On April 13, 2029, the asteroid Apophis, approximately 350 meters in diameter, will closely approach the Earth, bringing it within the orbits of the geostationary satellites. The probability of collision with Earth is zero. However, this is valid in the hypothesis that Apophis is not hit by other asteroids that it encounters on its path between now and 2029. Could it happen? The answer is theoretically yes, but it won’t happen, and in this news, we will see why. The Italian version was published on Media INAF.

On June 19, 2004, almost 20 years ago, Roy Tucker, David Tholen and Fabrizio Bernardi from the Kitt Peak Observatory in Arizona discovered the near-Earth asteroid 2004 MN4, now known as (99942) Apophis. For several months after its discovery, this asteroid was the minor body with the highest probability of impacting the Earth, which collapsed to zero as soon as a sufficient number of astrometric observations became available. From a dynamic point of view, the asteroid moves on an orbit inclined by a few degrees on the ecliptic plane with a semi-major axis of 0.9227 au and an eccentricity of 0.1914, which is traversed in approximately 324 days. This means that at perihelion, the asteroid reaches up to 0.746 au from the Sun, while at aphelion, it reaches 1.099 au. As can be seen, it is an asteroid on an Aten-type orbit; as such, it spends most of its time inside the Earth’s orbit, and it is for this reason that astrometric observations are difficult: mostly the asteroid is too close to the Sun to be easily observed.

From Celestial Mechanics, we know that there is no risk that Apophis could hit our planet for at least a century, even if on the evening of April 13, 2029, at 21:46 UTC, the asteroid will pass approximately 37,400 km from the centre of the Earth. Considering the considerable dimensions, approximately 350 meters in diameter, and the short distance, Apophis can be seen in the sky, with the naked eye, as bright as a third magnitude star and in noticeable movement on the celestial sphere: it will be the first asteroid to be visible to the naked eye.

While waiting for Apophis’ flyby, we can reflect on the fact that the phrase “Apophis will not collide with the Earth for at least a century” is valid only in the hypothesis – implied – that there is no “interference” from other minor bodies in the Solar System. Usually, when propagating the position of an asteroid forward in time, both the gravity of the Sun and that of planets and larger asteroids and the effect of solar radiation are considered. Still, numerical simulations do not include all the known minor bodies. The latter could also collide with our asteroid and change its orbit, as happened to Dimorphos when NASA’s DART probe hit it. Could something like this happen to Apophis? Could a collision with another asteroid put Apophis into an Earth-impact orbit? This is a very unlikely event, but considering the considerable size of Apophis and the short distance of the flyby from Earth, it is better to be sure. It is not just a problem of direct collision of Apophis with another asteroid; even close passes could be risky due to the possible presence of satellites or boulders due to a previous collision or surface activity of the asteroid.

This is the question that astronomer Paul Wiegert of the Department of Physics and Astronomy of the University of Western Ontario (Canada) asked himself and, together with his colleague Ben Hyatt (University of Waterloo, Canada), tried to obtain an answer. The problem, in principle, is easy: take all the known minor bodies (about 1.3 million asteroids and comets) and propagate their orbit forward in time to see if, from now to the flyby of 2029, some could be in close range with Apophis. From a practical point of view, something like this would require a lot of computational time, so we must first reduce the number of potential asteroids that can interfere with Apophis. Wiegert and Hyatt took into consideration two catalogues of minor bodies: The Small-Body Database, maintained by NASA’s JET Propulsion Laboratory (with 1.3 million asteroids and comets) and the NeoDyS-2 database, maintained by SpaceDyS of Pisa for ESA account (with approximately 32,000 near-Earth objects).

From these catalogues, asteroids and comets that cannot intersect the orbit of Apophis because they have a perihelion larger than our aphelion or vice versa were immediately eliminated. This first screening reduced the JPL catalogue to around 30,000 objects, which were further analysed, together with all those of NeoDyS-2, calculating their MOID (Minimum Orbit Intersection Distance) with the orbit of Apophis. The MOID, the minimum distance that two bodies moving on different orbits can reach, has been set at an upper limit of 0.001 au: all minor bodies with MOID higher than this limit (equal to approximately 150,000 km) have been eliminated. In this step, the orbital elements of each object were kept constant, i.e. the gravitational perturbations of the planets were neglected. Thus, 376 objects remained from the JPL catalogue and 396 from the NeoDyS-2 catalogue, with 322 commons to the two lists.

At this point, the orbits of the remaining bodies were numerically propagated into the future (this time taking into account planetary gravitational perturbations, but without including the Yarkovsky effect) using 2000 clones for each asteroid generated by the covariance matrix to take into account the orbitals uncertainties for each asteroid. From the numerical simulations, only asteroids with at least one clone with a MOID less than 10,000 km compared to the orbit of Apophis and with a flight time difference of less than 12 hours were extracted. The flight time is the time from the current position to the MOID; the collision occurs when the difference between the Apophis and asteroid values is zero.

The only asteroid that has as many as 1000 clones meeting these conditions in both databases was the near-Earth (4544) Xanthus for December 25, 2026, with a MOID of 9600 km and a flight time difference of -4 hours: Xanthus, which has an estimated diameter of 1.3 km, will pass to the MOID 4 hours after Apophis and the minimum distance between the two asteroids will be over 500,000 km. There are no known satellites for Xanthus; even if there were, they would be too close to the main body to collide with Apophis. Furthermore, it is an asteroid that has never been active. Even the gravitational perturbation it will exert on Apophis in its passage to minimum distance will have negligible effects. The calculations resulted in other asteroids that meet the above criteria, such as 2009 JG2, 2016 FB12, 2022 KN3 or 2016 CL18, but the collision probabilities are in all cases zero. Eventually, these asteroids can be observed to see if they are active asteroids that may have strewn the orbit with boulders of various sizes that could hit Apophis, but the probability is extremely low. Until 2029, we can rest assured that Apophis will not deviate from its orbit due to collisions with other known minor bodies.

Of course, we do not know all the minor bodies in the Solar System. For example, it is estimated that there are around a couple of billion small near-Earth asteroids of a few meters in diameter. These asteroids hit the Earth and disintegrate in the atmosphere on average every two weeks. They are sometimes discovered a few hours before hitting our planet, as recently happened with 2023 CX1 and 2024 BX1. Apophis could collide with a still unknown small asteroid, and this could change the orbit just enough for it to collide with Earth in 2029. How likely is such a scenario? Considering that the ratio between the “cross-section” of the Earth and Apophis is about 1 billion, we can expect a collision of Apophis with small near-Earth asteroids about once every 10 million years. Even the unexpected is very unlikely; as far as Apophis is concerned, we can rest assured.

For those wishing to learn more, I recommend reading the original article: Paul Wiegert, Ben Hyatt, Encounter circumstances of asteroid 99942 Apophis with the catalogue of known asteroids, accepted in the Planetary Science Journal.