How sunlight changes asteroid Bennu’s orbit

Bennu gives off heat as it spins, and the escaping infrared light nudges it backward. Radio signals from OSIRIS-REx helped scientists measure the slow change in its orbit.

Sunlight can change the orbit of an asteroid long after it strikes the surface. On Bennu, the warmest ground turns away from the Sun before it finishes cooling. The infrared light that escapes carries momentum, giving the asteroid a small but persistent recoil. Astronomers call this the Yarkovsky effect.

A warm afternoon on a spinning rock

Think of a patch of rock as it rotates into daylight. The Sun is highest over it at local noon, but the ground takes time to heat up. As the patch turns into afternoon, it can still be warmer than a patch on the morning side. It therefore emits more infrared light. A photon carries momentum even though it has no rest mass, so the asteroid recoils in the opposite direction to the light it emits. Emission across the surface is uneven: the little pushes do not all cancel.

Part of that recoil can act along the orbit or against it. For the daily heating cycle, a spin that goes with the orbit generally gives a push along the direction of travel, widening the orbit; a spin that goes against the orbit generally does the reverse. Bennu rotates the second way. The diagram shows this simplified geometry. The actual push also depends on the tilt of the spin axis and how quickly the surface absorbs and gives up heat. Review of the thermal recoil physics.

How scientists measured the drift

Bennu is about 500 meters across. Long before OSIRIS-REx got there, telescopes and radar had tracked it across multiple returns near Earth. Optical observations gave its direction against the stars; radar echoes constrained its distance. With those observations, astronomers could test whether a model with thermal recoil matched Bennu's positions better than a model without it. The 2014 orbit study found a measurable inward drift.

OSIRIS-REx gave researchers two things that were hard to get from Earth. Radio ranging to the spacecraft, combined with its measured position relative to Bennu, gave a much tighter estimate of the asteroid's distance from Earth. Close observations also helped the team model Bennu's shape, spin and surface temperatures. That mattered because a boulder and a bed of fine grains need not store and release heat at the same rate. The PolyCam photograph shows the rocky terrain; the temperatures came from infrared observations. Those infrared measurements showed that different boulders can retain heat differently, even on the same asteroid.

In the 2021 orbit analysis, Davide Farnocchia and colleagues estimated that thermal recoil was shrinking Bennu's semimajor axis by 284.6 ± 0.2 meters a year, evaluated for the orbit on January 1, 2011. The semimajor axis is half the long diameter of an orbit's ellipse. This number describes a change in the size of the orbit. It does not mean that Bennu is 285 meters closer to the Sun at every point after a year, and the rate varies slightly as the orbit changes.

Why a small push matters over a century

A slightly smaller orbit also changes how long a lap takes. Over many laps, that changes when the asteroid reaches a given point. The distinction matters at a close flyby: arriving a little earlier or later changes how Earth pulls on the asteroid and where it goes afterward.

Bennu's 2135 Earth flyby is one example. The 2021 analysis found no impact on that pass. It used the tighter approach prediction to rule out some narrow paths through the flyby that could have led to later impacts. Gravity still dominates Bennu's motion. The lesson is that a force too small to see in a single image can matter when scientists are predicting where the asteroid will be after decades of orbits.