Asteroid Apophis’ 2029 Flyby Will Provide a Bonanza of Asteroid Science

If NASA and other space agencies don’t want us to freak out about asteroids colliding with Earth, why do they give them names like Apophis? It sounds apocalyptic.

Apophis was the ancient Egyptian god of Chaos. He was an evil serpent that dwelled in endless darkness, the enemy of light and truth. So when they informed us that an asteroid named Apophis was due for a close encounter with Earth in 2029, people were understandably anxious. After all, Earth’s previous dominant inhabitants were evicted by an asteroid.

We’ve been tracking Apophis, or 99942 Apophis as it’s known in space circles, since its discovery in 2004. At that time, it looked like there was a chance that Apophis’ would pass through a “gravitational keyhole” which would alter its trajectory. Some worried that it could strike Earth on subsequent rendezvous. But now we know that we’re safe for at least 100 years. Maybe by that time, we’ll have our planetary defence system operational.

Apophis’ next close approach in 2029 is not something to fear, but something to embrace. The event is an opportunity to study the rock, and to see how it responds to its encounter with Earth.

A new paper outlines what the encounter might be like for Apophis. The paper’s titled “APOPHIS — Effects of the 2029 Earth’s Encounter on the Surface and Nearby Dynamics.” It’s an accepted manuscript in the Monthly Notices of the Royal Astronomical Society. The first author is Giulia Valvano from the Sao Paulo State University in Brazil.

Nobody’s observed Apophis up close, but it has been studied. It has a diameter of about 370 metres (1,210 feet). It’s an S-type asteroid, so it has a siliceous or “stony” composition, rather than a carbonaceous composition. About 17% of asteroids are S-types. S-type asteroids like Apophis are dominant in the inner Solar System but are rare further out.

Apophis is likely a bilobed body, and thermal observations show that it’s also likely a rubble-pile asteroid, similar to Itokawa. The asteroid is a tumbling retrograde rotator too. During its 2029 approach, Apophis will be visible to the naked eye, if you know where to look.

Apophis has been observed in detail and scientists have constructed models of its motion, orbit, shape, density, composition, and surface characteristics. They’ve also modelled what might happen to the asteroid when it comes close to Earth in 2029. When Apophis does its 2029 flyby, it’s an opportunity to test and refine those models.

This figure from the study shows Apophis’ trajectory past Earth in 2029. The green line and circle represent the Moon’s orbit and the equatorial plane respectively. The dotted blue line is Apophis’ path below the equatorial, and the solid blue line is the path above the equatorial. Image Credit: Valvano et al 2021.

Apophis’ closest approach will be on April 13th, 2029. On that day the asteroid will approach Earth at a distance of about 38,000 km (23,600 mi). At that time it’ll be subjected to not only Earth’s gravity but also that of the Moon and the Sun. It’ll also be exposed to solar radiation pressure.

According to the authors of the paper, it’ll be so close that its axis of rotation will be altered by tidal forces, and its surface will likely be affected, too. Models show that Earth’s gravitational tug on Apophis will cause landslides on its surface and likely send some particles up from its surface. These particles were modelled with different parameters, and while some escape into a disk surrounding Apophis, most fell back to the surface.

When 2029 arrives, and Apophis with it, the scientific astronomy community will engage in a massive detailed observational campaign. So will amateurs. By 2029, some of the powerful telescopes that are under construction now will be operational. We have a lot to learn about asteroids, and when one comes this close it’s an opportunity that can’t be ignored. Asteroids will always pose a risk to Earth, and the better we understand them the better we can deal with them.

In their paper, the authors say that all of their analyses may contribute to the 2029 observation campaign. The 2029 campaign may also validate their results, strengthening scientific models of asteroids and improving our ability to mitigate the threat posed by any dangerous ones.

Evan Gough

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