Big relief: Once a potential threat, 'city-killer' asteroid 2024 YR4 now poses almost no risk to Earth

New information is pushing asteroid 2024 YR4 off the front pages. Initially, it had a 2.8% chance of hitting Earth in 2032. Now, the European Space Agency (ESA) has slashed that probability to a mere 0.001%.  

Discovered on December 27, 2024, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 2024 YR4 initially raised concerns. ATLAS, designed to spot smaller asteroids on short notice, flagged it as a potential threat. Early observations suggested an impact probability above 1%, triggering efforts to refine its trajectory.  

Though not massive — measuring between 40 and 90 meters (130 to 300 feet) across — 2024 YR4 was still a cause for concern. An asteroid of that size could cause significant damage in a populated area. At its highest recorded risk on February 18, the chance of impact reached 2.8%. The next day, observations from the European Southern Observatory’s Very Large Telescope cut that estimate in half.  

Understanding an asteroid’s path isn’t easy. Space is dynamic, and these objects are small, dark, and difficult to track. Even with some of the world’s best telescopes, 2024 YR4 was barely visible. Over two months, the ESA’s Near-Earth Object Coordination Center and other institutions gathered more data, refining the asteroid’s orbit and significantly reducing its projected risk.  

2024 YR4 follows an elliptical orbit around the Sun, crossing Earth’s path every four years. It last came close to Earth on Christmas 2024, just two days before its discovery, passing within 830,000 km. In December 2028, it will come no closer than 8 million km. Unfortunately, from April until its next approach, ground-based telescopes won’t be able to track it.  

One challenge in predicting asteroid paths is the influence of gravity. When an asteroid like 2024 YR4 passes near Earth or the Moon, their gravitational forces can alter its trajectory — a phenomenon known as a "gravitational keyhole." These unpredictable shifts complicate long-term impact predictions.  

Each asteroid approach sparks debate about planetary defense. Should we destroy it with a nuclear explosion? Use a kinetic impactor to alter its course? Evacuate impact zones? NASA’s Double Asteroid Redirection Test (DART) proved that a small, well-aimed kinetic impactor can change an asteroid’s path—without the unpredictable risks of nuclear explosions. Future strategies may involve keeping kinetic impactors on standby at key locations, such as a Lagrange point or the Moon, ready to deploy when needed.  

For now, the priority remains finding and tracking potentially hazardous asteroids. The upcoming Vera Rubin Observatory is expected to significantly improve asteroid detection and orbital predictions. Unlike the 10-15 km asteroid that wiped out the dinosaurs, the largest space rocks tend to stay in the asteroid belt. It’s the smaller, decameter-sized objects — like 2024 YR4 — that pose the most immediate risk.