The invisible threat: Assessing the collisional hazard posed by the undiscovered Venus co-orbital asteroids

Currently, there are twenty known Venus co-orbital asteroids, but only one has an eccentricity (e) lower than 0.38 (Carruba et al. 2024; Pan & Gallardo 2024 and references therein). This distribution is likely an observational bias, as asteroids with higher eccentricities may come closer to Earth, making them easier to detect. Three known co-orbitals can potentially soon become PHAs (Carruba et al. 2025). The main objective of this work is to assess the threat that the undetected Venus co-orbital population may pose to Earth and to investigate their detectability.

Fig. (1): Left panel: the distribution in the (H, e) plane of known asteroids near Venus (blue full circles) and its co-orbital asteroids (red stars). Right panel: the distribution of 14382 simulated NEAs obtained from the NEOMOD3 model (black dots).

The expected orbital distribution of near-Earth objects (NEAs) in the vicinity of Venus obtained using the NEOMOD3 model (Nesvorný et al. 2024) does not show a dynamical preference for producing asteroids with high eccentricities (Fig. (1)). While the left panel shows that the majority of known co-orbitals have e > 0.38, the right panel reveals a large simulated population with e < 0.38. This suggests that observational biases are the most likely explanation for the lack of detection of low-eccentricity co-orbitals.

Pan & Gallardo (2024) recently developed a semi-analytical model to describe the coorbital motion in the 1:1 resonances. This permits describing the structure of the resonance under the assumption that the orbital elements of the asteroid remain fixed for a short period. Different types of coorbital orbits are possible, including tadpole (TL₄ and TL₅), horseshoe (H), and retrograde satellites (RS) orbits, compound configurations such as H-RS and T-RS, as well as transitions between these configurations. Known Venus coorbitals typically alternate between these configurations in a co-orbital cycle of approximately (12000 ± 6000) years.

Fig. (2): Projections in the (σ, a) plane of the Hamiltonian levels and the output of numerical simulations (black dots) for an asteroid in a TL₄ configuration (top left panel), in an RS orbit (top right panel), in an HRS configuration (bottom left panel) and a TRS orbit (bottom right panel).

To validate the Pan & Gallardo (2024) model, Carruba et al. (2025) applied it to the initial conditions of the 20 known Venus coorbitals and compared the results with 1000-year numerical simulations. Fig. (2) compares the projections in the (σ, a) plane of the Hamiltonian levels with the orbital evolution obtained in the numerical simulations for asteroids in different coorbital configurations (TL₄, RS, HRS, and TRS). The agreement between the semi-analytical model and the numerical simulations suggests that the model can be used to generate initial conditions for undetected Venus coorbitals.

To assess the collision risk, Carruba et al. (2025) performed 36000-year numerical simulations for several fictitious co-orbital asteroid clones distributed on a grid of initial eccentricity and inclination values. The simulations monitored close encounters with Earth, defined as an orbital distance smaller than the radius of Earth's Hill sphere, and tracked the minimum orbital intersection distance (MOID).

Fig. (3): Contour plot of the number of close encounters with Earth as a function of the initial (e, inc) values. The black stars display the location of the real co-orbitals of Venus with an MOID with Earth of 0.0005 au or less. The red stars show the orbital location of five test particles at e < 0.38 that experienced low MOIDs in our simulations.

Fig. (3) shows a contour map of the minimum MOID with Earth in the initial plane (e, inc). As expected, most close encounters occur near e = 0.38. However, there is a significant region in phase space at lower eccentricities (e < 0.38) where potential coorbitals may experience several close encounters and possibly collisions with Earth. 

Carruba et al. (2025) also investigated the observability of these potential undetected coorbitals from Earth, focusing on observing programs of the Vera C. Rubin Observatory. They analyzed a statistical sample of test particles that experienced low MOIDs with Earth, identified by the dashed line in Fig. (3). For each object, they computed ephemerides from the Vera C. Rubin Observatory site, applying sequential visibility filters: objects above the horizon, with elevation > 20◦ and apparent magnitude < 23.5. We calculated the "visibility percentage" – defined as the fraction of total ephemeris entries that satisfy all observability criteria – as a function of orbital elements. The analysis reveals a strong positive correlation between eccentricity and visibility percentage, with higher eccentricity objects being observable longer. 

Fig. (4): Fig. 9: Visibility percentage of Venus co-orbital asteroids as observed from the Rubin Observatory site as a function of orbital parameters.

Finally, observations conducted from Venus orbit, looking away from the Sun, could significantly improve the detection of these bodies. A Venus-based observer would have more favorable and consistent opportunities to detect and track co-orbital objects, although still subject to fundamental visibility limitations due to solar elongation limits. Proposed missions include space telescopes orbiting the Sun-Venus L₂ Lagrange point and telescope constellations such as the CROWN mission (Zhou et al. 2022, Fig. (5)). Such missions could be crucial to map and discover all the still “unseen” PHAs among Venus co-orbital asteroids.

Fig. (5): The CROWN telescope constellation in the Sun-Venus three-body system. Adapted from Zhou et al. (2022).

References

Carruba, V., Moreira Morais, M. H., Mourão, D. C., et al. 2024, RNAAS, 8, 213.

Carruba, V., Di Ruzza, S., Caritá, G., et al. 2025, Icarus, A116508.

Carruba, V., Sfair R., Araujo R. A. N.. et al. 2025, A&A, under review.

Nesvorný, D., Vokrouhlický, D., Shelly, F., et al. 2024, Icarus, 417, 116110.

Zhou, X., Li, X., Huo, Z., Meng, L., & Huang, J. 2022, Space: Science & Technology, A9864937.