Orbital Mechanics
What Is a Gravitational Keyhole?
During a very close flyby, Earth's gravity can slightly change an asteroid's orbit. In rare cases, if the asteroid passes through an extremely small region of space called a gravitational keyhole, that deflection could place it on a trajectory that returns years later for an impact.
Apophis, Earth and the 2029 Flyby
ESA visualization of asteroid 99942 Apophis passing near Earth.
In this ESA animation, the small blue sphere represents Earth, while the red circle is a visual representation of the so-called b-plane (or target plane), a mathematical reference plane or the critical passage region used by astronomers to analyze close asteroid encounters and identify potential gravitational keyholes.
What is the red circle around Earth?
The red circle is not Earth's surface and it is not the atmosphere. It is a visual reference region used to show the close-approach geometry around Earth. In real orbital analysis, scientists use a mathematical target plane called the b-plane to study exactly where the asteroid passes during the encounter.
A gravitational keyhole is a very small region within that close-approach geometry. If an asteroid were to pass through such a keyhole, Earth's gravity could bend its orbit in just the right way to make it return on a later impact trajectory.
For Apophis, one of the historically studied scenarios involved a possible return impact in 2036, after the 2029 close approach. Later observations ruled out that scenario.
In simple terms: the 2029 flyby acts like a gravitational steering event. A tiny difference in the asteroid's path near Earth could, in theory, lead to a very different orbit afterward.
Today, modern orbital calculations confirm that Apophis will safely miss Earth in 2029 and will not impact Earth for at least the next century.
2029 flyby
~32,000 km
Approximate distance above Earth's surface.
Keyhole concept
Tiny region
A narrow gravitational corridor that could alter a future orbit.
Current risk
Ruled out
No known impact risk for at least the next century.