Apophis: A Dance Before Its Date with Earth express
A spinning top launched into weightlessness doesn't just rotate around its axis: it also wobbles, like a drunken sailor. That's exactly how the asteroid Apophis behaves: it doesn't spin neatly but tumbles through space. On April 13, 2029, this celestial wanderer will whiz past at about 38,000 kilometers from Earth — closer than communications satellites. Our planet's gravity will yank it like a strong gust of wind tugging a weather vane, forcing it into a new spin.
To predict how Apophis's dance will change after the encounter, we need to know its current motions precisely. Astronomers took old photographs of the asteroid from 2012–2013 and 2020–2021 and applied the light-curve method (graphs of brightness variation). These brightness fluctuations, like fingerprints, reveal how the object tumbles. This yielded several rotation models, but none could be singled out — the data weren't enough. It's like trying to guess how a coin is spinning when you see it through frosted glass.
The puzzle is that all viable models show roughly the same orientation of the asteroid before the encounter. The reason is a lucky coincidence: the interval between the 2012–2013 and 2020–2021 observations (8 years) equals the gap from 2021 to 2029. Nature seems to have given us a synchronization: the asteroid is now facing the same way it did 8 years ago. But after the April encounter, uncertainty will spike: Earth's gravitational pull could set Apophis spinning in any of several directions. Fortunately, there's good news: most likely, the asteroid will keep rotating around its shortest axis, like a figure skater pulling in their arms.
Original abstract
as written by the authors arXiv:2604.24566
On April 13, 2029, the asteroid Apophis will pass near Earth at a geocentric distance of about 38,000 km. Numerical models have suggested that the post-encounter spin state will critically depend on the orientation of Apophis during the flyby. We aim to determine the spin state of Apophis from its photometric observations collected during two apparitions in 2012-2013 and 2020-2021. This will enable us to accurately predict the pre-encounter rotation state and, by accounting for Earth's gravitational torque, predict a range of possible post-encounter states. We used the light curve inversion method for tumbling asteroids to reconstruct the spin state of Apophis and its convex shape model. The result is adopted as the initial condition of a numerical model describing Apophis's future rotation state. The data from the two apparitions are insufficient to determine Apophis's rotation and precession periods uniquely. The formally best-fit solution is 27.374 +/- 0.001 h for the precession period and 262.2 +/- 0.1 h for the rotation period, but at least two other combinations of the periods provide a similarly good fit to the available data. All the currently acceptable models result in approximately the same pre-encounter orientation of Apophis in early 2029. This is because the accurate photometric data were collected during two apparitions separated by 8 years, which is the same interval as from 2021 to 2029. Although the close encounter with Earth in April 2029 hugely increases the post-encounter uncertainty of Apophis's spin state, the short-axis spin mode will be preserved with a high likelihood. Additional observations taken in 2027 and 2028 will break the ambiguity in Apophis's pre-encounter spin solution and allow us to get a more accurate post-encounter spin state prediction
Asteroid Apophis is named after the ancient Egyptian serpent of destruction, who tries to swallow the sun's barge every night. But unlike the myth, the real Apophis poses no threat to Earth for at least the next hundred years.
In Arthur C. Clarke's novel Rendezvous with Rama, humanity also tracks a cosmic wanderer whose rotation and trajectory hold a mystery. Apophis, like Rama, makes us gaze at the sky with awe and curiosity.