Yarkovsky Effect: Orbit and Rotation of Asteroids

Solar energy and asteroids

What is the Yarkovsky effect?

Solar energy significantly influences asteroids through two related thermal effects: the Yarkovsky effect, which changes their orbit, and the YORP effect, which acts on their rotation. The Yarkovsky effect is a non-gravitational force caused by an asteroid absorbing sunlight and then re-emitting it as heat.

It is not the absorbed or reflected light that produces the lasting effect, but the re-emitted thermal radiation. The effect therefore requires a surface with non-zero thermal inertia, that is, one capable of storing heat before releasing it.

A tiny but cumulative thrust

The force is minuscule: at most about 0.1 N for a 500 m near-Earth asteroid, an acceleration of the order of 10^-12 m/s². Yet it adds up with every revolution and can shift such an asteroid by several tens of kilometres in a decade.

Mechanism of the thermal thrust

When an asteroid is exposed to the Sun, its surface absorbs solar energy. Because the ground takes some time to heat up, the hottest region is that of the “afternoon” and not the one facing the Sun. As it spins, the asteroid therefore emits its infrared radiation asymmetrically, which exerts a weak recoil thrust. This thrust very slowly changes the asteroid's orbit.

Diurnal and seasonal components

Two components add up. The diurnal component, linked to the asteroid's rotation, depends on the cosine of the tilt of its axis (obliquity): an asteroid with prograde rotation slowly moves away from the Sun, while one with retrograde rotation moves closer. The seasonal component, linked to the revolution around the Sun, on the contrary always tends to bring the asteroid closer, but it is generally weaker. An obliquity close to 90° strongly attenuates the diurnal effect.

Influence of size and composition

The effect depends on the asteroid's size, shape, rotation and composition. The drift of the semi-major axis varies roughly as the inverse of the radius: the smaller the body, the faster it drifts. The effect concerns bodies smaller than 30 to 40 km in diameter and becomes negligible beyond that. It is greatest for an intermediate thermal inertia; a surface with very low or very high inertia (fine regolith or bare rock, for example) reduces it.

For asteroids from a few hundred metres to a few kilometres across, it causes measurable orbital changes within a few decades and considerable displacements over millions of years. It can bring them closer to or move them away from the Sun, altering their trajectory and orbital stability. For very small fragments, heat spreads throughout the volume of the body and the effect eventually vanishes.

History of a forgotten idea: from Yarkovsky to YORP

A pamphlet from 1901

The effect is named after Ivan Osipovich Yarkovsky, a Russian civil engineer from a family of Polish descent. In a privately published pamphlet in 1901, he noted that the heating of a planet with prograde rotation should produce a transverse acceleration of its motion. He thus thought he could offset the supposed drag of the ether, a then-popular hypothesis. This context was wrong and Yarkovsky could only roughly estimate the magnitude of the effect, but his idea had been launched.

Öpik, Radzievskii, Paddack and O'Keefe

It was mainly Ernst Öpik who brought this work to the Western literature, in 1951, when the original pamphlet had been lost. At the same time, Vladimir Radzievskii and his collaborators were publishing similar ideas in Russia. Radzievskii was the first to consider the effect of light thrust on the rotation of a body, but assuming albedo variations on its surface. Stephen Paddack and John O'Keefe then realised that it is the irregular shape and the thermal radiation that change rotation most effectively. The YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack) was born, a little more than half a century after Yarkovsky.

The revival of the 1990s

In the second half of the 1990s, David Rubincam and Paolo Farinella, who were studying thermal perturbations of the motion of artificial satellites such as LAGEOS, made the link with the small bodies of the solar system. A flowering of work followed on the dynamics of small asteroids and meteoroids. The Dutch amateur astronomer George Beekman also found in the Russian archives Yarkovsky's pamphlet, which had been thought lost (Beekman, 2006).

Measuring the Yarkovsky effect: from Golevka to Bennu

A drift to be detected over decades

Detecting the effect requires predicting an asteroid's position with a precision better than the expected drift. For a half-kilometre near-Earth asteroid, this takes at least three radar measurement campaigns spread over about ten years, or several decades of optical observations. The observation does not directly give the variation of the semi-major axis: it reveals an offset of the position along the orbit, which grows as the square of the elapsed time.

Golevka, the first direct detection (2003)

The asteroid (6489) Golevka, about 0.5 km across, provided the first unambiguous detection. Radar ranging at Arecibo during its approaches in 1991, 1995 and 2003 revealed a non-gravitational acceleration consistent with predictions published in 2000, with an offset of about fifteen kilometres from a purely gravitational trajectory. The measurement also made it possible to estimate Golevka's bulk density, about 2.7 g/cm³ (Chesley et al., 2003).

Weighing an asteroid through its drift

The drift depends on the mass, size, rotation and thermal properties of the surface. By measuring it, one can therefore work back to the density of an asteroid, provided the other parameters are known. This is what was done for Bennu before the OSIRIS-REx spacecraft arrived: from a drift of 284 ± 1.5 m/yr, the bulk density was estimated at 1260 ± 70 kg/m³, a macroporosity of about 40%, a sign of a rubble pile (Chesley et al., 2014).

Bennu, the most precise measurement

Tracking the OSIRIS-REx spacecraft around Bennu, from January 2019 to October 2020, made it possible to measure the Earth-Bennu distance to within a few metres. The Yarkovsky drift obtained is −284.6 ± 0.2 m/yr, with a signal-to-noise ratio of about 1400 (Farnocchia et al., 2021). The negative sign indicates that Bennu is slowly moving closer to the Sun, which corresponds to retrograde rotation. The effect is comparable to the weight of a few grapes, but it is enough to noticeably alter the orbit within a few centuries.

An excess of retrograde-rotating asteroids

Among the 36 valid detections listed in 2014, 28 show a negative drift, that is about 78% of asteroids with retrograde rotation. This imbalance is explained by the transport of asteroids from the main belt to the near-Earth space: the ν₆ secular resonance, at the inner edge of the belt, is reached only by asteroids whose semi-major axis decreases, hence with retrograde rotation, whereas the 3:1 resonance with Jupiter can be reached in both directions.

Detections at all scales

The list of detections covers a wide range of sizes. At the time of the 2015 review, the smallest object concerned was 2009 BD, about 4 m across, while (2100) Ra-Shalom and (4179) Toutatis, 2 to 3 km across, were among the largest.

The Yarkovsky effect on Apophis

A weak but constant force

The Yarkovsky effect exerts a very weak but constant force on the asteroid Apophis, about 340 m in diameter (± 50 m). Thanks to observations from 2004 to 2021, the drift of its semi-major axis is now measured at −199 ± 1 m/yr (Farnocchia et al., 2024). This non-gravitational force, although invisible in the short term, causes considerable orbital displacements over time.

Consequences for Apophis's trajectory

A drift of 199 m/yr amounts to about 200 km per millennium on the semi-major axis, an offset that builds up along the orbit. After Apophis's discovery in 2004, an impact in 2029 was considered possible (1 chance in 37, or 2.7%), before being ruled out. The Yarkovsky effect then remained a major source of uncertainty for modelling its trajectory, because the 2029 flyby amplifies any error, which left open the possibility of later impacts, notably in 2036 and 2068.

Accuracy of impact predictions

Optical observations from late 2020 and new radar measurements in March 2021 (Goldstone and Green Bank) reduced the uncertainty on Apophis's position in 2029 from several hundred to a few kilometres. Any impact risk was thus ruled out for at least a century, and Apophis was removed from the risk lists, including ESA's, on 29 March 2021. Its trajectory is now fully deterministic over the coming century.

The flyby of 13 April 2029

On Friday 13 April 2029, Apophis will pass Earth at about 38,000 km from the centre of the planet, or about 31,600 km in altitude, below the orbit of geostationary satellites. It will be visible to the naked eye from much of Europe and Africa. This encounter is a natural experiment: Earth's attraction may change Apophis's rotation and, consequently, its Yarkovsky effect.

The American spacecraft OSIRIS-APEX, which will arrive about a month after the flyby, is to measure this possible change, with a sensitivity of the order of 4% in the mission's current configuration. The European Ramses mission, whose launch is planned between 20 April and 15 May 2028, will reach the asteroid before the flyby and accompany it until August 2029.

N.B.:
The dates of the Ramses and OSIRIS-APEX missions are those announced in spring 2026 and may change.

Yarkovsky and planetary defense: from Bennu to 2024 YR4

Bennu and the impact risk calculation

For a few asteroids with extremely well-known orbits, the Yarkovsky effect is an essential ingredient of the risk calculation. For Bennu, the measurement made with OSIRIS-REx improved by a factor of 20 the knowledge of the circumstances of the close approach in 2135, which will strongly perturb its orbit. The cumulative impact probability through 2300 reaches 5.7×10^-4 (1 chance in 1750), and the most likely scenario concerns September 2182, with 1 chance in 2700 (Farnocchia et al., 2021).

2024 YR4: when the Yarkovsky effect becomes negligible

The asteroid 2024 YR4, about 60 m across, discovered in late 2024, had been associated with a 4.3% probability of impact with the Moon on 22 December 2032. The James Webb Space Telescope observations of 18 and 26 February 2026 extended the observation arc by eight months. They place the pass at 22,900 ± 800 km from the centre of the Moon, which rules out any lunar impact. Any impact with Earth is also ruled out for the next 100 years (de Wit et al., 2026).

For such a small object, the Yarkovsky effect could have complicated the prediction. However, the data show no sign of drift, and the expected dispersion increases the uncertainty on the 2032 position by only about 5%, a negligible effect.

Why small asteroids pose a new challenge

Upcoming large surveys will multiply the discoveries of asteroids a few tens of metres across. These objects, more numerous, quickly become too faint for ground-based telescopes. Their orbit then remains uncertain for longer, and the Yarkovsky effect, stronger for small bodies, will have to be taken into account carefully when assessing their risk.

Why does solar irradiation change an asteroid's rotation?

The YORP effect, an extension of the Yarkovsky effect

The YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack) is the equivalent of the Yarkovsky effect for rotation: solar irradiation and thermal re-emission change the rotation and the axis orientation of an asteroid. Its first direct detection dates from 2007, on (54509) 2000 PH5, renamed (54509) YORP, whose rotation was accelerating by 2.0×10^-4 degrees per day squared (Lowry et al., 2007). This asteroid, about a hundred metres across, spins in about twelve minutes.

Thermal asymmetry and torque

Irregularities in the asteroid's shape (craters, bumps, etc.) cause an asymmetry in the re-emission of heat. For a body of very regular shape, such as an ellipsoid of revolution, the effect vanishes. Otherwise, this asymmetry creates a torque that speeds up or slows down the asteroid's rotation and makes the tilt of its axis evolve towards extreme values.

Sensitivity of small asteroids

The YORP effect varies as the inverse of the square of the radius: it increases very quickly as size decreases. Small asteroids are therefore much more sensitive to it, to the point of having their rotation axis tip over, or their rotation speed up until fission. This effect plays a key role in the long-term evolution of near-Earth asteroids (NEA). In the cases detected up to 2014, all the asteroids concerned were speeding up their rotation.

Other effects linked to solar energy

Sublimation of volatile materials

Some asteroids containing ices or volatile materials can release gas under the effect of solar heat. This sublimation can create jets that change their trajectory or rotation, through a mechanism close to that of comets. The phenomenon remains rare and concerns only a small minority of bodies.

Thermal weathering

Heating and cooling cycles due to solar irradiation cause thermal fractures in surface rocks. The team of Marco Delbo (Côte d'Azur Observatory) showed in 2014 that this thermal fatigue breaks up rocks larger than a few centimetres faster than micrometeoroid impacts. It is therefore the main process producing the regolith of small asteroids (Delbo et al., 2014).

Astrophysical implications

Changing asteroid populations

The Yarkovsky effect slowly pushes asteroids towards gravitational resonances, notably the ν₆ secular resonance and the 3:1 resonance with Jupiter. From there, they can be ejected or injected onto orbits crossing those of the terrestrial planets, thus becoming near-Earth asteroids.

Spreading and dating of asteroid families

Asteroid families are born from collisions. Afterwards, the Yarkovsky effect spreads their members on either side of the initial position, with the smaller bodies drifting the farthest. The shape of this spreading makes it possible to date the families: the Koronis family, for example, would be of the order of 2.5 to 3 billion years old.

Binary asteroids and asteroid pairs

The YORP effect favours the fission of rapidly rotating asteroids, which contributes to the formation of binary asteroids and asteroid pairs. In a binary system, a variant called BYORP then modifies the mutual orbit of the two bodies.

The origin of meteorites

Öpik thought that the Yarkovsky effect brought meteorites directly from the main belt to Earth, but this transport is too slow. The current model combines the two mechanisms: the Yarkovsky effect pushes fragments towards the resonances, which then send them onto orbits crossing Earth's.

Impact risks for Earth

These effects are essential for predicting the trajectory of near-Earth asteroids. Ignoring these forces can lead to errors in forecast models and distort the assessment of impact risks, as the cases of Bennu and Apophis have shown.

Orders of magnitude of the effects

Yarkovsky effect

The force is at most of the order of 0.1 N for a 500 m asteroid, an acceleration of about 10^-12 m/s². It translates into a drift of the semi-major axis of a few tens to a few hundred metres per year for a near-Earth asteroid a few hundred metres across: −284.6 m/yr for Bennu (about 490 m) and −199 m/yr for Apophis (about 340 m). Over a million years, such a drift amounts to several hundred thousand kilometres.

N.B.:
Publications often express the drift in 10^-4 au/Myr. This unit is equivalent to about 15 m/yr.

YORP effect

Changes in the rotation period are counted in milliseconds per year: from 1.25 ms/yr for (54509) YORP to 45 ms/yr for Itokawa. This corresponds to changes in rotation speed of about 10^-10 rad/s per year (Itokawa) to 10^-8 rad/s per year ((54509) YORP), but these changes build up into significant effects over millions of years.

Key takeaways

The major role of solar energy

Solar energy plays a major role in the dynamical evolution of asteroids. The Yarkovsky and YORP effects influence their trajectories, their rotation and even their structure. Measured directly since 2003, they are now built into the orbit calculations of the best-observed asteroids.

Importance for evolution and risks

These mechanisms are crucial for understanding the evolution of asteroids in the solar system, the origin of near-Earth asteroids and meteorites, and for assessing impact risks on Earth. The flyby of Apophis in 2029, observed by the Ramses and OSIRIS-APEX spacecraft, will offer a unique test to refine these models.