Paul Wiegert
Dept. of Physics and Astronomy, The University of Western Ontario, London ON CANADA
Ben Hyatt
Dept of Physics and Astronomy, The University of Waterloo, Waterloo ON CANADA
4 March 2024
Asteroid 99942 Apophis will pass near the Earth in April 2029. Expected to miss our planet by a safe margin, that could change if Apophis' path was perturbed by a collision with another asteroid in the interim. Though the statistical chance of such a collision is minuscule, the high risk associated with this 340 meter object motivated us to examine even this very unlikely scenario.
In a paper accepted by the Planetary Science Journal , we identify encounters between known asteroids and Apophis up to April 2029. An open-access version of the paper is available on Arxiv or here.
We show that Apophis will encounter the 1300 meter diameter asteroid 4544 Xanthus in December 2026. Their Minimum Orbit Intersection Distance (MOID) is less than 10,000 kilometers with Apophis passing that closest point just four hours after Xanthus. Though a direct collision is ruled out, the encounter is close enough -the asteroids will get within roughly 500,000 km of each other- that material accompanying Xanthus (if any exists) could strike Apophis. We also identify other asteroid encounters that deserve monitoring.
The likelihood of an asteroid (or any material from an asteroid) colliding with and deflecting Apophis is extremely low. The most likely outcome of careful monitoring of the encounters between Apophis and the asteroids and comets discussed in this work is that no impact will be observed. The small probability of collision is however counter-balanced by disproportionately large consequences. Because of the hazard associated with even a small perturbation to this Earth-threatening asteroid, there is ample motivation to determine the risk as precisely as possible.
Video illustrations of a sample of the encounter circumstances
If we could watch Apophis as it travels along and passes by other asteroids, what would the closest encounters look like? These animations show us what we would see if we could watch them close up, and in particular illustrate different possibilities associated with the uncertainty in some of the asteroid orbits. Click on the illustration below to see an animation of the encounter circumstances on YouTube.
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What do we mean "uncertainty"? Simply that small or distant asteroids are often too faint to be seen by even our best telescopes. Many of the asteroids we have discovered were seen as they passed through the Earth's broad neighbourhood, but then become too faint to see when they continued on their way. But they are still out there. And our estimates of where they are during these times when we cannot see them has some uncertainty in it, because we have to extrapolate from what we could measure when we could see them. As a result, exactly how close these asteroids could pass to Apophis is not always precisely known.