Until the 2010s, there was a remote possibility that the Earth would be struck by Apophis, a NEO (Near-Earth Object) asteroid discovered in 2004 by planetologist David Tholen of the University of Hawaii’s IfA. Note that Apophis is also referenced in the catalogs under the NEO code 99942 and 2004 NM4. Apophis will already stress us somewhat in 2029, when it will pass within 31,000 km of the Earth’s surface, that is to say lower than the orbit of most communications satellites. But it will come back in 2036 then in 2068.

Before considering a disaster scenario, let’s see what are the probabilities of collision during the closest passages of Apophis to the Earth.

Predictions for 2029 and 2036

The company Astrium of the European consortium EADS had proposed in 2013 to launch a space probe which was to meet Apophis when it was less than 66 million kilometers from Earth. But this project was abandoned. The same year the CNES proposed a mission to Apophis for its next visit expected on April 13, 2029. Other groups took up this idea because the Planetary Society offered a prize of $ 50,000 to the author of the project which would be selected. for a space mission. The first prize was won by an American team.

“It is imperative to gather data on Apophis as soon as we can, because once we know he is on a collision course with Earth, the surest way to avoid disaster will be to push bend the asteroid so that it changes orbit, ”said Mike Healy, director of the Astronautics department at Astrium.

“If we wait too long, it will be impossible to build a spacecraft powerful enough to change its trajectory. At best, we should push it back before 2025 to make sure we miss it,” he said. precise.

However, there is still an unknown for 2029, when Apophis will get close enough to Earth to be deviated from its path. If this deviation causes it to pass through a particular point in space called “the keyhole”, it risks hitting the Earth during its next passage in 2036. But nothing is confirmed.

In 2007, astronomers had calculated that Apophis had a 1 in 5500 chance of going through “the keyhole”, which corresponds to an area of ​​space that extends over 600 meters. The possible resulting impact is predicted on April 13, 2036 and would have a 1 in 12,000 chance of occurring.

From the data collected in December 2004, the probability calculations indicated that Apophis had a 2.7% (bad) chance of hitting the Earth in 2029. This probability fell to 2.4% in 2013 (cf. D. Farnocchia et al. , 2013). New measures have further reduced this risk.

Predictions for 2068

In early 2020, Tholen and his colleagues spotted Apophis at 21st magnitude using the 820m Subaru telescope in Hawaii. It was located 0.5 AU from Earth or 75 million km. His position was slightly different from the forecast. Astronomers determined that it picked up speed following the Yarkovsky effect or YORP effect. As a reminder, when a body such as an asteroid is exposed to the radiation of a star, in this case the Sun, it absorbs its radiation and re-emits it in the form of heat which acts as a tiny propellant. This emission produces a slow drift of its trajectory which modifies the major axis of its orbit.

The Yarkovsky effect can affect asteroids hundreds of astronomical units from the Sun, much further than Pluto. For a body 500 m in length, the effect manifests itself up to about 50 AU.

Before astronomers found out that Apophis was experiencing this effect, this asteroid had no chance of colliding with Earth in 2068. But taking into account the Yarkovsky effect, the researchers changed their minds although the result is little different.

According to Tholen, “The new observations we obtained with the Subaru telescope earlier this year were good enough to reveal Apophis’ Yarkovsky acceleration, and they show that the asteroid is moving away from a purely gravitational orbit of around 170 meters per year, which is enough to keep the 2068 impact scenario in play. ”

In 2020, Apophis was the third biggest threat in NASA’s NEO surveillance program risk table, behind Bennu and 1950 DA.

According to simulations performed in 2020, Apophis had a one in 110,000 chance of hitting Earth in 2068, but Tholen estimated that the odds are closer to one in 530,000 if you take the Yarkovsky effect into account.

New simulations carried out in February 2021 indicated that Apophis had a one in 380,000 chance of hitting Earth in 2068. In theory, if the risk of collision is not zero, in practice it is just like. The researchers nevertheless specified that other observations aiming to refine the amplitude of the Yarkovksy effect and the way in which it affects the orbit of Apophis were in progress.


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