Введение
Сила, действующая на вращающееся тело в пространстве
Эффект Ярковского — это сила, действующая на вращающееся тело в пространстве, возникающая из-за анизотропного излучения тепловых фотонов, переносящих импульс. Обычно он рассматривается применительно к метеороидам или небольшим астероидам (диаметром от 10 см до 10 км), поскольку для этих тел его влияние наиболее заметно.
История открытия
Эффект был открыт польским русским инженером-строителем Иваном Осиповичем Ярковским (1844–1902), который в свободное время занимался научными проблемами в России. В памфлете, написанном примерно в 1900 году, Ярковский отметил, что ежедневный нагрев вращающегося объекта в космосе вызывает появление силы, которая, хотя и незначительна, может приводить к существенным долгосрочным изменениям орбит небольших тел, особенно метеороидов и малых астероидов. Идеи Ярковского могли бы быть забыты, если бы не эстонский астроном Эрнст Юрьевич Опик (1893–1985), который ознакомился с памфлетом Ярковского приблизительно в 1909 году. Спустя десятилетия, Опик, вспомнив содержание памфлета по памяти, обсудил потенциальную важность эффекта Ярковского для движения метеороидов в Солнечной системе.
Механизм
Эффект Ярковского является следствием того, что изменение температуры объекта, нагретого излучением (и, следовательно, интенсивность теплового излучения от объекта) отстает от изменений входящего излучения. То есть, поверхности объекта требуется время, чтобы нагреться при первом освещении, и требуется время, чтобы остыть при прекращении освещения. В целом, эффект имеет два компонента:
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 warmest point on a rotating body occurs around the "2 PM" site on the surface, or slightly after noon. 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, the force is in the direction of motion of the orbit, and causes the semi major axis 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. 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. 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. 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, utilizing 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 mission, launched in September 2016, studies 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 observations to further refine 99942 Apophis's orbit and measure the Yarkovsky acceleration with high precision, to within 0.5%. With these, astronomers were able to eliminate the possibility of a collision with the Earth for at least the next 100 years.
Дневной эффект: На вращающемся теле, освещенном Солнцем (например, астероиде или Земле), поверхность нагревается солнечным излучением в течение дня и остывает ночью. Термические свойства поверхности вызывают задержку между поглощением излучения от Солнца и излучением тепла, поэтому самая теплая точка на вращающемся теле находится примерно в 14:00 (2 PM) на поверхности или немного после полудня. Это приводит к разнице между направлениями поглощения и переизлучения, что создает результирующую силу вдоль направления движения по орбите. Если объект является прямым ротатором, сила направлена в сторону движения по орбите, что приводит к устойчивому увеличению большой полуоси орбиты; объект спирально удаляется от Солнца. Обратный ротатор спирально сближается. Дневной эффект является доминирующим компонентом для тел диаметром более 100 м.
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 warmest point on a rotating body occurs around the "2 PM" site on the surface, or slightly after noon. 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, the force is in the direction of motion of the orbit, and causes the semi major axis 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. 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. 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. 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, utilizing 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 mission, launched in September 2016, studies 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 observations to further refine 99942 Apophis's orbit and measure the Yarkovsky acceleration with high precision, to within 0.5%. With these, astronomers were able to eliminate the possibility of a collision with the Earth for at least the next 100 years.
Сезонный эффект: Этот эффект легче всего понять на идеализированном примере невращающегося тела, обращающегося вокруг Солнца, для которого каждый "год" состоит ровно из одного "дня". Двигаясь по своей орбите, полушарие, обращенное от Солнца ("темная" сторона), которое нагревалось в течение длительного предшествующего периода времени, неизменно находится в направлении движения по орбите. Избыток теплового излучения в этом направлении создает тормозящую силу, которая всегда вызывает спиральное движение к Солнцу. На практике, для вращающихся тел, сезонный эффект усиливается с увеличением наклона оси вращения. Он доминирует только в том случае, если дневной эффект достаточно мал. Это может произойти из-за очень быстрого вращения (недостаточно времени для остывания на ночной стороне, что приводит к почти равномерному продольному распределению температуры), небольшого размера (все тело нагревается) или наклона оси, близкого к 90°. Сезонный эффект более важен для небольших фрагментов астероидов (от нескольких метров до примерно 100 м), при условии, что их поверхности не покрыты изолирующим слоем реголита и они не имеют чрезвычайно медленного вращения. Кроме того, на очень больших временных масштабах, когда ось вращения тела может неоднократно изменяться в результате столкновений (и, следовательно, меняется и направление дневного эффекта), сезонный эффект также будет иметь тенденцию доминировать. Без непосредственных измерений очень трудно предсказать точный результат эффекта Ярковского на орбите конкретного астероида. Это связано с тем, что величина эффекта зависит от множества переменных, которые трудно определить по ограниченным доступным наблюдательным данным. К ним относятся точная форма астероида, его ориентация и альбедо. Расчеты дополнительно усложняются эффектами затенения и теплового "переосвещения", вызванными местными кратерами или возможной общей вогнутой формой. Эффект Ярковского также конкурирует с давлением излучения, которое может создавать аналогичные небольшие долгосрочные силы для тел с вариациями альбедо или не сферической формой. Например, даже для простого случая чистого сезонного эффекта Ярковского на сферическом теле на круговой орбите с наклоном 90°, изменения большой полуоси могут отличаться до двух раз в зависимости от того, является ли альбедо однородным или наблюдается сильная асимметрия альбедо между северным и южным полушариями. В зависимости от орбиты объекта и оси вращения, изменение большой полуоси под действием эффекта Ярковского может быть обращено путем перехода от сферической к не сферической форме. Несмотря на эти трудности, использование эффекта Ярковского является одним из сценариев, рассматриваемых для изменения траектории потенциально опасных околоземных астероидов. Возможные стратегии отклонения астероида включают "окраску" поверхности астероида или фокусировку солнечного излучения на астероиде для изменения интенсивности эффекта Ярковского и, следовательно, изменения орбиты астероида, чтобы избежать столкновения с Землей. Миссия OSIRIS-REx, запущенная в сентябре 2016 года, изучает эффект Ярковского на астероиде Бенну. В 2020 году астрономы подтвердили ускорение Ярковского астероида 99942 Апофис. Эти данные важны для предотвращения падения астероидов, поскольку считалось, что у 99942 Апофиса есть очень небольшой шанс столкнуться с Землей в 2068 году, и эффект Ярковского был значительным источником неопределенности в прогнозах. В 2021 году многопрофильная коллаборация профессиональных астрономов-любителей объединила данные со спутника Gaia, наземных радиолокационных измерений и наблюдений звездных оккультаций, проведенных любителями, для дальнейшего уточнения орбиты 99942 Апофиса и измерения ускорения Ярковского с высокой точностью, в пределах 0,5%. Благодаря этим данным астрономы смогли исключить возможность столкновения с Землей как минимум на следующие 100 лет.
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 warmest point on a rotating body occurs around the "2 PM" site on the surface, or slightly after noon. 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, the force is in the direction of motion of the orbit, and causes the semi major axis 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. 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. 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. 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, utilizing 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 mission, launched in September 2016, studies 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 observations to further refine 99942 Apophis's orbit and measure the Yarkovsky acceleration with high precision, to within 0.5%. With these, astronomers were able to eliminate the possibility of a collision with the Earth for at least the next 100 years.