Apophis flyby in 2029: Super-close approach to Earth will forever change the orbit of the 'god of chaos' asteroid, scientists say

Illustration of ESA's RAMSES spacecraft approaching the potentially hazardous asteroid Apophis before its close approach to Earth in 2029. (Image credit: ESA Science Office)

When the potentially hazardous asteroid 99942 Apophis flies past Earth on April 13, 2029, more than 2 billion people in Africa and Western Europe will be able to watch it pass across the night sky. On a clear day, the asteroid will appear as a faint star – about as bright as the stars of the Big Dipper, and easily visible to the naked eye – gliding past us.

The Apophis flyby will be “the first time in cosmic history that a potentially hazardous asteroid will be visible to the naked eye,” Richard Binzel, a professor of planetary science at the Massachusetts Institute of Technology, said Monday (Sept. 8) during a keynote address at the European Planetary Science Congress in Helsinki, Finland. Astronomers estimate that a close approach by an asteroid of this size — 1,140 feet (340 meters) in diameter, about the height of the Eiffel Tower — happens only once every 7,500 years.

For the public, it will be a dazzling, once-in-a-lifetime spectacle. For scientists, it promises something even rarer: a natural experiment unique in a millennium, allowing them to watch in real time how Earth’s gravity is changing the shape of a huge asteroid. “We don’t know,” Binzel said, “and we won’t know until we look.”

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Binzel, a pioneer in asteroid hazard research and creator of the Torino Asteroid and Comet Impact Hazard Scale, emphasized one important point: “If you take nothing else away from this talk, I want you to take three things away,” he said during his talk. “Apophis will safely miss the Earth; Apophis will safely miss the Earth; Apophis will safely miss the Earth.”

But when Apophis was first discovered in 2004, the picture was far less certain. Initial calculations suggested a 2.7% chance of an impact on April 13, 2029, which would have placed it at level 4 on the Torino scale, the highest rating ever given to a near-Earth object. Scientists named the asteroid 99942 Apophis after the Egyptian god of the underworld, giving it the nickname “the chaos asteroid.”

Over the next two decades, continuous tracking and radar observations narrowed Apophis’s orbit from hundreds of miles of uncertainty to just a few. By 2021, Apophis had officially been removed from all risk lists, with scientists estimating that it posed no threat for at least the next 100 years. However, last September, a study noted that there was still a tiny chance that an unknown asteroid could nudge it onto a collision course before its close flyby of Earth in 2029. The odds are more than one in a billion, and while scientists won’t be able to completely rule out that scenario for the next three years, astronomers remain confident that Apophis poses no threat for the next century.

“A lot of people have done a lot of work to make sure that we can say with complete confidence that Apophis will safely fly by Earth – there is no doubt about that,” Binzel said.

“The Earth won't care, but Apophis won't care.”

While Earth itself is unlikely to notice the impact, Apophis will not leave the asteroid untouched. Passing just over 18,000 miles above the planet’s surface — closer than geostationary satellites — its Aten-class orbit, which lies mostly inside Earth’s path around the sun and is therefore often hidden in our star’s glare, will change shape to a wider Apollo-class trajectory. Its spin may also change, potentially sending the asteroid into another tumble, Binzel said.

“Earth won't care, but Apophis won't care because its orbit will change,” he said. “It's all about the physics of the collision.”

An illustration of a potentially hazardous asteroid approaching Earth.

To capture these changes firsthand, NASA has repurposed its OSIRIS-REx spacecraft, which recently completed a mission to asteroid Bennu, for a new role: OSIRIS-APEX. The probe will rendezvous with Apophis before the flyby, map its surface, track its rotation, and measure how Earth’s gravity affects the asteroid during its close flyby. Among the most tantalizing goals, Binzel says, is the ability to measure seismic vibrations inside Apophis.

“In 60 years of planetary science, we have measured seismicity on only two objects: the Moon and Mars,” he said. “This would open the door to another leap in seismic measurements and interpretation of subsurface properties.”

The jump could be made through the Rapid Apophis Mission for Space Safety (RAMSES) programme. The European Space Agency (ESA) mission, if approved by the ESA Council of Ministers in November, would launch in spring 2028 and reach the asteroid by February 2029. The mission’s goal is to observe Apophis before, during and after its flyby of Earth, Monica Lazzarin, a professor of physics and astronomy at the University of Padova in Italy and a member of the RAMSES scientific team, told the conference.

During its encounter with the asteroid on April 12-14, 2029, RAMSES will be just 3 miles (5 kilometers) away and will map Apophis' orbit, look for tidal-lifted dust clouds, and possibly deploy a small satellite called a CubeSat that will touch its surface and detect seismic waves, Lazzarin said.