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What Is the Yarkovsky Effect and the YORP Effect?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

YORP and Yarkovsky

YORP and Yarkovsky

How It Was Developed

The Yarkovsky effect was named after I.O Yarkovsky, an engineer who speculated in 1901 how an object moving through the ether of space would be affected by the heating of one side and the cooling of the other. Sunlight hitting anything heats up that surface and of course, anything that is heated eventually cools off. For small objects, this heat being radiated off can be of such concentration that it actually generates a small amount of thrust! His work, however, was flawed because he tried to make his calculations using the ether of space, something we now know is instead a vacuum. Years later, in 1951, E.J. Opik rediscovered the work and updated it with current astronomical understandings. His goal was to see how the effect could be used to nudge the orbits of space objects in the Asteroid belt toward Earth. Other scientists such as O’Keefe, Radzievskii, and Paddack added to the work by noting that the thermal thrust of the heat radiating out could cause bursts of rotational energy and lead to increases in rotation, sometimes with disintegration as a result. And the thermal energy radiated would be based on the distance from the sun because it affected the amount of optical light impacting our surface. This rotational insight expressed as torque was therefore nicknamed the YORP effect based on the 4 scientists behind it (Vokrouhlicky, Lauretta).

What It Affects

The Yarkovsky effect is felt by the smaller objects of the Universe, that being less than 40 kilometers in diameter. This isn’t to say that other objects don’t feel it, but as far as creating measurable differences in motion this is the range models show would cause an appreciable effect (over a range of millions to billions). Space satellites, therefore, fall under this purview also. However, measuring the effect has challenges including knowing the albedo, spin axis, surface irregularities, shadowed regions, internal layout, the geometry of the object, inclination to the ecliptic, and distance from the sun (Vokrouhlicky).

But knowing the effect has brought some interesting consequences. The semimajor axis, the elliptical feature of the object’s orbit, can drift out if the object spins prograde because the acceleration of the object increases against the direction of motion (since that is the part of the spin that has cooled the most since facing the sun). If retrograde, then the semimajor axis will decrease, for the acceleration will work with the spin of the object. Seasonal drift (north facing summer vs. south facing winter) causes hemispherical changes and changed along the spin axis, resulting in centrally-directed accelerations against the center, causing the orbit to decay. As we can see, this is complicated! (Vokrouhlicky, Lauretta)

Evidence for the Yarkovsky Effect

Trying to see the effects of the Yarkovsky effect can be challenging with all the noise our data has as well as the possibility of the effect being mistaken as a consequence of something else. Additionally, the object in question must be of sufficiently small size for the effect to take hold but be large enough for detection. To minimize these issues, a long data set can help reduce those random permutations and refined equipment can locate hard-to-see objects. One of the features unique to the Yarkovsky effect is its results on the semimajor axis, to which it can only be attributed. It causes a drift in the semimajor axis of about 0.0012 AU every million years, or about 590 feet each year, making precision critical. The first candidate object spotted was (6489) Golevka. Since this, many others have been spotted (Vokrouhlicky).



Evidence for the YORP Effect

If finding the Yarkovsky effect was challenging, then the YORP effect is even more so. So many things cause other things to spin, so isolating the YORP from the rest can be tricky. And it’s harder to spot because the torque is so small. And the same criteria for size and placement from the Yarkovsky effect still holds. To assist in this search, optical and radar data can be used to find Doppler shifts on either side of the object to determine the rotational mechanics at any given time and two different wavelengths being used gives us better data to compare with (Vokrouhlicky).

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The first confirmed asteroid with the YORP effect detected was 2000 PH5, later renamed (54509) YORP (of course). Other interesting cases have been spotted, including P/2013 R3. This was an asteroid that was spotted by Hubble to be flying apart at 1,500 meters per hour. At first, scientists felt that a collision was responsible for the breakup but the vectors did not match such a scenario nor the size of the debris seen. Nor was it likely from ices sublimating and losing the structural integrity of the asteroid. Models show that the likely culprit was the YORP effect taken to the extreme, increasing the rotational rate to the point of breakup (Vokrouhlicky, “Hubble,” Lauretta).

Asteroid Bennu, a potential Earth impactor of the future, displays multiple signs of the YORP effect. For starters, it may have been a part of its formation. Simulations show that the YORP effect could have caused asteroids to migrate outward toward their current positions. It also gave the asteroids a preferred spin axis that has caused many to develop bulges along their equators as a result of these angular momentum changes. All of these things have caused Bennu to be of great interest to science, hence the OSIRUS-REx mission to visit and sample from it (Lauretta).

And this is but a sampling of the known applications and results of this effect. With it, our understanding of the Universe has grown that extra bit more. Or is that thrust forward?

P/2013 R3

P/2013 R3

Work Cited

“Hubble witnesses an asteroid mysteriously disintegrating.” Space and Telescope, 06 Mar. 2014. Web. 09 Nov. 2018.

Lauretta, Dante. “The YORP Effect and Bennu.” The Planetary Society, 11 Dec. 2014. Web. 12 Nov. 2018.

Vokrouhlicky, David and William F. Bottke. “Yarkovsky and YORP effects.” Scholarpedia, 22 Feb. 2010. Web. 07 Nov. 2018.

© 2019 Leonard Kelley

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