Motions Within the Solar System

The sun and planets each rotate on their axes. Because they formed from the same rotating disk, the planets, most of their satellites, and the asteroids, all revolve around the sun in the same direction as it rotates, and in nearly circular orbits. The planets orbit the sun in or near the same plane, called the ecliptic (because it is where eclipses occur). 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 its orbit is so eccentric, Pluto sometimes comes closer to the sun than does Neptune. It's interesting to note that most planets rotate in or near the plane in which they orbit the sun, since they formed, rotating, out of the same dust ring. Uranus must have suffered a whopping collision, though, that set it rotating on its side.

Distances Within the Solar System

The most common unit of measurement for distances within the solar system is the astronomical unit (AU). The AU is based on the mean distance from the sun to Earth, roughly 150,000,000 km. JPL's Deep Space Network refined the precise value of the AU in the 1960s by obtaining radar echoes from Venus. This measurement was important since spacecraft navigation depends on accurate knowledge of the AU. 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. 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 about 4 light years away, is over 265,000 AU from the sun.

In Cosmic Perspective

The Sun

The sun is a typical star. Its spectral classification is "G2 V." G2 basically means it's a yellow-white star, and the roman numeral V means it's a "main sequence" dwarf star (by far the most common) as opposed to supergiant, or sub-dwarf, etc. You can view current images of the sun as seen today by the suite of instruments aboard SOHO (Solar & Heliospheric Observatory) as it views the sun from the L1 Lagrange point between the Earth and the sun.

The sun dominates the gravitational field of the solar system; it contains about 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 about 0.135% of the mass of the solar system. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. Even though the planets make up only a small portion of the solar system's mass, they do 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 sun's gravity 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 10⁹ years ago in the sun, and will probably continue for another 5 x 10⁹ years. The apparent surface of the sun has no clean physical boundary, as solid planets do, although it appears as a sharp boundary when seen from the distance of Earth. Click the SOHO solar image at right for more details about the image.

The sun rotates on its axis with a period of approximately 25.4 days. This is the adopted value at 16° latitude. Because the sun is a gaseous body, not all its material rotates together. (This solar fact sheet describes how rotation varies with latitude). Solar matter at very high latitudes takes over 30 days to complete a rotation. Our star's output varies slightly over an 11-year cycle, during which the number of sunspots changes.

The sun's axis is tilted about 7.25 degrees to the axis of the Earth's orbit, so we see a little more of the sun's northern polar region each September and more of its southern region in March. As viewed from the Earth's surface, the Sun subtends roughly half a degree of arc upon the sky (as does the Moon, at this period in cosmic time.)

The sun has strong magnetic fields that are associated with sunspots and coronal mass ejections, CMEs (also called solar flares). A sunspot is a relatively cool area that appears dark against the hotter face of the sun. Sunspots are formed when magnetic field lines just below the sun's visible surface are twisted, and reach though the photosphere. CMEs are huge magnetic bubbles of plasma that erupt from the sun's corona and travel through space at high speed. View this spectacular 5-Mbyte SOHO movie of CMEs in August, 1999.

The solar magnetic field is not uniform, and it is very dynamic. Solar magnetic field variations and dynamics are targets of major interest in the exploration of the solar system

Our Bubble of Interplanetary Space

The "vacuum" of interplanetary space includes copious amounts of energy radiated from the sun, some interplanetary and interstellar dust (microscopic solid particles) and gas, and the solar wind. The solar wind, discovered by Eugene Parker in 1958, is a flow of lightweight ions and electrons (which together comprise plasma) thrown from the sun. The solar wind inflates a bubble, called the heliosphere, in the surrounding interstellar medium (ISM).

The solar wind has a visible effect on comet tails. It flows outward from our star at about 400 km per second, measured in the vicinity of Earth's orbit, and the Ulysses spacecraft found that it approximately doubles its speed at high solar latitudes.

The boundary at which the solar wind meets the ISM, containing the collective "solar" wind from other local stars in our galaxy, is called the heliopause. This is where the solar wind and the sun's magnetic field stop. The boundary is theorized to be roughly teardrop-shaped, because it gets "blown back" to form a heliotail, as the sun moves through the ISM (toward the right in the diagram above). The sun's relative motion may also create an advance bow shock, analogous to that of a moving boat. This is a matter of debate and depends partly on the strength of the interstellar magnetic field.

But before it gets out to the heliopause, the solar wind is thought to slow to subsonic speeds, creating a termination shock. This appears at the perimeter of the green circle in the diagram. Its actual shape, whether roughly spherical or teardrop, depends on magnetic field strengths, as yet unknown.

In the diagram above, temperatures are theorized; none have been actually measured beyond the termination shock. Note that even with the high particle temperatures, their density is so low that massive objects like spacecraft remain very cold (as long as they are shaded, or distant, from the sun).

The white lines in the diagram represent charged particles, mostly hydrogen ions, in the interstellar wind. They are deflected around the heliosphere's edge (the heliopause). The pink arrow shows how neutral particles penetrate the heliopause. These are primarily hydrogen and helium atoms, which are mostly not affected by magnetic fields, and there are also heavier dust grains. These interstellar neutral particles make up a substantial fraction of the material found within the heliosphere. The little black + in the green area represents the location of Voyager 1 at 80 AU in January of 2001. Voyager 1 is humanity's most distant object, and in 2004 the spacecraft is returning evidence that it is probably beginning to encounter the termination shock now, at a distance of just over 90 AU from the Sun.

The solar wind changes with the 11-year solar cycle, and the interstellar medium is not homogeneous, so the shape and size of the heliosphere probably fluctuate.

The solar magnetic field is the dominating magnetic field within the heliosphere, except in the immediate environment of planets which have their own magnetic fields. It can be measured by spacecraft throughout the solar system, but not here on earth, where we are shielded by our planet's own magnetic field.

The actual properties of the interstellar medium (outside the heliosphere), including the strength and orientation of its magnetic field, are important in determining the size and shape of the heliopause. Measurements that the two Voyager spacecraft will make in the region beyond the termination shock, and possibly beyond the heliopause, will provide important inputs to models of the termination shock and heliopause. Even though the Voyagers will sample these regions in discrete locations, this information will result in more robust overall models.

For further information on this vast subject and its many related topics, search the web for "heliosphere," "Alfven waves," "pickup ions," and "local interstellar cloud."

The Terrestrial Planets

The planets Mercury, Venus, Earth, and Mars, are called terrestrial because they have a compact, rocky surface like Earth's terra firma. The terrestrial planets are the four innermost planets in the solar system. None of the terrestrial planets have rings, although Earth does have belts of trapped radiation, as discussed below. Only Earth has a substantial planetary magnetic field. Mars and the Earth's Moon have localized regional magnetic fields at different places across their surfaces, but no global field.

Of the terrestrial planets, Venus, Earth, and Mars have significant atmospheres. The gases present in a planetary atmosphere are related to a planet's size, mass, temperature, how the planet was formed, and whether life is present. The temperature of gases may cause their molecules or atoms to achieve velocities that escape the planet's gravitational field. This contributes to Mercury's lack of a permanent atmosphere, as does its proximity to the source of the relentless solar wind.

The presence of life on Earth causes oxygen to be abundant in the atmosphere, and in this Earth is unique in our solar system. Without life, most of the oxygen would soon become part of the compounds on the planet's surface. Thus, the discovery of oxygen's signature in the atmosphere of an extrasolar planet would be significant.

Mercury lacks an atmosphere to speak of. Even though most of its surface is very hot, there is strong evidence that water ice exists in locations near its north and south poles which are kept permanently-shaded by crater walls. This evidence comes from Earth-based radar observations of the innermost planet. The discovery of permanently shaded ice at the poles of Earth's Moon strengthens arguments that the indications of ice on Mercury may be real.

Venus's atmosphere of carbon dioxide is dense, hot, and permanently cloudy, making the planet's surface invisible. Its best surface studies have come from landers and imaging radar from orbiting spacecraft.

Earth, as of November 2001, is still the only place known to harbor life. And life has flourished here since the planet was young. Our home planet is also unique in having large oceans of surface water, an oxygen-rich atmosphere, and shifting crustal sections floating on a hot mantle below, described by the theory of plate tectonics. Earth's Moon orbits the planet once every 27.3 days at an average distance of about 384,400 km. The Moon's orbital distance is steadily increasing at the very slow rate of 38 meters per millenium. Its distance at this point in its history makes the Moon appear in the sky to be about the same size as the Sun, subtending about half a degree.

Mars' atmosphere, also carbon dioxide, is much thinner than Earth's, but it sustains wispy clouds of water vapor. Mars has polar caps of carbon dioxide ice and water ice. The planet's surface shows strong evidence for extensive water coverage in its distant past, as well as possible evidence for water flow in small springs during recent times.

Earth's Radiation Environment

JPL's first spacecraft, Explorer 1, carried a single scientific instrument devised and operated by James Van Allen and his team from the University of Iowa. Early in 1958 the experiment discovered bands of rapidly moving charged particles trapped by Earth's magnetic field in toroidal, or doughnut-shaped regions surrounding the equator. The illustration below shows these belts only in two dimensions, as if they were sliced into thin cross-sections.

The belts that carry Van Allen's name have two areas of maximum density. The inner region, consisting largely of protons with an energy greater than 30 million EV, is centered about 3,000 km above Earth's surface. The outer belt is centered about 15,000 to 20,000 km up, and contains electrons with energies in the hundreds of millions of EV. It also has a high flux of protons, although of lower energies than those in the inner belt.

Flight within these belts can be dangerous to electronics and to humans because of the destructive effects the particles have as they penetrate microelectronic circuits or living cells. Most Earth-orbiting spacecraft are operated high enough, or low enough, to avoid the belts. The inner belt, however, has an annoying portion called the South Atlantic Anomaly (SAA) which extends down into low-earth-orbital altitudes. The SAA can be expected to cause problems with spacecraft that pass through it.

Terrestrial Planetary Data

This table compares features of the terrestrial planets in terms of the values for Earth. Light minutes are often used to express distances within the region of the terrestrial planets, useful because they indicate the time required for radio communication with spacecraft at their distances. If you click on the planet's name at the top of the table, you'll see a complete set of technical data for the planet, with a comparison to Earth. Here is a more extensive table of planetary data.

The Jovian Planets

Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets, because they are all gigantic compared with Earth, and they have a gaseous nature like Jupiter's -- mostly hydrogen, with some helium and trace gases and ices. The Jovian planets are thus referred to as the "gas giants" because gas is what they are mostly made of, although some or all of them probably have small solid cores. All have significant planetary magnetic fields, rings, and lots of satellites.

Jupiter is more massive than all the other planets combined. It emits electromagnetic energy from charged atomic particles spiraling through its strong magnetic field. If this sizzling magnetosphere were visible to our eyes, Jupiter would appear larger then the full Moon in Earth's sky. The trapped radiation belts near Jupiter present a hazard to spacecraft as do Earth's Van Allen belts, although the Jovian particle flux and distribution differ from Earth's. Bringing a spacecraft close to Jupiter presents a hazard mostly from ionized particles. Spacecraft intended to fly close to Jupiter must be designed with radiation-hardened components and shielding. Spacecraft using Jupiter for gravity assist may also be exposed to a harsh radiation dose. Instruments not intended to operate at Jupiter must be protected by being powered off and by having detectors covered.

Saturn, the farthest planet easily visible to the unaided eye, is known for its extensive, complex system of rings, which are very impressive even in a small telescope. Using a small telescope one can also discern the planet's oblateness, or flattening at the poles. Continued study of Saturn's ring system can yield new understandings of orbital dynamics, applicable to any system of orbiting bodies, from newly forming solar systems to galaxies.

Uranus, which rotates on its side, and Neptune are of similar size and color, although Neptune seems to have a more active atmosphere despite its much greater distance from the sun. Both planets are composed primarily of rock and various ices. Their extensive atmosphere, which makes up about 15% the mass of each planet, is hydrogen with a little helium. Neither of these planets were known to the ancients; Uranus was discovered in 1781, Neptune in 1846.

...And Pluto

Satellites of the Jovian Planets

The gas giants have numerous satellites, many of which are large, and seem as interesting as any planet. Small "new" satellites of the Jovian planets are being discovered every few years.

Jupiter's Galilean satellites, so named because Galileo Galilei discovered them in 1610, exhibit great diversity from each other. All four can be easily seen in a small telescope or binoculars. Io (pictured here) is the closest of these to Jupiter. Io is the most volcanically active body in the solar system, due to heat resulting from tidal forces (discussed further in Chapter 3) which flex its crust. Powerful Earth-based telescopes can observe volcanoes resurfacing Io continuously. Europa is covered with an extremely smooth shell of water ice. There is probably an ocean of liquid water below the shell, warmed by the same forces that heat Io's volcanoes. Ganymede has mountains, valleys, craters, and cooled lava flows. Its ancient surface resembles Earth's Moon, and it is also suspected of having a sub-surface ocean. Callisto, the outermost Galilean moon, is pocked all over with impact craters, indicating that its surface has changed little since the early days of its formation.

Saturn's largest moon, enigmatic Titan, is larger than the planet Mercury. Almost a terrestrial planet itself, Titan has a hazy nitrogen atmosphere denser than Earth's. The Huygens Probe executed a spectacurly successful mission in January 2005, revealing rivers and lakebeds on the surface, and extensive details of its atmosphere. Saturn also has many smaller satellites made largely of

TITAN, FALSE COLOR IMAGED BY CASSINI. CLICK FOR MORE INFO.

water ice. The "front," or leading, side of Saturn's icy satellite Iapetus is covered in dark material of some kind, and an equatorial mountain range higher than Olympus Mons on Mars was recently discovered on this 1450-km diameter moon. Icy Enceladus orbits within the densest part of Saturn's E Ring, and may somehow be the source of that ring's fine ice-particle makeup.

All of Uranus's five largest moons have extremely different characteristics. The surface of Miranda, the smallest of these, shows evidence of extensive geologic activity. Umbriel's surface is dark, Titania and Ariel have trenches and faults, and Oberon's impact craters show bright rays similar to those on Callisto.

Neptune's largest moon Triton is partly covered with nitrogen ice and snow, and has currently active nitrogen geysers that leave sooty deposits on the surface downwind.

Rings

SATURN'S RINGS IN FALSE COLOR

Jupiter's equatorial dust rings can be detected at close range in visible light and from Earth in the infrared. They show up best when viewed from behind, in forward scattered sunlight. Saturn, Uranus, and Neptune all have rings made up of myriad particles of ice ranging in size from dust and sand to boulders. Each particle in a ring is an individual satellite of the planet in its own right. Ring particles interact with each other in complex ways, affected by gravity and electrical charge. They also interact with the thin extended atmospheres of the planets. Saturn's magnificent ring system, as visible from Earth, spans about 280,000 km, yet its thickness is only around 200 meters! The A-ring, measured at several points, was found to be only ten meters thick.

When two satellites occupy orbits very close to each other within a ring system, one orbiting farther from the planet than a ring, and the other one orbiting closer to the planet than that ring, they confine particles between their orbits into a narrow ring, by gravitationally interacting with the ring particles. Thus these satellites are called shepherd moons.

 

Inferior and Superior Planets

Mercury and Venus are referred to as inferior planets, not because they are any less important, but because their orbits are closer to the sun than is Earth's orbit. They always appear close to the sun in Earth's morning or evening sky; their apparent angle from the sun is called elongation. The outer planets, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are all known as superior planets because their orbits are farther from the sun than the Earth's.

Planet or Moon appears in crescent phase when nearly between observer and sun.

Phases of Illumination

Inferior planets may pass between the Earth and the sun on part of their orbits, so they can exhibit nearly the complete range of phases from the earth's point of view... from the dark "new" phase, to slim "crescent" phase, to the mostly lit "gibbous" phase (approximating the fully illuminated "full" phase when approaching the other side of the sun). Our own Moon, of course, exhibits all the phases. Superior planets, though, usually appear gibbous, and appear full only when at opposition (see below), from our earthly point of view.

Viewed from superior planets, Earth goes through phases. Superior planets can be seen as crescents only from the vantage point of a spacecraft that is beyond them.

Conjunction, Transit, Occultation, Opposition

When two bodies appear to pass closest together in the sky, they are said to be in conjunction. When a planet passes closest to the sun as seen from Earth and all three bodies are approximately in a straight line, the planet is said to be in solar conjunction. The inferior planets Venus and Mercury can have two kinds of conjunctions with the Sun: (1) An inferior conjunction, when the planet passes approximately between Earth and Sun (if it passes exactly between them, moving across the Sun's face as seen from Earth, it is said to be in transit); and (2) A superior conjunction when Earth and the other planet are on opposite sides of the Sun and all three bodies are again nearly in a straight line. If a planet disappears behind the sun because the sun is exactly between the planets, it is said to be in occultation.

Superior planets can have only superior conjunctions with the sun. At superior conjunction the outer planet appears near its completely illuminated full phase. The planet is said to be at opposition to the sun when both it and the Earth are on the same side of the sun, all three in line. (The Moon, when full, is in opposition to the sun; the Earth is then approximately between them.)

Opposition is a good time to observe an outer planet with Earth-based instruments, because it is at its nearest point to the Earth and it is in its fullest phase.

Inferior planets can never be at opposition to the sun, from Earth's point of view.

Occultations, transits, conjunctions, and oppositions offer special opportunities for scientific observations by spacecraft. Studies of the solar corona and tests of general relativity can be done at superior conjunctions. Superior conjunctions also present challenges communicating with a spacecraft nearly behind the sun, which is overwhelmingly noisy at the same radio frequencies as those used for communications. At opposition, such radio noise is at a minimum, presenting ideal conditions for gravitational wave searches. These special opportunities and challenges are further discussed in later chapters.