The near-Earth asteroid Apophis will make an extremely close flyby of Earth on April 13, 2029, at a distance of approximately 38,000 km—an event that occurs on average only once every 7,500 years, offering an unprecedented opportunity for planetary science and planetary defense. However, significant uncertainties remain regarding the physical parameters of Apophis, such as its shape, spin state, internal structure, and material composition, and predictions of tidal effects—including potential orbital changes, spin acceleration, surface material displacement, and dust ejection during the flyby—are highly dependent on model assumptions. Although multiple nations have planned exploration missions, how to coordinate global observational resources within the limited time window, optimize rapid-response mission architectures, and translate scientific understanding into effective planetary defense strategies remain critical challenges to be addressed.
In a recent review published in Space: Science & Technology, the team led by Li Jianyang from the School of Atmospheric Sciences, Sun Yat-sen University, systematically synthesizes the scientific opportunities and planetary defense value of the 2029 Apophis flyby event. The study integrates radar observations, light-curve and spectral data to summarize the current state of knowledge about Apophis, and analyzes the expected effects of the Earth flyby from four dimensions: orbital and spin changes, surface material displacement, internal structural response, and dust ejection. Through mission trajectory design and launch window calculations, the study evaluates the feasibility and scientific return of various exploration modes, including rendezvous, flyby, sample return, and impactor experiments, and reviews the approved or planned exploration missions globally, such as OSIRIS-APEX and RAMSES, along with their payload configurations. The study points out that during the flyby, surface changes at the centimeter-to-decimeter scale, spin-period alterations on the order of hours, and potential dust ejection phenomena are expected to be detectable, providing direct evidence for the structural evolution of rubble-pile asteroids. Furthermore, the coordinated multi-national detection and observation activities will serve as the first global joint exercise without an actual threat under the framework of the International Asteroid Warning Network, accumulating engineering experience for rapid response to near-Earth asteroid threats. The study also emphasizes that the Apophis event is comparable in significance to the historic moments of the Voyager mission in 1977 and the multinational joint exploration of Halley's Comet in 1986, and will lay the foundation for the scientific exploration and resource utilization of asteroids that frequently fly by within the Earth–Moon system, holding substantial strategic value for the substantive advancement of planetary defense capabilities.
First, this paper focuses on the fundamental physical state of Apophis and the scientific significance of its flyby event. With a diameter of approximately 340 meters, Apophis is classified as a potentially hazardous asteroid. Since its discovery in 2004, the possible impact risk it poses has once attracted widespread attention. As illustrated in Fig. 1, Apophis will fly past Earth on April 13, 2029, at a distance of about 38,000 km—merely one-tenth of the Earth–Moon distance. Such a close flyby event is extremely rare; an asteroid of comparable size passing Earth at such a close distance occurs on average only once every 7,500 years. Fig. 2 presents a distribution diagram of near-Earth objects in terms of flyby distance and size, clearly demonstrating the uniqueness and scientific value of the Apophis flyby event. Current knowledge of Apophis is derived primarily from radar and light-curve observations: Table 1 summarizes its physical parameters, including a size of approximately 340 m, a non-principal-axis rotation state, a rotation period of about 30.6 hours, a density of about 1.95 g/cm³, and a porosity of approximately 55%, suggesting that it may possess a rubble-pile structure; Fig. 3 presents a shape model based on radar data, exhibiting an elongated, asymmetric, and slightly bifurcated morphology. Spectroscopic observations classify it as an Sq-type asteroid, similar to ordinary chondrites, implying that its physical properties are comparable to those of asteroid Itokawa.
Second, the paper provides a detailed analysis of the expected effects of the Earth flyby on Apophis, encompassing four aspects: orbital and spin changes, surface material displacement, internal structural response, and dust ejection. In terms of orbital and spin effects, Earth's gravity will transfer Apophis from an Aten-type orbit to an Apollo-type orbit; the orbital uncertainty is expected to increase dramatically from approximately 1 km before the flyby to 6,000 km one year after the flyby, and the spin period is projected to change by −7.6 to +14.4 hours. Regarding surface material displacement, numerical simulations indicate that although tidal forces will not cause structural disruption, local material movement may occur in about 1% of the surface area (with displacements not exceeding three times the maximum particle radius), potentially exposing fresh material and altering local spectral properties. In terms of internal structural response, by combining the shape model with spin-state variations, the mass distribution and center-of-mass location of Apophis can be retrieved. Fig. 4 illustrates the trajectory of Apophis as it traverses Earth's magnetosphere, passing sequentially through different space environment regions including the magnetosheath, magnetotail, and magnetosphere, providing an important reference for understanding the interaction between asteroids and the Earth's space environment. With respect to dust ejection, Apophis's extremely low gravity allows dust particles smaller than 50 μm to be ejected from its surface; after being captured by Earth's magnetosphere, these particles form high-speed dust streams that may produce observable electromagnetic disturbances on spacecraft.
Finally, the paper systematically reviews the scientific exploration opportunities, planetary defense implications, and prospects for international cooperation. In terms of exploration missions, Fig. 5 presents the distributions of C3 energy and flyby velocity for different launch windows, indicating that within specific windows, small launch vehicles can already enable Apophis exploration. Fig. 6 presents an example trajectory with a low flyby velocity of 1.88 km/s in January 2029. Currently approved or under-development missions include NASA's OSIRIS-APEX (a rendezvous mission arriving in June 2029), ESA's RAMSES (arriving before the flyby to monitor the entire process), and Japan's DESTINY+, among others; Chinese scientists have also proposed conceptual mission designs such as ARS and CROWN/Apophis. In terms of ground-based observations, Apophis will be resolved by multiple 10-meter-class telescopes during the flyby, and the under-construction China Fuyan radar is expected to achieve meter-level resolution and sub-centimeter surface deformation detection. At the planetary defense level, this event will serve as the first global joint exercise without an actual threat under the framework of the International Asteroid Warning Network, accumulating critical experience for rapid response to near-Earth asteroid threats. The establishment of an International Year of Planetary Defense will further promote public outreach and global cooperation, making the Apophis flyby a milestone event in the history of planetary science and planetary defense.
