The invisible threat of Venus co-orbital asteroids and their potential risk to Earth

Astronomers currently know of only 20 co-orbital asteroids of Venus. Nineteen of them have eccentricities greater than 0.38, meaning their orbits are elongated enough to carry them farther from the Sun in the sky, where telescopes can spot them.

However, computer models suggest a much larger population may exist at lower eccentricities. These near-circular orbits keep the asteroids close to the Sun’s glare when viewed from Earth, making them nearly invisible.

Numerical simulations show that some of these hidden objects may evolve into trajectories that bring them close to Earth. Over tens of thousands of years, orbital shifts could turn once-stable companions of Venus into potential Earth-crossers.

Unlike Jupiter’s Trojan asteroids, which are relatively stable, Venus’ co-orbitals are highly unstable. They cycle between orbital states over roughly 12 000 years, sometimes creating conditions for close planetary encounters.

Eccentricity measures how stretched an orbit is compared to a circle. Earth’s orbit has an eccentricity of just 0.017, nearly circular. The known Venus co-orbitals almost all exceed 0.38, meaning they are significantly elongated.

These high-eccentricity orbits are easier to detect because they move farther from the Sun, creating observing windows for telescopes. By contrast, lower-eccentricity orbits remain hidden close to the Sun’s position in the sky.

The study found that some low-eccentricity co-orbitals may enter “risk regions,” where their paths bring them within 75 000 km (46 600 miles) of Earth’s orbit. At such distances, long-term simulations show that collisions become statistically possible over millennial timescales.

This does not mean every low-eccentricity Venus co-orbital is dangerous. But it highlights a population that, if it exists, could remain unseen while posing genuine planetary defense concerns.

The study identified several known Venus co-orbitals that could evolve into potentially hazardous asteroids. Based on their brightness and assumed reflectivity, some measure about 300–390 m (984–1 280 feet) in diameter.

If such an object struck Earth, it could excavate a crater 2.2–3.4 km (1.4–2.1 miles) wide. The impact would release 150–410 megatons of energy, far exceeding the yield of the largest nuclear weapons.

On the Torino impact hazard scale, these scenarios correspond to level 8, impacts capable of causing localized destruction on land or generating destructive tsunamis if occurring offshore.

Ground-based surveys can detect Venus co-orbitals only during brief periods near dusk or dawn, when the asteroids rise above 20° on the horizon and appear bright enough.

For low-eccentricity orbits, these favorable windows last only one to two weeks and may be separated by months or years of invisibility. Even the most advanced facilities, such as the Vera C. Rubin Observatory in Chile, would detect them only during rare alignments.

Simulations show that detection probability, the “visibility percentage,” is strongly tied to eccentricity. High-eccentricity objects are observable more often, while low-eccentricity ones may be visible less than 10% of the time.

This bias explains why nearly all catalogued Venus co-orbitals have high eccentricities: they are simply the ones Earth-based telescopes can reach.

Closing this observational blind spot will require telescopes in space. NASA’s NEO Surveyor mission, scheduled for launch after 2027, will operate from the Sun–Earth L1 point. It will observe at lower solar elongations, potentially revealing asteroids hidden in Earth’s daytime sky.

China’s proposed CROWN mission would go further. It envisions a constellation of six telescopes in Venus-like orbits, supported by a mothership. Simulations show this approach could track more than 94% of target asteroids for at least 100 days, enough time to compute reliable orbits.

Other mission concepts place telescopes at the Sun–Venus L2 point, where geometry favors long-duration coverage of the inner solar system. Together, these designs illustrate how space-based observatories could finally expose Venus’s “invisible” asteroid companions.

Venus co-orbitals likely formed in the main asteroid belt between Mars and Jupiter. Over billions of years, gravitational interactions with giant planets scattered some inward.

When they entered the inner solar system, a fraction were temporarily captured in 1:1 orbital resonance with Venus. These captures last on average 12 000 years before the objects drift into new orbits, some approaching Earth, others ejected into space.

This constant reshuffling ensures that new asteroids may enter Venus’s orbital region over time, maintaining a long-term collision risk.

The existence of an unseen population of Venus co-orbital asteroids underscores a major gap in planetary defense. Current ground-based networks are biased toward high-eccentricity objects that come close to Earth.

Low-eccentricity asteroids, harder to detect but still hazardous, remain beyond the reach of most observatories. Only space missions near Venus or operating at Sun–Earth L1 are likely to reveal their true numbers.

References:

¹ The invisible threat: Assessing the collisional hazard posed by undiscovered Venus co-orbital asteroids – V. Carruba et al. – Astronomy and Astrophysics – June 30, 2025 – https://doi.org/10.1051/0004-6361/202554320 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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