Apophis source population and Earth encounter frequency of Apophis-like bodies

¹ Charles University, Faculty of Mathematics and Physics, Astronomical Institute, V Holešovičkách 2, 18000 Praha, Czech Republic
² Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
³ Aix-Marseille University, CNRS, CNES, LAM, Institut Origines, Marseille, France
⁴ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁵ Trans-Tasman Occultation Alliance (TTOA), Wellington, PO Box 3181, New Zealand

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 November 2025
Accepted: 23 January 2026

Abstract

Context. Asteroid (99942) Apophis will safely pass by the Earth in April 2029. This extraordinary event will be observable by the naked eye from Africa and western Europe.

Aims. We provide context for the Apophis 2029 Earth passage by analyzing possible source populations for bodies in Apophis-like orbits, in particular, the Flora family, which has a mineralogical composition corresponding to LL chondrite meteorites, similar to measurements of Apophis itself.

Methods. We estimated the specific encounter probability of the present-day orbit of Apophis by classical Öpik and N-body methods. We then performed orbital simulations of the Flora family, initializing bodies between 2.1 and 2.3 au, and tracked their evolution into the near-Earth object (NEO) space to compute their mean and peak encounter probabilities with Earth.

Results. Out of an estimated population of ~3380 NEOs larger than or equal to Apophis (≥420 m), 610 ± 140 are LL-like NEOs from Flora. Their mean encounter probability is p = 86 × 10⁻¹⁸ km⁻² yr⁻¹, corresponding to a once per 13 000 y frequency of encounters closer than 38 000 km. However, this does not apply to Apophis alone, for which the specific encounter probability is higher p′ = 1603 × 10⁻¹⁸ km⁻² yr⁻¹, but the frequency is lower, only once per 430 000 y, when we consider it as a single object. Our simulation of the Flora family over ~1 billion years indicates that Apophis-like bodies from Flora have orbits that are particularly persistent in nearEarth space. The temporal distribution of encounter probabilities exhibits peaks (up to >10⁴ in the same units) and the specific value for Apophis is not unusual (occurring ~70% of time). In other words, there is always at least one Apophis-like body among NEOs. We find that such persistence also creates favorable opportunities for temporary capture as Earth coorbitals and Apophis-like bodies are ultimately removed from the inner Solar System by approaching the Sun or by impact into one of the terrestrial planets, where the relative split between these outcomes is (45 ± 2)% and (50 ± 2)%. While our current knowledge of the Apophis orbit guarantees no threat from Apophis in the next few centuries, we cannot predict any specific outcome for Apophis in the coming thousands or millions of years. Evaluating this statistically over the long term, we find that objects in Apophis-like orbits have a (19 ± 2)% chance of Earth impact over their lifetime of ~30Myr.

Conclusions. Apophis appears to be a prototypical example for the population of hundred-meter bodies intersecting Earth’s orbit and impacting the Earth, making it a particularly worthwhile target for investigations, thus advancing our knowledge for planetary defence.