“It’s all about the encounter physics.” With those words, MIT Professor of Planetary Sciences Richard Binzel emphasized the record-book quality of what will happen on April 13, 2029, when asteroid 99942 Apophis passes within slightly more than 18,600 miles of Earth closer than most geostationary satellites. For the first time in recorded history, a potentially hazardous asteroid of this magnitude will appear visible to the naked eye, a faint but persistent speck of light moving across the skies of Africa and Western Europe, as bright as the stars in the Big Dipper.
The 1,100-foot-long, cigar-shaped S-type asteroid about as tall as the Eiffel Tower won’t hit Earth. Binzel reiterated the guarantee three times during his keynote at the Europlanet Science Congress: Apophis will safely pass the Earth; Apophis will safely pass the Earth; Apophis will safely pass the Earth. But while the planet will be indifferent, Apophis won’t. Earth’s gravitational field will elongate and compress the asteroid, quietly changing its orbit, altering its status from an Aten to an Apollo object, and perhaps tipping its rotation into a novel tumbling mode.
The tidal forces at work are more than an intellectual curiosity. As the near side of Apophis is pulled tighter toward Earth than the far side, the induced stresses may cause seismic vibrations, landslides, and surface displacements. Such impacts have been hypothesized but never seen straight out on an asteroid during a close planetary encounter. The 2029 flyby constitutes a natural experiment of unusually high accuracy: astronomers calculate that an asteroid this size comes this close only once between 5,000 and 10,000 years.
To photograph the change in detail, two spacecraft will be in place. NASA’s OSIRIS-APEX, a modified version of the OSIRIS-REx spacecraft, which brought samples from asteroid Bennu in 2023, will arrive just after the flyby. It will pull within 5 meters of Apophis’s surface and fire its thrusters downward, catapulting dust and gravel up to expose underlying material. Armed with the Canadian-made OSIRIS Laser Altimeter, which can make billions of accurate range measurements, OSIRIS-APEX will chart the asteroid in three dimensions, monitor changes in its rotation, and study its chemical makeup.
ESA’s planned Rapid Apophis Mission for Space Safety (RAMSES), if funded later in 2025, would depart in April 2028 and arrive at Apophis by February 2029. Flying as close as 3 miles at the April 12–14 flyby, RAMSES would make an extensive before-and-after mapping, measuring the asteroid’s shape, surface topography, and gravitational field. The mission idea entails launching CubeSats with seismometers to measure tidal stress vibrations an ability that would be able to unveil the asteroid’s cohesion, porosity, and internal structure.
The scientific return is more than curiosity. Knowing how the orbit, spin, and surface of a stony asteroid react to gravitational tides will make planetary defense models more accurate. In an actual threat situation, this information would guide the decision on whether to try a kinetic impact, as with NASA’s DART mission, or some other deflection method. “Apophis is not a planetary defense crisis,” said Tom Statler, a planetary scientist at NASA Headquarters. It is an opportunity, and an historic one.
The chance is enhanced through the infrequency of the encounter. Apophis, an early solar system remnant dating back around 4.6 billion years ago in the inner asteroid belt, has been nudged into its present Earth-crossing path by series of gravitational pushes from Jupiter. Its 2029 flyby will increase its orbital period from 0.9 to 1.2 years, moving it into a broader solar orbit. Earth-based telescope, radar array, and spacecraft observations will all come together to document every change that can be measured.
For planetary scientists, the flyby is essentially a stress test of a large, old object one that may reveal new material from below its weathered surface and provide a direct glimpse of how rubble-pile asteroids respond to extreme tidal forces. For the general public, it is a cosmic event that will need no telescope, just clear skies and an eastern view. As Patrick Michel of the Observatoire de la Côte d’Azur described it, “For the first time ever, nature is bringing one to us and conducting the experiment itself. All we need to do is watch.”
