by 
A recent study led by Ronald-Louis Ballouz from Johns Hopkins University Applied Physics Laboratory suggests that the asteroid 99942 Apophis may experience tremors—similar to earthquakes—due to Earth’s gravitational pull during its close flyby on April 13, 2029, with simulations indicating significant surface changes. Apophis, approximately 340 meters in size, will pass within about 32,000 kilometers of Earth, closer than many satellites in orbit.
When Apophis was discovered on June 19, 2004, by Roy Tucker, David Tholen, and Fabrizio Bernardi during the University of Hawaii Asteroid Survey (UHAS), initial calculations indicated that it could approach Earth with a risk of collision, especially during its pass in 2029. It didn’t help that it is named after the Egyptian god of darkness and chaos.
At the time of its discovery, Apophis was considered the potentially “most dangerous” asteroid for Earth due to a possible impact that could kill millions of people. However, additional research allowed scientists to rule out the possibility of impact in 2029 and 2036, and calculations now show no impact risk for at least the next 100 years.
Despite the minimal risk of collision, the close approach of Apophis has captured the interest of the scientific community. The gravitational interaction with Earth could induce two main effects on Apophis: gravitational tidal forces could cause it to experience tremors, much like an earthquake, and a possible change in its rotation may refresh its surface, potentially exposing fresh layers beneath its weathered surface. Scientists call this shaking “astroquakes,” which could be triggered by Earth’s gravity during the close passage, potentially causing landslides and tremors on Apophis’s surface.
Ballouz and his team developed computational simulations to anticipate these phenomena. They found that Earth’s gravity can plausibly have a detectable effect on Apophis, inducing discrete seismic events driven by short-term tides. The astroquakes are expected to start about an hour before Apophis reaches its closest point to Earth and continue for some time after, with vibrations persisting briefly post-flyby. These tremors could shake Apophis enough to dislodge surface materials and even eject small rocks into space, potentially transforming the asteroid’s appearance.
This interaction between Apophis and Earth’s gravity will allow scientists to obtain information about the composition and internal structure of Apophis. Detailed information about Apophis’s spin and rotation state is fundamental for astronomers, as it will allow them to predict its behavior more accurately when it approaches Earth. Understanding how these asteroids can change due to gravitational forces is crucial for developing planetary defense strategies.
NASA’s OSIRIS-APEX mission is preparing to meet Apophis in 2029 to better understand these potential changes, allowing scientists to analyze in real time the effects of the gravitational interaction with Earth. Reusing the spacecraft from the OSIRIS-REx mission, which previously collected samples from the asteroid Bennu, OSIRIS-APEX will observe Apophis for 18 months after its approach. This mission could reveal why certain asteroids lack weathered appearances after planetary encounters, expanding our knowledge about space weathering.
Ballouz believes the findings may explain why some asteroids have surfaces younger than their time tumbling through space would indicate. Near-Earth asteroids like Apophis tend to have two properties: their surfaces are loose and rubbly, and they show less space weathering than asteroids that don’t experience planetary flybys. Recent research suggests that asteroids passing near planets tend to show less eroded surfaces than those remaining in deep space, which may provide valuable insights into how asteroids change when they come close to planets.