Planetary Defense: The Bennu Experiment

Credit: NASA

As a result, when this asteroid comes back near Earth in 2080, according to Chesley’s calculations, the best window we can get on its whereabouts is 14,000 kilometers (nearly 9,000 miles) wide. By 2135, when Bennu’s shifted orbit is expected to bring it closer than the Moon, its flyby window grows wider, to 160,000 kilometers (nearly 100,000 miles). This will be Bennu’s closest approach to Earth over the five centuries for which we have reliable calculations.

“Right now, Bennu has the best orbit of any asteroid in our database,” Chesley said. “And yet, after that encounter in 2135, we really can’t say exactly where it is headed.

There’s another phenomenon nudging Bennu’s orbit and muddying future impact projections. It’s called the Yarkovsky effect. Having nothing to do with gravity, the Yarkovsky effect sways Bennu’s orbit because of heat from the Sun.

“There are a lot of factors that might affect the predictability of Bennu’s trajectory in the future, but most of them are relatively small,” says William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado, and a participating scientist on the OSIRIS-REx mission. “The one that’s most sizeable is Yarkvovsky.”

This heat nudge was named after the Polish civil engineer who first described it in 1901: Ivan Osipovich Yarkovsky. He suggested thatsunlight warms one side of a small, dark asteroid and some hours later the heat radiates away as the asteroid rotates its hot side into cold darkness. This thrusts the rock pile a bit, either toward the Sun or away from it, depending on the direction of its rotation.

In Bennu’s case, astronomers have calculated that the Yarkovsky effect has shifted its orbit about 284 meters (0.18 miles) per year toward the Sunsince 1999. In fact, it helped deliver Bennu to our part of the solar system, in the first place, from the asteroid belt between Mars and Jupiter over billions of years. Now, Yarkovsky is complicating our efforts to make predictions about Bennu’s path relative to Earth.

Getting face-to-face with the asteroid will help

The OSIRIS-REx spacecraft will use its suite of instruments to transmit radio tracking signals and capture optical images of Bennu that will help NASA scientists determine its precise position in the solar system and its exact orbital path. Combined with existing, ground-based observations, the space measurements will help clarify how Bennu’s orbit is changing over time.

Additionally, astronomers will get to test their understanding of the Yarkovksy effect on a real-life asteroid for the first time. They will instruct the spacecraft to follow Bennu in its orbit about the Sun for about two years to see whether it’s moving along an expected path based on gravity and Yarkovsky theories. Any differences between the predictions and reality could be used to refine models of the Yarkovsky effect.

But even more significant to understanding Yarkovsky better will be the thermal measurements of Bennu. During its mission, OSIRIS-REx will track how much solar heat radiates off the asteroid, and where on the surface it’s coming from—data that will help confirm and refine calculations of the Yarkovsky effect on asteroids.

The spacecraft also will address some open questions about the Yarkovsky theory. One of them, said Chesley, is how do boulders and craters on the surface of an asteroid change the way photons scatter off of it as it cools, carrying away momentum from the hotter side and thereby nudging the asteroid in the opposite direction? OSIRIS-REx will help scientists understand by mapping the rockiness of Bennu’s surface.

“We know surface roughness is going to affect the Yarkovsky effect; we have models” said Chesley. “But the models are speculative. No one has been able to test them.”

After the OSIRIS-REx mission, Chesley said, NASA’s trajectory projections for Bennu will be about 60 times better than they are now.

This picture shows the OSIRIS-REx spacecraft’s view of Bennu during the final phase of its journey to the asteroid. From Aug. 17 through Nov. 27 the spacecraft’s PolyCam camera imaged Bennu almost daily as the spacecraft traveled 1.4 million miles (2.2 million km) toward the asteroid. The final images were obtained from a distance of around 40 miles (65 km). During this period, OSIRIS-REx completed four maneuvers slowing the spacecraft’s velocity from approximately 1,100 mph (491 m/sec) to 0.10 mph (0.04 m/sec) relative to Bennu, which resulted in the slower approach speed at the end of the video.