comet

History

In ancient times, comets were considered harbingers of disaster. Isaac Newton, using the law of universal gravitation, proved that comets move along orbits. In 1705, Edmond Halley calculated that the bright comets of 1531, 1607, and 1682 were the same one and predicted its return in 1758, which came true.

How it works

As a comet approaches the Sun, ice on its surface turns into gas, erupting in jets. Solar radiation and solar wind push away dust and ionized gas, forming two tails: a dust tail, curved along the orbit, and an ion tail, straight and pointing directly away from the Sun.

History

In ancient times, comets were considered harbingers of disaster. Isaac Newton, using the law of universal gravitation, proved that comets move along orbits. In 1705, Edmond Halley calculated that the bright comets of 1531, 1607, and 1682 were the same one and predicted its return in 1758, which came true.

How it works

As a comet approaches the Sun, ice on its surface turns into gas, erupting in jets. Solar radiation and solar wind push away dust and ionized gas, forming two tails: a dust tail, curved along the orbit, and an ion tail, straight and pointing directly away from the Sun.

History

Until the 17th century, comets were considered atmospheric phenomena. Tycho Brahe in 1577, by measuring parallax, showed their cosmic nature. Johannes Kepler suggested straight-line trajectories, but Isaac Newton in 'Principia' (1687) applied the law of universal gravitation to calculate parabolic orbits of comets. Edmond Halley, processing data on 24 comets, identified the comet of 1682 with previously observed ones and predicted its return in 1758. In the 1950s, Jan Oort proposed the hypothesis of a giant spherical cloud on the periphery. Fred Hoyle studied organic molecules in comets and their possible connection to the origin of life.

How it works

A comet's orbit is determined by initial conditions and perturbations from planets. Long-period comets come from the Oort cloud with nearly parabolic orbits. During sublimation, reactive forces alter the orbit (non-gravitational acceleration). The brightness of the coma is modeled by the radiation transfer equation, accounting for dust scattering and gas fluorescence (e.g., CN, C₂ radicals). Tails are classified by type: type I — plasma, formed by ion interaction with the solar wind magnetic field; type II — dust, where solar radiation pressure F_{\text{rad}} = (S/c) Q_{\text{pr}} A, where S is the radiation flux density, c is the speed of light, Q_{\text{pr}} is the radiation pressure efficiency, A is the cross-sectional area of a dust grain. The Doppler effect contributes to emission when scattering on fast-moving particles. Wien's displacement law is used to estimate dust temperature.

Open problems & fun facts

Origin of short-period comets: capture from the Oort cloud or formation in the Kuiper belt?; Nature of non-gravitational accelerations: detailed model of sublimation and surface activity.; Chemical composition of nuclei: the ratio of primordial ices and organics as a key to the early Solar System.; Why do some comets have no tail, being dead or covered with a crust?

💡 The nucleus of comet 67P/Churyumov-Gerasimenko, studied by the Rosetta probe, has the shape of a rubber duck and 'sings': the probe detected low-frequency plasma oscillations that were translated into audible sound.