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An international study led by researchers at São Paulo State University (UNESP) in Brazil has identified a little-known but potentially significant threat: Asteroids that share Venus's orbit and may completely escape current observational campaigns because of their position in the sky. These objects have not yet been observed, but they could strike Earth within a few thousand years. Their impacts could devastate large cities.

"Our study shows that there's a population of potentially dangerous asteroids that we can't detect with current telescopes. These objects orbit the sun, but aren't part of the asteroid belt, located between Mars and Jupiter. Instead, they're much closer, in resonance with Venus. But they're so difficult to observe that they remain invisible, even though they may pose a real risk of collision with our planet in the distant future," astronomer Valerio Carruba, a professor at the UNESP School of Engineering at the Guaratinguetá campus (FEG-UNESP) and first author of the study, told Agência FAPESP.

The study is in the journal Astronomy & Astrophysics. The work combined analytical modeling and long-term numerical simulations to track the dynamics of these objects and assess their potential to come dangerously close to Earth.

The so-called "Venusian co-orbital asteroids" circle the sun rather than the planet, but they share the same orbital region and similar periods.

"These objects enter into 1:1 resonance with Venus, which means that they complete one revolution around the sun in the same time as the planet," the researcher explains.

Unlike Jupiter Trojans, which tend to be more stable, the Venusian co-orbitals known to date are highly eccentric and unstable. They alternate between different orbital configurations in cycles that last, on average, about 12,000 years. These transitions mean that the same object can be in a safe configuration close to Venus one moment and pass close to Earth at another.

"During these transition phases, the asteroids can reach extremely small distances from Earth's orbit, potentially crossing it," Carruba warns.

The less eccentric, the more dangerous

The current catalog lists only 20 Venusian co-orbital asteroids—all but one of which have an eccentricity greater than 0.38. This means their orbits take them to regions of the sky farther from the sun, where they are more easily detected by ground-based observatories. However, computer models show that there must be a much larger population of asteroids with lower eccentricities that would remain virtually invisible from Earth.

"The absence of objects with an eccentricity of less than 0.38 is clearly the result of an observational bias," the researcher points out.

Eccentricity is a mathematical concept and parameter that measures how elongated an orbit is in relation to a perfect circle. Its value ranges from 0, which represents a circular orbit, to close to 1, which represents a highly elliptical orbit. For example, Earth's orbit has an eccentricity of approximately 0.017, making it nearly circular. The Venus co-orbital asteroids known to date have eccentricities greater than 0.38, indicating much more elongated trajectories. Asteroids with lower eccentricities tend to remain closer to their average orbits and are more difficult to detect when located near the sun.

In simulations with fictitious objects, the group identified risk regions where asteroids could come dangerously close to Earth. Some of these simulated objects reach minimum distances of around 5×10^−45 astronomical units—a distance so small that statistically, it would correspond to almost certain impacts on a millennial scale.

"Asteroids about 300 meters in diameter, which could form craters 3 to 4.5 kilometers wide and release energy equivalent to hundreds of megatons, may be hidden in this population," says Carruba. "An impact in a densely populated area would cause large-scale devastation."

The study analyzed the possibility of detecting these objects from Earth using the Vera Rubin Observatory (LSST), which was recently inaugurated in Chile. However, simulations indicate that even the brightest asteroids would only be visible for one to two weeks if they were above 20 degrees on the horizon. Additionally, these windows of visibility are separated by long periods of non-observation.

"Such asteroids can remain invisible for months or years and appear for only a few days under very specific conditions. This makes them effectively undetectable with Vera Rubin's regular programs," the researcher reveals.

One alternative would be to use space telescopes focused on regions close to the sun. Missions such as NASA's Neo Surveyor and China's proposed Crown could detect asteroids at low solar elongations from Venus-orbiting positions, providing more comprehensive and continuous coverage.

"Planetary defense needs to consider not only what we can see, but also what we can't yet see," Carruba argues.

The origin of asteroids has been attributed to the fragmentation of a hypothetical Earth-like planet by impact. However, the most widely accepted hypothesis today about the origin of objects in the asteroid belt, located between Mars and Jupiter, is that they are remnants from the formation of our solar system. These rocky bodies are fragments of planetesimals (the "building blocks" of planets) that failed to aggregate and form a planet due to Jupiter's strong gravitational influence.

This influence disrupted the orbits of objects in the region and prevented their coalescence, or the process by which they would eventually merge. Thus, the asteroid belt represents a kind of "fossil" of the protoplanetary disk, containing planetary building blocks in different states of evolution and composition.

As for the co-orbitals of Venus, it is believed that they originated in the main belt. Due to complex gravitational interactions, primarily with Jupiter and Saturn, they were gradually diverted to internal orbits. There, they were temporarily captured in resonance with Venus.

"These captures are ephemeral on an astronomical timescale, lasting—on average—about 12,000 years. And the objects may eventually evolve into trajectories close to Earth or be ejected from the solar system," explains Carruba.

The research was conducted by the Orbital Dynamics and Planetology Group (GDOP) at UNESP.