99942 Apophis: The Value of a Close Approach

The approach of the asteroid 99942 Apophis in April of 2029 offers an opportunity to study a sizeable asteroid through both radar and optical telescopes. Marina Brozović, a radar scientist at the Jet Propulsion Laboratory, points out that radar studies of the object might resolve surface details that are no more than a few meters in size. No surprise, then, that Apophis is the subject of much discussion at the 2019 Planetary Defense Conference in College Park, Maryland.

This is the same conference at which NASA Administrator Jim Bridenstine warned about the critical nature of planetary defense, noting the Chelyabinsk event in 2013 that delivered some 30 times the energy of the Hiroshima bomb. NASA has contracted with SpaceX to provide launch services for its Double Asteroid Redirection Test (DART), which is expected to launch in 2021 via a SpaceX Falcon 9 and test asteroid deflection through high-speed collision.

DART’s target will be the tiny moon of an asteroid called Didymos, which it will reach by solar electric propulsion in October of 2022, when the asteroid closes to within 11 million kilometers of Earth. Bridenstine pointed out that the NASA plan to detect and characterize 90 percent of near-Earth objects measuring 140 meters in diameter and above is “only about a third of the way there,” adding that events like Chelyabinsk are expected roughly every 60 years.

So it’s heartening to see missions like the Japanese Hayabusa2 and NASA’s OSIRIS-REx probing the nature of these objects, even as we look toward the Apophis opportunity in 2029. Numerous small objects on the order of 5-10 meters have been found passing as close to the Earth as Apophis, but the latter is substantial, a 340-meter asteroid that will be widely studied as it approaches to within 31,000 kilometers of the surface. The asteroid will become a naked eye object in the night sky over the southern hemisphere on April 13, 2029.

Image: This animation shows the distance between the Apophis asteroid and Earth at the time of the asteroid’s closest approach. The blue dots are the many man-made satellites that orbit our planet, and the pink represents the International Space Station. Credit: NASA/JPL-Caltech.

You may recall that following its discovery in 2004, early calculations showed a 2.7% chance that Apophis might impact the Earth in 2029, but follow-up observations have ruled that out. Now we can take advantage of the close passage to study Apophis’ size, shape and composition. A key question: Can we use the flyby to learn more about the asteroid’s interior?

“We already know that the close encounter with Earth will change Apophis’ orbit, but our models also show the close approach could change the way this asteroid spins, and it is possible that there will be some surface changes, like small avalanches,” said Davide Farnocchia, an astronomer at JPL’s Center for Near Earth Objects Studies (CNEOS), who co-chaired the April 30 session on Apophis with Marina Brozović.

Apophis’ passage in 2029 will take it within the distance some spacecraft orbit the Earth, and there remains the possibility of a mission to the object. As to future collision risks, the trajectory of Apophis is well established, but gravitational interactions between asteroid and Earth make it necessary to continue to recalculate the orbit. As we assemble the catalog of potentially hazardous objects, the need for missions like DART — and others testing a range of mitigation strategies — is clear. Because if we ever do find an asteroid of this size that presents a danger to our planet, we need to know what we’re going to do.

You can watch video from the 2019 Planetary Defense Conference here.