-REx high-gain antenna tracking, each with 15 ns uncertainty (~2 m in radial distance). The span covers more than 17 orbital periods in the fit (the orbit precesses and is not closed). - Force model and integration: The JPL Comet and Asteroid Orbit Determination Package is used with a high-fidelity model: relativistic gravity from the Sun, eight planets, Pluto, and the Moon via the EIH N-body formulation; point-mass Newtonian perturbations from 343 small-body perturbers; Earth’s oblateness; and non-gravitational accelerations (Yarkovsky, solar radiation pressure, Poynting–Robertson drag). Numerical integration uses the DIVA variable-order Adams integrator in quadruple precision with a tolerance of 1e-15. The fifth-force acceleration is included for Bennu by differentiating the Yukawa potential and dividing by Bennu’s mass. Given the stringent limits found, applying the fifth-force term only to Bennu (not all other bodies) is justified a posteriori. - Parameters and fitting: The fit varies Bennu’s heliocentric orbital elements at osculating epoch 2011-01-01.0 TDB (eccentricity, perihelion distance, time of perihelion, longitude of node, argument of perihelion, inclination), Bennu’s bulk density, area-to-mass ratio, the masses of the 343 small-body perturbers, and a constant delay bias for pseudo-range points. Fifth-force parameters are characterized by range λ (denoted A in the text) and strength α. λ is fixed to 51 logarithmically spaced values between 10 au and 10^7 au; for each fixed λ, all other parameters (including α) are estimated via a global least-squares fit. In total, 353 parameters are explicitly varied for each λ grid point. Confidence intervals for α are derived; consistency with α = 0 indicates no detection.

- No evidence for a fifth force: For all tested ranges λ, the 95% confidence interval for α is compatible with α = 0. Other orbital and physical parameters agree with earlier estimates that assumed no fifth force. - Strongest sensitivity near m ≈ 10^-17 eV: Bennu tracking yields the tightest bounds in the mediator mass range m ~ 10^-18–10^-17 eV, with peak sensitivity at m ≈ 10^-17 eV (λ ≈ 0.1 au). Bennu’s orbital parameters (a = 1.1264 au, e = 0.20375) enhance sensitivity by probing shorter heliocentric distances within its eccentric orbit. - Dataset-driven precision: The inclusion of 36 OSIRIS-REx pseudo-range points (≈2 m radial uncertainty each), alongside 489 optical observations and 29 radar measurements (7 Doppler, 22 delay), significantly sharpens trajectory constraints and, in turn, fifth-force limits over more than 17 orbital periods. - Comparative constraints: Fits to Apophis tracking data yield weaker limits than Bennu over much of the considered range, reflecting OSIRIS-REx’s meter-level sensitivity. Bennu limits are competitive with, and in the mass window m ~ 10^-18–10^-17 eV stronger than, existing laboratory/space tests. - Scaling relation in short-range regime: A fit in the λ ≪ au regime provides a simple scaling relation between λ and α (consistent with planetary-ephemerides results), enabling quick estimates of constraints at small ranges. - Model translation: Constraints on α–λ can be mapped to couplings in models with Yukawa-type fifth forces, including U(1) dark photons and baryon-coupled scalars. The analysis does not require assuming that the mediator constitutes dark matter, making the limits broadly applicable.

By augmenting a high-fidelity solar-system dynamics model with a Yukawa fifth-force term and fitting directly to OSIRIS-REx and ground-based tracking of Bennu, the study robustly tests deviations from Newtonian gravity. The null result (α consistent with zero) translates into stringent upper limits on the strength–range parameter space of fifth forces. These limits address the central question of whether ultralight bosons mediating long-range forces affect asteroid orbits: within the probed ranges, they do not at a detectable level. The resulting constraints are most powerful in the mediator mass region m ~ 10^-18–10^-17 eV, complementing and, in that window, improving upon constraints from planetary precession, LLR, and EP tests. The results are directly applicable to broad classes of theories (e.g., dark photons and baryon-coupled scalars) through model-dependent mappings from α–λ to particle couplings, while remaining independent of any assumption that the mediator comprises the dark matter. Comparisons to other probes (e.g., MICROSCOPE, LLR, planetary ephemerides, superradiance bounds for vectors) highlight complementarity and differing model assumptions and systematics.

High-precision tracking of asteroid Bennu by OSIRIS-REx, combined with ground-based observations and a comprehensive force model, yields stringent, model-agnostic constraints on Yukawa-type fifth forces, with strongest sensitivity for mediator masses m ~ 10^-18–10^-17 eV (peak near ~10^-17 eV). The study demonstrates the power of asteroid orbit determination to probe extensions of the Standard Model involving ultralight bosons (e.g., dark photons, baryon-coupled scalars), without assuming a dark matter abundance. Future improvements are expected from: (i) OSIRIS-APEX tracking of Apophis; (ii) incorporating data from broader asteroid populations (NEOs, TNOs, Trojans, Hildas, main-belt) to cover wider orbital configurations and mediator ranges; and (iii) enhanced tracking with advanced timing/quantum technologies (e.g., deep space atomic clocks).

- Fifth-force acceleration applied only to Bennu in the force model; interactions among other bodies via the fifth force are neglected. Given the tight limits obtained, this is argued to be a good approximation, but it remains an assumption. - Composition dependence for mapping α–λ to specific gauge couplings (e.g., U(1)B−L) introduces uncertainty if precise bulk compositions are imperfectly known; the paper remains largely agnostic by reporting α–λ limits. - Potential plasma screening and kinetic mixing effects are neglected in the chosen scenarios (e.g., focusing on U(1)B where plasma effects are unimportant); inclusion would require detailed plasma modeling and could be model dependent. - Sensitivity reported on a λ grid (51 logarithmically spaced values between 10 and 10^7 au); off-grid behaviors are inferred by interpolation/extrapolation. - The fitted short-range scaling relation is approximate and applies in the λ ≪ au regime; comparison with other ephemeris-based results indicates consistency but differing systematic assumptions.