Once-in-a-millennium Apophis flyby in 2029 will be visible to billions
On Sunday, April 13, 2029, an asteroid the size of the Empire State Building will pass within 31,000 kilometers (19,000 miles) of Earth, closer than geosynchronous satellites and roughly one-tenth the distance to the Moon. At its brightest, the asteroid 99942 Apophis will be visible to the naked eye for an estimated 3.9 billion people across Africa, Asia, eastern South America, and parts of Europe.
The flyby is the closest predicted passage of an object this large in recorded human history. Events of this scale occur roughly once every 5,000 to 10,000 years.
“There has never before been a time in human history that we could predict an asteroid visibly passing by the Earth,” Richard Binzel, a planetary scientist at MIT, told Space.com.
At closest approach, at approximately 21:45 UTC on April 13, the asteroid will be over the North Atlantic Ocean, traveling at roughly one full-Moon width per minute across the sky. It will be visible to the naked eye from dark-sky locations for roughly seven hours total, starting over Australia at approximately 15:00 UTC and ending over the North Atlantic around 22:00 UTC.
At its brightest, Apophis will reach magnitude 3, roughly one-third as bright as Polaris. In total, an estimated 90% of the world’s population, or roughly 7.6 billion people, will be within the viewing area for at least some portion of the flyby, though local weather and light pollution will determine how many actually see it.
Zero impact risk
Discovered in 2004 by astronomers at Kitt Peak National Observatory, Apophis was initially given a 1-in-37 chance of impacting Earth in 2029, the highest probability ever recorded for a large asteroid at the time. Follow-up radar observations using the Goldstone and Green Bank telescopes in March 2021 eliminated any risk of impact for at least the next 100 years.
“Apophis will safely pass the Earth,” Binzel emphasized.
If it were to hit, the consequences would be devastating. Apophis measures 340 to 375 meters (1,115 to 1,230 feet) across, and an impact would release roughly 1,000 megatons of energy, roughly 20 times the yield of the largest nuclear bomb ever detonated.
A natural planetary defense experiment
The flyby is not just a spectacle. Earth’s gravity will significantly alter Apophis’s orbit, moving it from the Aten group of Earth-crossing asteroids to the Apollo group. The gravitational forces may also alter the asteroid’s spin rate and could trigger quakes or landslides, exposing pristine subsurface material.
“We simply do not know what is going to happen,” Binzel said. The flyby is a natural experiment in how large asteroids respond to close gravitational encounters, data that is critical for designing future planetary defense deflection strategies.
Two spacecraft will be waiting
NASA’s OSIRIS-APEX mission, formerly the OSIRIS-REx spacecraft that collected samples from asteroid Bennu, will rendezvous with Apophis in June 2029, a few weeks after the flyby. It will map the asteroid’s surface, measure its chemical composition down to millimeter scale, and perform a maneuver in which it fires thrusters near the surface to kick up loose rocks, exposing subsurface material.
ESA’s RAMSES (Rapid Apophis Mission for Space Safety) will arrive even earlier, reaching the asteroid in February 2029, before the close approach. Approved for launch in spring 2028 aboard a JAXA H3 rocket, RAMSES will carry two CubeSats and will observe how Earth’s gravity alters the asteroid in real time, measuring changes in shape, spin, orientation, and orbital path before, during, and after the flyby.
The 2029 encounter is the first time humanity has both predicted and prepared to observe an asteroid flyby of this magnitude. Scientists expect it to yield insights into asteroid structure, planetary defense, and the early formation of the solar system.
Sources: Space.com (Sharmila Kuthunur), NASA Science, The Planetary Society, ESA, MIT