(Very) Small Chance of Apophis Asteroid Impact in 2068

Near-Earth asteroid 99942 Apophis may crash into our planet in April 2068 after all. For the first time ever, astronomers have measured how the asteroid’s orbit is slowly shrinking. Taking this non-gravitational effect into account, “the 2068 impact scenario needs to be looked at again,” team leader David Tholen (University of Hawai‘i) told the online meeting of the Division of Planetary Sciences of the American Astronomical Society on October 26th.

Apophis was discovered in 2004. Initially, it looked like the roughly 350-meter-wide rock might crash into Earth on April 13, 2029, or maybe in April 2036. More precise observations subsequently revealed that in both cases, the chance of an actual impact is zero, although the 2029 flyby will take Apophis within the orbits of geostationary satellites — it will be visible as a fast-moving naked-eye object from Europe and Africa.

Working from the current uncertainty in the asteroid’s orbit and using just gravity, astronomers calculated that encounters later this century would also be harmless. However, in 2015, David Vokrouhlický (Charles University in Prague) and his colleagues predicted that Apophis should experience a tiny orbital change due to the Yarkovsky effect, leading to a possible impact in 2068.

The Yarkovsky effect is a subtle, net force on a small, rotating body caused by asymmetric heating. Tholen and his colleagues have now measured the strength of this effect on Apophis by making extremely precise positional observations using the Japanese 8.3-meter Subaru telescope on Mauna Kea, Hawai‘i. Their results indicate that the semi-major axis of the asteroid’s orbit is decreasing by 170 meters per year – in very good agreement with Vokrouhlický’s prediction.

The upshot: there’s indeed a small chance (currently about 1 in 150,000) of an actual impact in April 2068, a little more than 47 years from now.

“It is extremely important to recognize that warnings of a possibility of an impact are very different from predictions of impact,” says Steven Chesley (JPL). Then again, he adds that “just because some [possible impact scenarios] were eliminated in the past does not mean they will always be in the future.”

Much depends on Apophis’s “miss distance” in April 2029, and the precise way that Earth’s gravity deflects its orbit. “By studying the 2029 encounter closely and critically,” Tholen says, “it will be easier to predict future impact scenarios.”

---

---