Astronomy Essentials

Yarkovsky effect: Pushing asteroids with sunlight

Can sunlight move a rock?

Would you believe that sunlight has the ability to change the course of asteroids? It can and it does.

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. But – over many years – the tiny pushing effect of sunlight adds up so that asteroids have a tough time sticking to their orbits. Rotating asteroids drift widely over time. It’s a factor that complicates the picture for scientists trying to assess the long-term risk of asteroids on Earth-crossing orbits.

This is known as the Yarkovsky effect, and, yes, it can change the orbit of small asteroids. 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. The asteroid speeds up and moves out to a slightly larger orbit.

The opposite happens for an asteroid rotating in a retrograde direction, opposite from its orbital motion. The Yarkovsky effect pushes a retrograde rotator backward. It slows down and falls toward the sun on an increasingly smaller orbit.

The OSIRIS-REx team, which sent a spacecraft to the asteroid Bennu, explains how this works in the video below:

No place to hide from the sun

As 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. 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.

Why the Yarkovsky effect matters

Astronomy often focuses on the large, the vast, and the highly energetic. But sometimes, very small forces can alter the evolution of an entire planetary system. The Yarkovsky effect is one example. An imbalance in the radiation of heat of an asteroid changes its orbit.

And that change can make the difference between the status quo and mass extinction.

Squarish gray asteroid with rough surface scattered thickly with individual rocks.
The OSIRIS-REx spacecraft and its PolyCam instrument collected 12 images to make this mosaic of asteroid Bennu on December 2, 2018. The spacecraft was just 15 miles (24 km) from the asteroid. Scientists were surprised to see Bennu’s surface strewn with boulders. Image via NASA/ University of Arizona.

The Yarkovsky effect on Asteroid Bennu

Consider 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 have known since at least 2012 that Bennu undergoes the Yarkovsky effect’s delicate nudge. It’s a minuscule push on an asteroid, imparted by nothing more than sunlight.

Knowing the precise orbit of the asteroid is essential to a successful spacecraft encounter. 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. By measuring the delay in the return signal, researchers were able to measure accurately how far the asteroid was from Earth. 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.

100 miles in a dozen years

Between 1999 and 2012 – 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. This is faster than a snail’s pace. The discrepancy is entirely the result of heat radiating from the asteroid’s surface.

Like the proverbial tortoise racing the hare, slow and steady is the way the Yarkovsky effect manifests itself. If you guessed that the thrust imparted by radiation is tiny, you would be right. 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. That’s about 1/2 an ounce (14 grams).

Understanding the evolution of our solar system requires taking into account all the forces at play, no matter how small. If the weight of three grapes can shove an entire asteroid off course by 100 miles (160 km) over a dozen years, what about over 1,000 years? Or a 100,000? Or a billion?

Yarkovsky effect vs the YORP effect

Do not confuse the Yarkovsky effect, which changes the orbit of an asteroid, with the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, which changes the rotational period and spin axis of the asteroid. Because of the YORP effect, scientists have found that Bennu’s rotation is speeding up by about one second per century. In other words, Bennu’s rotation period is getting shorter by about one second every 100 years.

How are these two forces intertwined? The YORP effect, changing the spin speed and rotational axis of the asteroid, can also alter the Yarkovsky effect.

Sentry-II, a new computer program to calculate the Yarkovsky effect

NASA announced on December 6, 2021, that a new asteroid monitoring computer program, called Sentry-II, can now calculate the Yarkovsky effect for asteroids. Read about the program here.

Scientists used the Sentry-II program to show a reduced possibility of the Earth colliding with asteroid 1950 AD.

Read more: Astronomers find a Yarkovsky effect for infamous asteroid Apophis

Who was Ivan Yarkovsky?

Yarkovsky effect: A Victorian-era portrait of a bearded man in a dark suit.
Ivan Yarkovsky (1844-1902). Image via Wikipedia.

The Polish-Russian civil engineer Ivan Yarkovsky first described the effect that bears his name around the year 1900. Yarkovsky, born in 1844, worked for the Alexandrovsk railway company for more than 20 years, exploring railroad technology. During that time, he also dabbled in other scientific pursuits. His interest in the motions of the planets led to the publishing of a pamphlet describing the effect that would come to bear his name.

His work would have been lost, but Ernst Opik rediscovered it and made it widely known in 1951.

In the years since Yarkovsky published his musings, planetary astronomers have come to realize that his effect has most likely dramatically changed entire families of asteroids and played an essential role in the movement of objects from the main asteroid belt to Earth. In fact, without this effect, the Earth would have experienced fewer asteroid impacts over its history. It’s hard not to wonder if any mass extinctions were the result of just half an ounce (14 grams) of pressure on one side of a rock quietly orbiting between Mars and Jupiter.

Title page, in Russian, of Ivan Yarkovsky's pamphlet.
Title page of Ivan Yarkovsky’s pamphlet, privately published in Bryansk, Russia, in 1901. This is where the concept of his effect appeared for the first time (although in a context that’s now obsolete; he assumed that ether existed between solar system bodies). Image via Scholarpedia.

Bottom line: An ounce (28 grams) of force from an imbalance in sunlight can steer asteroids into (or out of) Earth-crossing orbits. This is the Yarkovsky effect, a minuscule push on an asteroid, imparted by nothing more than sunlight. Given a long-enough time, the effect can drastically alter the layout of the solar system.

April 13, 2022
Astronomy Essentials

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