The solar system has been a topic of study from the beginning of history. For nearly all that time, people have had to rely on long-range and indirect measurements of its objects. At first, almost all observations were based on visible light and, later, on radio waves received here on Earth from the objects under investigation. However, with the emergence of space flight, instruments can be sent to many solar system objects to measure their physical properties and dynamics directly and at close range. With the data collected from these measurements, knowledge of the solar system is advancing at an unprecedented rate.
The solar system consists of an average star we call the sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes the satellites of the planets, numerous comets, asteroids, meteoroids, and the interplanetary medium. The sun is the richest source of electromagnetic energy in the solar system. The sun's nearest known stellar neighbor is a red dwarf star called Proxima Centauri, at a distance of 4.3 light years away (a light year is the distance light travels in a year, at the rate of 300,000 km per second). The whole solar system, together with the local stars visible on a clear night, orbits the center of our home galaxy, a spiral disk of 200 billion stars we call the Milky Way. The Milky Way has two small galaxies orbiting it nearby, which are visible from the southern hemisphere. They are called the Large Magellanic Cloud and the Small Magellanic Cloud. Our galaxy, one of billions of galaxies known, is travelling through intergalactic space. On a cosmic scale, all galaxies are receding from each other. Galaxies relatively close together may exhibit motion toward or away from each other on a local scale.
The planets, most of the satellites of the planets, and the asteroids revolve around the sun in the same direction, in nearly circular orbits. The sun and planets rotate on their axes. The planets orbit the sun in or near the same plane, called the ecliptic. Pluto is a special case in that its orbit is the most highly inclined (17 degrees) and the most highly elliptical of all the planets. Because of this, for part of its orbit, Pluto is closer to the sun than is Neptune.
The most common unit of measurement for distances within the solar system is the astronomical unit (AU). One AU equals the mean distance from the sun to Earth, about 150,000,000 km. JPL refined the precise value of the AU in the 1960s using radar echoes from Venus, since spacecraft navigation depended on its accuracy. Another way to indicate distances within the solar system is terms of light time, which is the distance light travels in a unit of time at the rate of 300,000 km per second. Distances within the solar system, while vast compared to our travels on Earth's surface, are comparatively small-scale in astronomical terms. For reference, Proxima Centauri, the nearest star at 4 light years away, is about 250,000 AU distant from the sun.
|Light Time||Approximate Distance||Example|
|1 second||299,792 km||~0.75 Earth-Moon distance|
|1 minute||18,000,000 km||0.125 AU|
|8.3 minutes||150,000,000 km||Earth-Sun distance (1 AU)|
|1 hour||1,000,000,000 km||~1.5 x Sun-Jupiter Distance|
|4 years||(Included for reference)||Distance to nearest star|
The sun is best characterized as a typical star. The sun dominates the gravitational field of the solar system; it contains 99.85% of the solar system's mass. The planets, which condensed out of the same disk of material that formed the sun, contain only 0.135% of the mass of the solar system. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining fraction. Even though the planets make up a small portion of the solar system's mass, they retain the vast majority of the solar system's angular momentum. This storehouse of momentum can be utilized by interplanetary spacecraft on so-called "gravity-assist" trajectories.
The gravity of the sun creates extreme pressures and temperatures within itself, sustaining a thermonuclear reaction fusing hydrogen nuclei and producing helium nuclei. This reaction yields tremendous amounts of energy, causing the material of the sun to be plasma and gas. These thermonuclear reactions began about 5 x 10 years ago in the sun, and will probably continue for another 5 x 10 years. The sun has no distinct surface. The apparent surface of the sun is optical only and has no discrete physical boundary.
The sun rotates once on its axis within a period of approximately 28 days at its equator. Because the sun is a gaseous body, rotation speed varies with latitude, being slower at higher latitudes.
The sun has strong magnetic fields that are associated with sunspots. The solar magnetic field is not uniform and is very dynamic. Solar magnetic field variations and dynamics are targets of major interest in the exploration of the solar system.
Nearly all the solar system by volume appears to be an empty void. Far from being nothingness, this vacuum of "space" comprises the interplanetary medium. It includes various forms of electromagnetic radiation and at least two material components: interplanetary dust and interplanetary gas. Interplanetary dust consists of microscopic solid particles. Interplanetary gas is a tenuous flow of gas and charged particles, mostly protons and electrons--plasma--which stream from the sun, called the solar wind.
The solar wind can be measured by spacecraft, and it has a large effect on comet tails. It also has a measurable effect on the motion of spacecraft. The speed of the solar wind is about 400 km per second in the vicinity of Earth's orbit. The speed approximately doubles at high solar latitudes. The point at which the solar wind meets the interstellar medium, which is the "solar" wind from other stars, is called the heliopause. It is a boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the sun's influence perhaps 100 AU from the sun. The space within the boundary of the heliopause, containing the sun and solar system, is referred to as the heliosphere.
The solar magnetic field extends outward into interplanetary space; it can be measured on Earth and by spacecraft. The solar magnetic field is the dominating magnetic field throughout the interplanetary regions of the solar system, except in the immediate environment of planets which have their own magnetic fields.