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Researchers have used precision tracking of the asteroid Bennu to test whether a hypothetical fifth force could subtly alter its orbit. They found no confirmed signal. Instead, their 2024 analysis set upper limits on certain proposed forces, narrowing the range of strengths and particle masses that remain possible.
What physicists mean by a “fifth force”
Physics describes four familiar fundamental interactions: gravity, electromagnetism, the strong nuclear force and the weak nuclear force. A “fifth force” is a broad label for a possible additional interaction, not the name of one established theory.
The Bennu study tested selected models in which a new, very light particle could mediate a long-range interaction. The models include a dark photon, a baryon-coupled scalar and interactions associated with gauged U(1)B. Such particles appear in some theories beyond the Standard Model and may be connected to dark matter, but the study does not establish that they exist or make up dark matter. The study in Communications Physics treats them as possibilities to constrain.
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How an extra force could affect an asteroid
A fifth force would not necessarily make an asteroid visibly swerve. A tiny additional acceleration could instead accumulate over time, changing the predicted position or subtly shifting orbital parameters and close-approach timing. Comparing a precise orbit model with years of observations lets researchers test for those small differences.
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For the models they considered, the researchers added a Yukawa-like acceleration to the standard orbital calculation:
a(r) = α̃(GM☉/r3)e−r/λ(1 + r/λ)r
- α̃ describes the proposed force’s relative strength.
- λ is its characteristic range.
- The exponential factor suppresses the interaction at distances much greater than that range.
The test is to determine which values of strength and range still fit the observed trajectory. Those allowed and excluded regions are constraints; they are not evidence that the extra term is present.
Why Bennu offers a precise test
101955 Bennu has been observed with optical and radar measurements since its discovery in 1999. NASA’s OSIRIS-REx spacecraft added high-precision navigation and radiometric data while studying the asteroid. The mission arrived in December 2018, collected a sample in October 2020 and returned it to Earth in September 2023.
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Bennu is also important to impact-hazard calculations, which has encouraged careful long-term tracking. Its orbit is eccentric: the study gives a semimajor axis of about 1.1264 astronomical units (AU) and an eccentricity of about 0.20375. That changing distance from the Sun helps test forces whose effects depend on distance.
To distinguish a possible new interaction from ordinary orbital physics, the model included gravitational perturbations from the Sun, planets, Pluto, the Moon and hundreds of smaller bodies. It also accounted for non-gravitational effects such as thermal recoil (the Yarkovsky effect), solar-radiation pressure, Poynting–Robertson drag and Earth’s oblateness. Without such modeling, an apparent mismatch could reflect familiar forces, uncertain asteroid properties or measurement and ephemeris errors rather than new physics.
What the Bennu analysis found—and what it did not
The 2024 paper reports 2-sigma upper limits on the strength of the modeled forces. In practical terms, force strengths above the relevant limit are excluded under the paper’s assumptions; weaker forces remain possible. The strongest sensitivity is around a mediator mass of 10−17 electronvolts (eV), corresponding roughly to a range of 0.1 AU. The study is especially sensitive across a broader mass region of approximately 10−18 to 10−16 eV.
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The authors report that, in roughly the 10−18–10−17 eV mediator-mass range, their Bennu constraints are stronger than existing laboratory and space-test bounds for the models compared. That is a parameter-specific comparison, not a universal claim that asteroid tracking beats every other test.
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This was not a detection
The analysis did not find a statistically significant unexplained deviation in Bennu’s orbit that requires a new particle or interaction. It searched for particular signatures and found that any force in those model families must remain below the reported limits. It does not test or rule out every conceivable additional force.
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Why orbital systematics matter
More precise measurements reduce observational uncertainty, but the result still depends on how well ordinary effects are modeled. Thermal emission, radiation pressure, outgassing, an asteroid’s shape and mass distribution, small-body perturbations, radar systematics, spacecraft navigation and planetary ephemerides can all affect a fitted trajectory.
The Bennu team checked how the results changed with different planetary ephemerides. Switching from DE424 to DE440 shifted fitted best values by about 0.1 to 1.9 sigma, depending on the assumed force range. The authors used conservative 2-sigma limits to account for this uncertainty floor. A credible future detection would need to survive comparable checks, appear consistently across independent observations and objects, and follow the distance dependence predicted by the proposed interaction.
How asteroid tests complement other experiments
Laboratory experiments, lunar laser ranging, planetary ephemerides and spacecraft tests probe different distances, materials, force ranges and coupling assumptions. Asteroid tracking is useful for interactions whose ranges are comparable to solar-system scales: it follows an orbit over a long baseline in the Sun’s gravitational field. It is a complementary method, not automatically the best test at every range.
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An earlier theoretical proposal examined nine near-Earth asteroids as probes of Yukawa-type forces across mediator masses around 10−21–10−15 eV, and suggested extending the method to main-belt asteroids, Hildas, Jupiter Trojans and trans-Neptunian objects. That proposal describes a broader research opportunity, not a result from the Bennu analysis. The earlier study is catalogued by the U.S. Department of Energy’s OSTI.
What Apophis could add
The researchers also analyzed optical and radar observations of 99942 Apophis collected from 2004 to 2021. For ranges above approximately 3 × 10−2 AU in the region they analyzed, Bennu’s OSIRIS-REx tracking produced stronger constraints than the Apophis dataset.
Apophis will make a close approach to Earth in 2029, and NASA’s OSIRIS-APEX mission is intended to study it afterward. New tracking around the encounter could improve constraints by adding observations of a well-measured orbit under strong Earth-encounter dynamics. That is a future opportunity, not evidence that Apophis is being influenced by a fifth force; any analysis will need to model Earth’s gravity and measurement uncertainties carefully. A Los Alamos-linked explainer outlines the Apophis opportunity.
Why a null result still matters
Asteroid tracking has not revealed a new fundamental interaction, but it can eliminate parts of the parameter space for specific theories. The Bennu result shows how data gathered for asteroid science and planetary defense can also test ideas about ultralight particles and long-range forces—while keeping a strict distinction between a possible explanation and an observed discovery. The publication record for the study is available from OSTI.
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