Sky & Telescope - February 2020 - 37

THE YORP EFFECT

Sunlight

Sunlight warms an irregularly
shaped asteroid.

The asteroid reradiates the
light as heat. The photons
act like little thrusters ﬁring
off from every square meter
of surface.

Boulder). Over long periods, the YORP effect can spin up a
rubble-pile asteroid enough for the centrifugal force at its
equator to exceed the body's gravitational attraction, and
pieces can drift away.
The strength of the effect depends on the intensity of the
incident light. It's stronger for near-Earth objects and much
weaker in the main belt, which lies farther from the Sun.
Also, the smaller the object, the more a given amount of sunlight can increase its spin. "This means that these effects are
only active on small bodies," Scheeres says. His students have
measured the YORP effect as it spun up small defunct artiﬁcial satellites in geostationary orbit within months to years.
Asteroids are more massive, so it takes hundreds of thousands
to hundreds of millions of years to spin them up. But the
Sun shines relentlessly. Many single near-Earth asteroids are
spinning nearly fast enough to start shedding material, and
all the near-Earth binaries have close orbits, both outcomes
expected from the YORP effect.
Asteroid rubble that has spun off may drift away, but it
generally does not escape permanently. Instead, it goes into
orbit around the asteroid, where in theory escaped pieces

Because of the asteroid's
odd shape, the tiny thrusts
don't cancel out. Instead,
they create a net torque. The
torque spins the asteroid up
or down, depending on the
orientation of the spin axis
with respect to the torque.

might accrete to form a smaller, stable companion for the
original asteroid. Details are unclear, including how much
material would escape, how much would stay to form the
resulting binary companion, and how large it would be.
However, Scheeres says, formation of a new object in orbit
could in theory trigger the related "binary YORP effect,"
which can cause the pair to either spiral in or out depending
on details. If the two spiraled out, they could become separate objects. But if they spiraled in, they could merge with
each other to re-form a single object, even though some mass
would be lost.

Main Belt and Comets
The YORP effect is much weaker farther out in the main
asteroid belt, so other effects are likely responsible for most
of its 169 orbital binary asteroids. Estimates of the fraction of
binaries in the main belt vary widely because few have been
observed closely enough to detect whether these are multiple
objects. Main-belt binaries include a wide variety of relative
sizes and orbital spacings, and many are much larger and
more widely spaced than objects with orbits near Earth's.

YORP EFFECT: G REGG DINDER M A N / S&T; ASTEROID
BRE A K UP: N ASA / ESA / D. JE WIT T (UCL A)

q ASTEROID BREAKUP Main belt object P/2013 R3 disintegrated unexpectedly in 2013. Each piece has its own comet-like dust tail. Researchers
suspect the YORP effect might have spun the asteroid up until it flew apart.

Oct. 29, 2013

Nov. 15, 2013

Dec. 13, 2013

Jan. 14, 2014

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Sky & Telescope - February 2020

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