Twenty years ago, the US Congress instructed NASA to find 90% of near-Earth asteroids threatening Earth. They've made progress finding these asteroids that orbit the Sun and come to within 1.3 astronomical units of Earth. However, they may have to expand their search since astronomers are now finding asteroids co-orbiting Venus that could pose a threat.
New research tries to understand how many more may co-orbit Venus and how we can detect them. They can be hidden in the Sun's glare and resist our efforts to find them. It comes down to observability windows and how the asteroids' brightness changes.
The research is titled " The invisible threat: assessing the collisional hazard posed by the undiscovered Venus co-orbital asteroids," and has been submitted to the journal Astronomy and Astrophysics. The lead author is Valerio Carruba, an assistant professor at São Paolo University in Brazil. The paper is currently available at arxiv.org.
"Twenty co-orbital asteroids of Venus are currently known," the authors write. "Co-orbital status protects these asteroids from close approaches to Venus, but it does not protect them from encountering Earth." Venus's co-orbital asteroids are considered potentially hazardous asteroids (PHA) if they have "a minimum diameter of about 140 meters and come within 0.05 astronomical units (au) of Earth's orbit," they explain.
The big question is, do these pose a collisional threat to Earth?
"We aim to assess the possible threat that the yet undetected population of Venus co-orbiters may pose to Earth, and to investigate their detectability from Earth and space observatories," the authors write.
Only one of the 20 known asteroids has an orbital eccentricity below 0.38. This makes sense since asteroids with wider orbits come closer to Earth and are easier to detect. So its detection is likely the result of an observational bias. Unfortunately, it also means there could be many more of them with minor orbital eccentricities that are very difficult to detect.
Most of the Solar System's asteroids are in the main belt between Mars and Jupiter. However, others are co-orbital with planets, like the Jupiter Trojans, which form two groups: one behind and one ahead of Jupiter. Astronomers are finding more asteroids co-orbiting with Venus, posing a threat to Earth. Image Credit: NASA/LPI
One problem in determining their danger is that co-orbitals have unpredictable orbits. "The co-orbital asteroids of Venus are highly chaotic, with Lyapunov times of the order of 150 years," the authors explain. The Lyapunov time refers to how long an object's orbit takes to become unpredictable because of chaotic dynamics.
This means that studying a single orbit of an object doesn't tell us much about what its orbit will evolve into in more than about 150 years. The authors write that a statistical study of 'clone' asteroids provides a clearer picture.
The researchers created a grid with different orbital inclinations and populated it with 26 cloned asteroids with different orbital characteristics. They then integrated them with the orbits of the Solar System's planets for 36,000 simulated years. Then they checked to see if any cloned asteroids had a close encounter with Earth.
"There is a range of orbits with eccentricity < 0.38, larger at lower inclinations, for which Venus' co-orbitals can pose a collisional hazard to Earth," the authors write.
Then they checked to see if they are observable from Earth with the upcoming Vera Rubin Observatory. They found that these objects are only observable periodically due to the Sun's glare. These observational windows mostly occur when the objects are near their closest approach to Earth.
The Vera Rubin Observatory will see first light in July 2025. Once it gets going, it will release a flood of data and discoveries and find more potentially hazardous objects, including those co-orbiting Venus. Image Credit: Rubin Observatory/NSF/AURA.
"The combination of elevation constraints and solar elongation limitations restricts our observations to specific periods throughout the year," the authors write. Solar elongation means the angular distance between one of these asteroids and the Sun, as measured from Earth's perspective.
The study shows how difficult it can be to detect these dangerous asteroids from Earth. One solution might be to send a spacecraft to Venus' orbit. "However, observations conducted from Venus' orbit, positioned facing away from the Sun, may enhance the detection of these bodies," the researchers explain. Several missions have been proposed, including to the Sun-Earth or Sun-Venus L1 or L2 halo orbit.
We know there are asteroids out there with considerable chances to strike Earth. Some of them are large enough to destroy entire cities. Even a relatively small asteroid 150 meters in diameter can strike Earth with a force equal to hundreds of megatons of TNT. That's thousands of times more potent than the atomic bombs dropped in World War 2. "Among these, low-e Venus co-orbitals pose a unique challenge, because of the difficulties in detecting and following these objects from Earth," the authors write in their conclusion.