The Yarkovsky effect is a
force
In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
acting on a rotating body in space caused by the
anisotropic
Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
emission of
thermal photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
s, which carry
momentum
In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. ...
. It is usually considered in relation to
meteoroid
A meteoroid ( ) is a small rocky or metallic body in outer space.
Meteoroids are distinguished as objects significantly smaller than ''asteroids'', ranging in size from grains to objects up to wide. Objects smaller than meteoroids are classifie ...
s or small
asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.
History of discovery
The effect was discovered by the Polish-Russian civil engineer
Ivan Osipovich Yarkovsky (1844–1902), who worked in Russia on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the daily heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large long-term effects in the orbits of small bodies, especially
meteoroid
A meteoroid ( ) is a small rocky or metallic body in outer space.
Meteoroids are distinguished as objects significantly smaller than ''asteroids'', ranging in size from grains to objects up to wide. Objects smaller than meteoroids are classifie ...
s and small
asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s. Yarkovsky's insight would have been forgotten had it not been for the
Estonia
Estonia, officially the Republic of Estonia, is a country in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the Baltic Sea across from Sweden, to the south by Latvia, and to the east by Ru ...
n astronomer
Ernst J. Öpik (1893–1985), who read Yarkovsky's pamphlet sometime around 1909. Decades later, Öpik, recalling the pamphlet from memory, discussed the possible importance of the Yarkovsky effect on movement of meteoroids about the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
.
Mechanism
The Yarkovsky effect is a consequence of the fact that change in the temperature of an object warmed by radiation (and therefore the intensity of thermal radiation from the object) lags behind changes in the incoming radiation. That is, the surface of the object takes time to become warm when first illuminated, and takes time to cool down when illumination stops. In general there are two components to the effect:
* Diurnal effect: On a rotating body illuminated by the Sun (e.g. an asteroid or the Earth), the surface is warmed by solar radiation during the day, and cools at night. The thermal properties of the surface cause a lag between the absorption of radiation from the Sun and the emission of radiation as heat, so the surface is warmest not when the Sun is at its peak but slightly later. This results in a difference between the directions of absorption and re-emission of radiation, which yields a net force along the direction of motion of the orbit. If the object is a
prograde rotator, then the force is in the direction of motion of the orbit, and causes the
semi-major axis
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...
of the orbit to increase steadily: the object spirals away from the Sun. A
retrograde rotator spirals inward. The diurnal effect is the dominant component for bodies with diameter greater than about 100 m.
* Seasonal effect: This is easiest to understand for the idealised case of a non-rotating body orbiting the Sun, for which each "year" consists of exactly one "day". As it travels around its orbit, the "dusk" hemisphere which has been heated over a long preceding time period is invariably in the direction of orbital motion. The excess of thermal radiation in this direction causes a braking force that always causes spiraling inward toward the Sun. In practice, for rotating bodies, this seasonal effect increases along with the
axial tilt
In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...
. It dominates only if the diurnal effect is small enough. This may occur because of very rapid rotation (no time to cool off on the night side, hence an almost-uniform
longitudinal temperature distribution), small size (the whole body is heated throughout), or an axial tilt close to 90°. The seasonal effect is more important for smaller asteroid fragments (from a few metres up to about 100 m), provided their surfaces are not covered by an insulating
regolith layer and they do not have exceedingly slow rotations. Additionally, on very long timescales over which the spin axis of the body may be repeatedly changed by collisions (and hence also the direction of the diurnal effect changes), the seasonal effect will also tend to dominate.
In general, the effect is size-dependent, and will affect the semi-major axis of smaller asteroids, while leaving large asteroids practically unaffected. For kilometre-sized asteroids, the Yarkovsky effect is minuscule over short periods: the force on asteroid
6489 Golevka has been estimated at 0.25
newtons
The newton (symbol: N) is the unit of force in the International System of Units (SI). Expressed in terms of SI base units, it is 1 kg⋅m/s2, the force that accelerates a mass of one kilogram at one metre per second squared.
The unit i ...
, for a net acceleration of 10
−12 m/s
2. But it is steady; over millions of years an asteroid's orbit can be perturbed enough to transport it from the
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
to the inner Solar System.
The mechanism is more complicated for bodies in strongly
eccentric orbits.
Measurement
The effect was first measured in 1991–2003 on the asteroid
6489 Golevka. The asteroid drifted 15 km from its predicted position over twelve years (the orbit was established with great precision by a series of radar observations in 1991, 1995, and 1999 from the
Arecibo radio telescope).
Without direct measurement, it is very hard to predict the exact result of the Yarkovsky effect on a given asteroid's orbit. This is because the magnitude of the effect depends on many variables that are hard to determine from the limited observational information that is available. These include the exact shape of the asteroid, its orientation, and its
albedo
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
. Calculations are further complicated by the effects of shadowing and thermal "reillumination", whether caused by local craters or a possible overall concave shape. The Yarkovsky effect also competes with
radiation pressure, whose net effect may cause similar small long-term forces for bodies with albedo variations or non-spherical shapes.
As an example, even for the simple case of the pure seasonal Yarkovsky effect on a spherical body in a circular orbit with 90°
obliquity, semi-major axis changes could differ by as much as a factor of two between the case of a uniform albedo and the case of a strong north-south albedo asymmetry. Depending on the object's orbit and
spin axis, the Yarkovsky change of the semi-major axis may be reversed simply by changing from a spherical to a non-spherical shape.
Despite these difficulties, using the Yarkovsky effect is one scenario under investigation to alter the course of potentially Earth-impacting
near-Earth asteroids. Possible
asteroid deflection strategies include "painting" the surface of the asteroid or focusing solar radiation onto the asteroid to alter the intensity of the Yarkovsky effect and so alter the orbit of the asteroid away from a collision with Earth. The
OSIRIS-REx
OSIRIS-REx was a NASA asteroid-study and sample-return mission that visited and collected samples from 101955 Bennu, a C-type asteroid, carbonaceous near-Earth object, near-Earth asteroid. The material, returned in September 2023, is expected ...
mission, launched in September 2016, studied the Yarkovsky effect on
asteroid Bennu.
In 2020, astronomers confirmed Yarkovsky acceleration of the asteroid
99942 Apophis. The findings are relevant to
asteroid impact avoidance as 99942 Apophis was thought to have a very small chance of Earth impact in 2068, and the Yarkovsky effect was a significant source of prediction uncertainty. In 2021, a multidisciplinary professional-amateur collaboration combined
Gaia satellite and ground-based radar measurements with amateur stellar
occultation
An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks f ...
observations to further refine 99942 Apophis's orbit and measure the Yarkovsky acceleration with high precision, to within 0.5%. With these data, astronomers were able to eliminate the possibility of a collision with the Earth for at least the next 100 years.
See also
*
Asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
*
Poynting–Robertson effect
The Poynting–Robertson effect, also known as Poynting–Robertson drag, named after John Henry Poynting and Howard P. Robertson, is a process by which solar radiation causes a dust grain orbiting a star to lose angular momentum relative to its or ...
*
Radiation pressure
*
YORP effect
References
External links
*
Asteroid Nudged by Sunlight: Most Precise Measurement of Yarkovsky Effect– (
ScienceDaily
''ScienceDaily'' is an American website launched in 1995 that aggregates press releases and publishes lightly edited press releases (a practice called churnalism) about science, similar to Phys.org and EurekAlert!.
History
The site was f ...
2012-05-24)
{{DEFAULTSORT:Yarkovsky Effect
Asteroids
Concepts in astrophysics
Orbital perturbations
Radiation effects
Rotation