Asteroids crossing Earth's orbit have long been recognized as one of the most dangerous space threats. Most of them are tracked using telescopes, including the state-of-the-art Vera Rubin Observatory. But a new study has revealed that there is a class of objects that remain virtually "invisible" to our instruments.
These are asteroids that move in resonance with Venus - they orbit the Sun in the same time as the planet. These bodies are much closer to the Sun than the main asteroid belt between Mars and Jupiter, but due to the peculiarities of the lighting they are almost impossible to see from Earth.
Co-author of the study, Professor Valerio Carruba from the University of the State of São Paulo, explained that the orbits of such objects are very unstable. Although they “share” the path with Venus, the trajectory often takes them far beyond the planet’s orbit – into the zone of Mercury or even Earth. Currently, only 20 such asteroids are known, the most famous of which is Zoozve, with a diameter of up to 500 meters.
The study shows that about once every 12,000 years, the orbits of these asteroids change shape dramatically, at which time they can cross the Earth's orbit or approach at a distance five times smaller than the distance to the Moon. Although most of them are small, among the "invisible" objects there may be giants with a diameter of 200–500 meters, the impact of which in a densely populated region could cause a catastrophe of global scale.
Asteroids are not transparent or disappearing, they just almost never enter the zone of full illumination. Like Venus or Mercury, they are constantly observed only in phases - half-illuminated or with a thin crescent, which makes them practically invisible even to the most powerful ground-based telescopes.
According to Karruba and colleagues, even Vera Rubin can only see them during short time windows of each orbit. To effectively track such objects, scientists believe that a space telescope closer to the Sun, such as at the L2 point of Venus, is needed. Only then can the true scale of the danger be assessed.
