The asteroid Apophis could hit Earth again in 2068, new measures...

Artist’s representation of an object close to the earth.
Image: NASA / JPL-Caltech

Only a small handful of objects are known to pose a serious threat to Earth, and the gigantic asteroid Apophis is one of them. Scientists are currently reassessing its potential to strike our planet in 48 years, thanks to improved observations of the problematic asteroid.

Observations made earlier this year by the Subaru Telescope in Hawai’i give astronomers a better idea of ​​how the Yarkovsky effect influences the orbital path of asteroid 99942 Apophis. This effect is like an integrated propulsion system for asteroids, in which traces of leaking radiation can alter the momentum of an object in space, causing it to drift very slightly from the path that gravity would otherwise choose.

“Without taking Yarkovsky’s drift into account, Apophis is still a threatening object, but not in 2068,” says Dave Tholen, researcher at the Institute of Astronomy at the University of Hawaii and co-author of the study. In progress. an email. “With Yarkovsky taken into account, the 2068 impact scenario is still in play. Small, but different from zero. “

Tholen, along with Davide Farnocchia of NASA’s Jet Propulsion Laboratory, analyzed the new numbers, finding that Yarkovsky’s acceleration keeps the Apophis threat in window 2068. Their findings are on display in new research presented at the 2020 virtual meeting of the American Astronomical Society Division of Planetary Sciences.

steroid apophis (circled) as it appeared when it was discovered in 2004.
Image: UH / IA

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Apophis currently holds the title of third highest threat to NASA Sentry Risk Table. The Palermo Technical Impact Risk Scale estimate suggests that there is a 1 in 150,000 chance that Apophis will strike Earth on April 12, 2068 (mark your calendars). Or, if you prefer percentages, that’s a 0.00067% chance of impacting Earth. Tholen said the odds are actually closer to 1 in 530,000, a figure used by the NEODyS impact monitoring service, which includes a nominal Yarkovsky drift rate. The reanalysis will result in a revised threat risk for Apophis, but even then Tholen said we will have to “be careful with this calculation” as there will be other variables to consider. And indeed, we should expect to see the odds change over time as astronomers get a better handle on this asteroid’s route.

A collision with Earth – unlikely as it sounds – would be very serious. Apophis, filled with nickel and iron, is over 300 meters wide, or more than three football fields, if you like to imagine it. An impact with the surface would release the equivalent of 1,151 megatons of TNT. Such a calamitous event occurs on Earth approximately once every 80,000 years.

So it’s understandable that scientists are watching Apophis closely to improve their estimates. When the near-Earth asteroid was discovered in 2004, for example, astronomers attributed a horrendous 2.7% probability of impact on Earth in 2029. Astronomers have since excluded that as a possibility, le long de with a possible impact in 2036. As for the 2068 encounter, that cannot be ruled out, at least not yet, due to how the Yarkovsky effect influences Apophis.

Asteroids, because they are exposed to the sun’s rays, absorb a lot of energy. Eventually, however, this excess heat is redirected back into space, but not perfectly evenly across the asteroid’s body. This results in additional acceleration, which can change the path of the object.

“The irradiated light of a body gives that body a very small push. The hotter side of an asteroid pushes a little harder than the colder side because the hotter side emits more light (at invisible infrared wavelengths), so there is a non-gravitational force. net acting on the body, ”explained Tholen, a co-discoverer of Apophis. “It is such a tiny force that it is not noticeable for larger objects, but the smaller the object, the easier it is to detect the effect.”

Tholen and his colleagues have followed Apophis’ position for 16 years, and they have now noticed a slight deviation from an orbital path constrained exclusively by gravity.

“The observations made with Subaru in January and March of this year were critical to the success of this effort, as they enabled us to measure the position of the asteroid with an accuracy of approximately twice the size of the asteroid itself. – even, ”he said. “Apophis is about 300 meters in diameter and we measured the position at about 700 meters, even though we were about 70 million kilometers from the object.”

His team’s calculations show that the semi-major axis (half the longest length of an elliptical orbit) of Apophis’ orbit is currently shrinking at a rate of around 170 meters each year due to the ‘Yarkovsky effect and not because of gravity. When Apophis passes Earth in 2029, its semi-major axis will increase dramatically due to the gravity of our planet, he said.

More observations should improve estimates, including a better characterization of how the Yarkovsky effect influences the rate of Apophis drift. It’s pretty safe to say that astronomers will know if an impact is inevitable long before 2068.

In case you were wondering, the two NEOs with higher risk scores on the Palermo Technical Impact Risk Scale are asteroids. 29075 (1950 DA) and asteroid Determining. 29075 (1950 DA) has a 1 in 8300 (0.012%) chance of hitting Earth in 2880, and Bennu, who is currently investigation by NASA’s OSIRIS-REx spacecraft, has a 1 in 2700 (0.037%) chance of impacting Earth between the years 2175 and 2199. There are objects with a greater chance of hitting us, but the The Palermo scale takes into account other factors, such as the potential of an asteroid to cause catastrophic damage.

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