Invisible ‘city killer’ asteroids orbiting Venus could hit Earth without warning, scientists warn

Astronomers led by Valerio Carruba of São Paulo State University (UNESP) have identified a largely unseen population of asteroids co‑orbiting the Sun in resonance with Venus, objects few have noticed, but which may one day pose serious risk to Earth, his team published findings in Astronomy & Astrophysics. These Venus co‑orbital asteroids, also known as 1:1 mean‑motion resonant objects, share roughly the same orbital period around the Sun as Venus, without being satellites of the planet itself.

There are currently around 20 known co‑orbitals of Venus. Most of them have high orbital eccentricity (e > 0.38), meaning their paths around the Sun are quite elongated, bringing them far enough from Venus’s path (and the glare of the Sun) that telescopes can sometimes pick them up. But the key result of the recent study is that this observed distribution is almost certainly biased, there may be many more with lower eccentricities that are effectively invisible with current ground‑based methods.

The study emphasises that many of these asteroids are large enough, at least 140 metres wide, to be classified as ‘city killers’ if they were to strike Earth. Such an impact could devastate a large urban area. While none of the known co‑orbitals currently pose an imminent threat, simulations spanning several co‑orbital cycles (on the order of 12,000 years each) show that some could evolve into Earth‑crossing orbits under gravitational perturbations. In particular, six known co‑orbitals were identified as having future potential to become what astronomers term Potentially Hazardous Asteroids (PHAs), with minimum orbital intersection distances small enough to merit attention. The study covers long‑term dynamics over up to about 36,000 years, three average co‑orbital cycles, to estimate how often transitions to risky orbits might happen.

The greatest challenge is observational: these low‑eccentricity co‑orbitals spend much of their orbits very close to the Sun in our sky, where Sun glare severely limits what ground‑based telescopes can see. Generally, objects with high eccentricity wander farther from the Sun’s vicinity, making them more likely to be found. But those with nearly circular orbits, which never stray far, remain hidden. Another factor is the timing and geometry: when these bodies are observable, it may be only during brief windows, just after sunset or before sunrise, and only at certain angles relative to the Sun when glare is less overwhelming.

Future telescopes and missions could close some of these observational gaps. NASA’s NEO Surveyor, slated to launch in the next few years, is designed to view in infrared wavelengths, allowing detection of objects that are faint, dark, or hidden by sunlight, and to spot asteroids coming from close to the Sun. The Vera C. Rubin Observatory may also catch some of these during favourable configurations. Carruba’s team indicates that while many low‑eccentricity Venus co‑orbitals are currently undetectable, some will enter favourable viewing windows, and new survey capabilities could reveal them.

No immediate danger: The current known co‑orbitals are not on a collision course with Earth for the foreseeable future.

Long‑term risk: Over thousands of years, gravitational effects can shift orbits. Objects that are safe now may become hazardous.

Need for better surveys: Effectively defending Earth against such threats depends not only on what we see, but on estimating what we cannot yet see. Space‑based infrared observations could prove crucial.