On April 13, 2029, a chunk of rock roughly the size of the Eiffel Tower will thread the gap between Earth and the ring of communications satellites that orbit our planet. The asteroid, named 99942 Apophis, will pass within about 19,800 miles of the surface, closer than any object of its size has ever been observed. NASA says there is zero chance of impact, but the agency is preparing for the encounter as both a scientific windfall and a real-world stress test of its planetary defense capabilities.
Why Apophis is famous
Apophis earned its notoriety almost immediately after its discovery in June 2004. Within months, preliminary orbit calculations gave it a 2.7 percent chance of striking Earth in 2029, the highest impact probability ever assigned to a known asteroid and enough to push it to Level 4 on the Torino Impact Hazard Scale. That brief window of alarm galvanized the planetary science community. Additional observations quickly shrank the uncertainty, and by 2021, radar measurements from NASA’s Goldstone Deep Space Communications Complex eliminated any remaining impact risk for at least the next century.
But the close pass itself never went away. Apophis will still skim Earth at a distance of roughly 31,600 km from the planet’s center, or about 19,800 miles from the surface, according to orbit analysis from NASA’s Jet Propulsion Laboratory. JPL’s radar-based modeling sets a firm minimum approach of 29,470 km (18,300 miles) from the surface. For perspective, geostationary satellites orbit at about 22,236 miles out. Apophis will pass inside that shell.
What NASA is sending to meet it
NASA already has a spacecraft en route. OSIRIS-REx, which collected and delivered a sample from asteroid Bennu in September 2023, was redirected toward Apophis immediately after releasing its return capsule over the Utah desert. The probe fired its engines for a diversion burn and was rechristened OSIRIS-APEX (Origins, Spectral Interpretation, Resource Identification, and Security – Apophis Explorer) to mark its new assignment.
OSIRIS-APEX is designed to arrive at Apophis shortly after the 2029 flyby and study what Earth’s gravity does to the asteroid at close range. During the encounter, tidal forces from our planet are expected to tug on Apophis’s loosely packed, rubble-pile body. Scientists want to know whether that gravitational squeeze will trigger landslides, expose fresh subsurface material, or subtly alter the asteroid’s spin and shape. OSIRIS-APEX will map the surface in detail, measure changes to the regolith, and track shifts in the asteroid’s rotation and orbit, data that could reshape how researchers model the long-term paths of near-Earth objects.
The mission builds on momentum from NASA’s DART experiment in September 2022, which deliberately slammed a spacecraft into the small asteroid Dimorphos and successfully changed its orbit. DART proved that kinetic impact could work as a deflection technique. OSIRIS-APEX tackles a different question: what happens to an asteroid’s structure and trajectory when a planet’s gravity, rather than a spacecraft, does the pushing.
A planetary defense dry run
Federal agencies are treating the Apophis flyby as more than a science mission. NASA has already used the asteroid as a simulated threat in planetary defense tabletop exercises, running through the full detection-to-assessment pipeline that would activate if a genuinely dangerous object were discovered. Those drills practice astrometry, orbit determination, impact probability calculations, and interagency coordination, the same operational steps being applied to Apophis tracking in real time.
The U.S. Geological Survey is also involved, framing the flyby within its broader work on cascading natural hazards and preparedness. While the USGS relies on NASA’s orbital data rather than generating its own, the agency contributes to characterization and impact-consequence modeling that would be critical in an actual threat scenario.
Together, these efforts amount to a live rehearsal. Every radar ping, every orbit refinement, and every coordination protocol exercised between now and April 2029 strengthens the infrastructure that would need to function flawlessly if a future asteroid were found on a collision course.
What scientists still want to learn
Several open questions will only be answered by the flyby itself. Chief among them: will the 2029 encounter alter Apophis’s orbit enough to change its collision probability with Earth in future centuries? JPL confirms the flyby will shift the asteroid’s path, and ground-based radar will help quantify the change, but a specific post-flyby impact probability for later dates has not yet been published. That calculation will depend on OSIRIS-APEX measurements, follow-up radar tracking, and extensive modeling campaigns that will unfold after the encounter.
There is also genuine suspense around how visibly the asteroid’s surface will change. Simulations suggest tidal forces could rearrange boulders, open fresh fractures, or loft small particles from the surface. But the threshold at which those effects become dramatic, rather than subtle, remains uncertain. Only direct before-and-after imaging will settle the question, and OSIRIS-APEX is positioned to deliver exactly that comparison.
As of spring 2026, NASA’s Apophis overview confirms that tracking and characterization work continues. Updated radar observations from Goldstone and other facilities are expected to further tighten the orbital uncertainty window over the next three years, producing revised closest-approach distances that will remain well within the safe range already established.
How to watch it happen
For the public, the most striking detail may be the simplest: Apophis is expected to be visible to the naked eye. During its closest approach on the evening of April 13, 2029, observers across parts of Europe, Africa, and western Asia should be able to spot it as a fast-moving point of light crossing the sky without a telescope. Exact visibility will depend on local weather and final trajectory refinements, but the opportunity is unprecedented. No known asteroid this large has ever passed this close while humanity had the tools to watch, measure, and send a spacecraft to greet it.
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*This article was researched with the help of AI, with human editors creating the final content.
