Sciency Words: The Yarkovsky Effect

Hello, friends!  Welcome to another episode of Sciency Words, a special series here on Planet Pailly where we discuss the definitions and etymologies of scientific terms, in order to expand our scientific vocabularies together!  Today’s Sciency Word is:

THE YARKOVSKY EFFECT

Imagine an asteroid orbiting the Sun.  Every once in a while, this asteroid passes alarmingly close to Earth.  If you’re familiar with Kepler’s laws of planetary motion, you may expect that scientists could predict, with pinpoint accuracy, where that asteroid will be years, decades, or even centuries into the future.  However, there are certain physical forces acting on asteroids that are not accounted for in Kepler’s laws.  One of those physical forces is known as the Yarkovsky Effect.

Definition of the Yarkovsky Effect: In astrophysics, the Yarkovsky Effect is a thermal force that affects the orbit of asteroids.  Like most planets, asteroids rotate; therefore, you could say that asteroids have day-night cycles.  During daytime, the surface of an asteroid absorbs heat from the Sun.  At night, the asteroid’s surface cools off by radiating heat out into space.  This radiating heat generates a very, very, very small amount of thrust.  Over time, that small amount of thrust can dramatically change the orbital trajectory of an asteroid.

Etymology of the Yarkovsky Effect: The Yarkovsky Effect is named in honor of Polish/Russian civil engineer Ivan Yarkovsky, who first described a similar “heat engine” effect in 1888, and who later published a pamphlet on the topic in 1901.  Yarkovsky’s work would have been lost to history, except that Estonian physicist Ernst Öpik recalled reading Yarkovsky’s 1901 pamphlet and reintroduced the idea to the physics community in 1951.

Yarkovsky was more of a science hobbyist than a professional scientist.  He had a day job working on railroads.  In his free time, he read a lot about science, and he did a lot of thinking.  He performed his own experiments, occasionally, and he came up with some interesting ideas that sound like utter nonsense today, but which must have made sense in the context of late 19th Century science.  Even the Yarkovsky Effect, as Yarkovsky originally described it, was tied up with a now defunct scientific theory called ether theory.

Still, even if his starting assumptions were off track, Yarkovsky stumbled upon the truth at least one time.  Asteroids do have “heat engines,” as Yarkovsky described it.  Asteroids do have these naturally occurring thermal propulsion systems, powered by sunlight, which can mess with their orbits.  The challenge for astrophysicists today is that the Yarkovsky Effect is kind of random (or if it isn’t random, in the truest sense of the word, then it may as well be).

Asteroids are irregularly shaped.  Sometimes, they rotate on more than one axis (I once read a paper that called this multiple axis rotation “chaotic tumbling”).  And in terms of mineral composition, asteroids are made of all sorts of crazy stuff.  Different minerals can absorb and radiate heat in different ways.  So the Yarkovsky Effect pushes asteroids around, but because of all the variables I just mentioned, it’s hard to say which direction the Yarkovsky Effect will push at any given time.  It’s also hard to say how hard of a push the Yarkovsky Effect might give.

Which is why missions to study asteroids—missions like the recent ORISIR-REx Mission or the upcoming DART Mission—are so important.  We may never understand asteroids perfectly, but we do need to understand them better.  There are so many asteroids that fly alarmingly close to Earth.  It would be nice if astrophysicists could predict, with pinpoint accuracy or something near to it, where those asteroids will be years, decades or centuries into the future.

WANT TO LEARN MORE?

I used the following sources to write this blog post.  The one at the bottom is kind of a long read, but it tells the fascinating story of Ivan Yarkovsky, a man who was nearly forgotten by history.  For those of you who are interested in the history of science, it is well worth a read.

• How Sunlight Pushes Asteroids from NASA Science.
• Yarkovsky Effect: Pushing Asteroids with Sunlight from EarthSky.
• I.O. Yarkovsky and the Discovery of “His” Effect from the Journal for the History of Astronomy.