![]() Asteroid Bennu – 68 million tons of mass, a 1/3 of a mile (1/2 a kilometer) wide – is being pushed around by a force equal to, as team member Steven Chesley said in 2012, the weight of three grapes on Earth. If you guessed that the thrust imparted by radiation is tiny, you would be right. Like the proverbial tortoise racing the hare, slow and steady is the way the Yarkovsky effect manifests itself. The discrepancy is entirely the result of heat radiating from the asteroid’s surface. 100 miles in a dozen yearsīetween 19 – when scientists first announced a measurement of the Yarkovsky effect for asteroid Bennu – Bennu had wandered by about 100 miles (160 km) from where it otherwise would have been. Repeated observations beginning in 1999 (the year of the asteroid’s discovery) and into this century – using the Arecibo and Goldstone radio telescopes – revealed the Yarkovsky effect acting on asteroid Bennu. By measuring the delay in the return signal, researchers were able to measure accurately how far the asteroid was from Earth. That’s why – for asteroid Bennu, the target of the OSIRIS-REx spacecraft mission – astronomers observed every close passage of the asteroid and bounced signals from radio telescopes off the asteroid’s surface. Knowing the precise orbit of the asteroid is essential to a successful spacecraft encounter. It’s a minuscule push on an asteroid, imparted by nothing more than sunlight. Scientists have known since at least 2012 that Bennu undergoes the Yarkovsky effect’s delicate nudge. Image via NASA/ University of Arizona.Ĭonsider the example of asteroid Bennu, which made headlines two years ago when NASA’s OSIRIS-REx spacecraft successfully collected a sample of dust from its surface (the spacecraft is due to return to Earth on September 24, 2023). Scientists were surprised to see Bennu’s surface strewn with boulders. The spacecraft was just 15 miles (24 km) from the asteroid. The OSIRIS-REx spacecraft and its Pol圜am instrument collected 12 images to make this mosaic of asteroid Bennu on December 2, 2018. ![]() An imbalance in the radiation of heat of an asteroid changes its orbit.Īnd that change can make the difference between the status quo and mass extinction. But sometimes, very small forces can alter the evolution of an entire planetary system. Why the Yarkovsky effect mattersĪstronomy often focuses on the large, the vast, and the highly energetic. As the surface heats up during the day and cools down – expels heat – at night, the rotating asteroid gives off radiation that can act as a sort of mini-thruster. The OSIRIS-REx team, which sent a spacecraft to the asteroid Bennu, explains how this works in the video below:Īs the asteroid rotates, the part of the asteroid’s surface that faces the sun is constantly shifting and it continuously expels heat from its ever-shifting sunlit side. It slows down and falls toward the sun on an increasingly smaller orbit. The Yarkovsky effect pushes a retrograde rotator backward. The opposite happens for an asteroid rotating in a retrograde direction, opposite from its orbital motion. The asteroid speeds up and moves out to a slightly larger orbit. For asteroids that spin in a prograde direction – that is, the asteroid spins in the same direction it’s orbiting – the asteroid gets a push in the direction of its orbital motion. This is known as the Yarkovsky effect, and, yes, it can change the orbit of small asteroids. It’s a factor that complicates the picture for scientists trying to assess the long-term risk of asteroids on Earth-crossing orbits. ![]() Rotating asteroids drift widely over time. But – over many years – the tiny pushing effect of sunlight adds up so that asteroids have a tough time sticking to their orbits. The effect of sunlight on rotating asteroids is tiny in contrast to the gravitational forces acting on asteroids as they move through the solar system. Would you believe that sunlight has the ability to change the course of asteroids? It can and it does.
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