Sciency Words: The YORP Effect

Hello, friends!  Welcome to another episode of Sciency Words, a special series here on Planet Pailly where we talk about the definitions and etymologies of scientific terms.  In today’s episode, we’re talking about:


Picture a windmill.  As the wind gets stronger or weaker, the windmill spins faster or slower, right?  Okay.  Now replace the windmill with an asteroid orbiting the Sun, and replace the wind with sunlight.  Over long periods of time, sunlight can make the asteroid spin faster or slower.  Sunlight can also change an asteroid’s axis of rotation.  This is known as the YORP Effect (not to be confused with the Yarkovsky Effect).

Definition of the YORP Effect: In astrophysics, the YORP effect is what happens when reflected and/or absorbed sunlight generates “thermal torque” on an asteroid.  Reflected sunlight exerts a very small (but non-zero) amount of force on the surface of an asteroid.  Absorbed sunlight radiates away from the surface of an asteroid as heat, exerting an additional small (but non-zero) amount of force.  Due to the irregular shapes and material consistencies of asteroids, it’s hard to predict exactly what this thermal torque will do, but over long enough periods of time it can dramatically change an asteroid’s rotation rate and axis of rotation.

Etymology of the YORP Effect: The term was coined in 1999 by American geophysicist David Rubincam.  The YORP Effect, as we currently know it, combines the previous research of Ivan Yarkovsky, John O’Keefe, Vladimir Radzievskii, and Stephen Paddack.  YORP is therefore an acronym of the names Yarkovsky, O’Keefe, Radzievskii, and Paddack.

This all started with Ivan Yarkovsky and his Yarkovsky Effect, which we talked about in last week’s Sciency Words post.  The Yarkovsky Effect has to do with the way sunlight affects the orbital trajectory of an asteroid.  The Yarkovsky Effect was lost to science for a while, then it was reintroduced in 1951.  Shortly after that reintroduction, other scientists started wondering what other effects sunlight might have on an asteroid, which ultimately led to this idea of a thermal torque effect, which we now call the YORP Effect.

To be clear, the Yarkovsky Effect and the YORP Effect are two different effects—one related to an asteroid’s orbital trajectory, the other to an asteroid’s rotation rate and axis of rotation.  They’re caused by the same thing—sunlight—but they are two different effects.

In 2007, observations of an asteroid named 2000 PH5 helped confirm that the YORP Effect is real.  The asteroid had been monitored closely over the course of about four years, and astronomers found that its rotation rate was steadily increasing.  This increase could not be explained by gravitational interactions alone, nor by collisions with other asteroids or any other known effects.  Therefore, by process of elimination, only the YORP effect was left as a possible explanation.  Asteroid 2000 PH5 was subsequently renamed 54509 YORP to honor its help in confirming the YORP Effect.

And in 2013, an asteroid named P/2013 R3 literally YORP-ed itself apart.  The YORP Effect caused the asteroid to spin so fast that it started flinging chunks of itself away.  There may have been some previous collision or other catastrophic event that made P/2013 R3 more fragile; still, in the end, it was the YORP Effect that caused the final destruction of that asteroid.

So if you’re an asteroid flying around in space, be careful.  It may be fun YORP-ing and Yarkovsky-ing around the Solar System, but you don’t want to Yarkovsky yourself into hitting a planet, and you don’t want to YORP yourself into self-disintegration either.


P.S.: The DART Mission is scheduled to crash itself into an asteroid tonight at 7:14 p.m. East Coast time in the U.S. (also known as 23:14 GMT).  If you’re interested, NASA TV will be live streaming the collision on their YouTube Channel.  It would not surprise me if the Yarkovsky and YORP Effects are mentioned as part of NASA TV’s science commentary.