Artist’s depiction of a near-earth-object.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 now re-evaluating its potential to strike our planet in 48 years, owing to improved observations of the problematic asteroid.
Observations made earlier this year from the Subaru Telescope in Hawai’i are providing astronomers with a better sense of how the Yarkovsky effect is influencing the orbital path of asteroid 99942 Apophis. This effect is like a built-in propulsion system for asteroids, in which trace amounts of leaking radiation can alter an object’s momentum in space, causing it to drift ever-so-slightly from the path otherwise chosen by gravity.
“Without taking Yarkovsky drift into account, Apophis is still a threatening object, just not in 2068,” Dave Tholen, a researcher from the Institute for Astronomy at the University of Hawai’i and a co-author of the pending study, explained in an email. “With Yarkovsky taken into account, the 2068 impact scenario is still in play. Small, but non-zero.”
Tholen, along with Davide Farnocchia from NASA’s Jet Propulsion Laboratory, crunched the new numbers, finding that Yarkovsky acceleration is keeping the Apophis threat inside the 2068 window. Their findings are laid out in new research presented at the 2020 virtual meeting of the Division for Planetary Sciences of the American Astronomical Society.
steroid apophis (circled) as it appeared during its discovery in 2004. Image: UH/IA
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Apophis currently owns the title of third-highest threat on NASA’s Sentry Risk Table. The estimate on the Palermo Technical Impact Hazard Scale suggests there’s a 1 in 150,000 chance of Apophis hitting Earth on April 12, 2068 (mark your calendars). Or, if you prefer percentages, that’s a 0.00067% chance of Earth impact. Tholen said the odds are actually closer to 1 in 530,000, a figure used by the NEODyS impact monitor service, which includes a nominal Yarkovsky drift rate. The new analysis will result in a revised threat risk for Apophis, but even then, Tholen said we’ll need “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 itinerary.
A collision with Earth—as improbable as it appears to be—would be seriously bad. Apophis, packed with nickel and iron, measures over 1,000 feet (300 meters) wide, or over three football fields, if that’s how you like to picture it. An impact with the surface would release the equivalent of 1,151 megatons of TNT. Such a calamitous event happens on Earth around once every 80,000 years.
Understandably, therefore, scientists are keeping a close watch on Apophis to improve their estimates. When the near-Earth asteroid was discovered in 2004, for example, astronomers initially assigned a horrific 2.7% chance of an Earth impact in 2029. Astronomers have since ruled this out as a possibility, along with a possible impact in 2036. As for the 2068 encounter, that cannot be ruled out, at least not yet, owing to how the Yarkovsky effect is influencing Apophis.
Asteroids, because they’re exposed to the Sun’s rays, absorb a lot of energy. Eventually, however, this excess heat is redirected back into space, but not in a perfectly uniform fashion across the asteroid’s body. This results in some added acceleration, which can change the object’s trajectory.
“Light radiated from a body gives that body a tiny, tiny push. The warmer side of an asteroid pushes a little harder than the cooler side because the warmer side emits more light (at invisible infrared wavelengths), so there is a net non-gravitational force acting on the body,” explained Tholen, a co-discoverer of Apophis. “It’s such a tiny force that it’s not noticeable for larger objects, but the smaller the object, the easier it is to detect the effect.”
Tholen and his colleagues have been tracking Apophis’s position for the past 16 years, and they’ve now noticed a slight departure 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 to a precision of about twice the size of the asteroid itself,” he said. “Apophis is roughly 300 meters in diameter, and we measured the position to about 700 meters, even though we were something like 70 million kilometers from the object.”
His team’s calculations show that the semi-major axis (half of the longest length of an elliptical orbit) of Apophis’s orbit is currently shrinking at a rate of around 170 meters each year as a result of the Yarkovsky effect and not because of gravity. When Apophis brushes past Earth in 2029, its semimajor axis will increase significantly on account of our planet’s gravity, he said.
More observations should improve the estimates, including a better characterization of how the Yarkovsky effect is influencing Apophis’s drift rate. It’s fairly safe to say that astronomers will know if an impact is inevitable well in advance of 2068.
In case you’re wondering, the two near-Earth objects with higher risk ratings on the Palermo Technical Impact Hazard Scale are asteroid 29075 (1950 DA) and asteroid Bennu. 29075 (1950 DA) has a 1 in 8,300 chance (0.012%) of hitting Earth in 2880, and Bennu, which is currently being investigated by NASA’s OSIRIS-REx spacecraft, has a 1 in 2,700 (0.037%) chance of an Earth impact between the years 2175 and 2199. There are objects with a greater chance of hitting us, but the Palermo scale takes other factors into account, such as an asteroid’s potential to do catastrophic damage.