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Chapter 1. The Solar System

Upon completion of this chapter, you will be able to state distances of objects within the solar system in terms of light-time, describe the sun as a typical star, relate its share of the mass within the solar system, and compare the terrestrial and jovian planets. You will be able to distinguish between inferior and superior planets, describe asteroids, comets, and the Oort cloud. You will be able to describe the environment in which the solar system resides.

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. For all of human history and pre-history, observations were based on visible light. Then in the 20th century people discovered how to use additional parts of the spectrum. Radio waves, received here on Earth, have been used since 1931 to investigate celestial objects. Starting with the emergence of space flight in 1957, instruments operating above Earth's obscuring atmosphere could take advantage not only of light and radio, but virtually the whole spectrum (the electromagnetic spectrum is the subject of a later chapter). At last, with interplanetary travel, instruments can be carried to many solar system objects, to measure their physical properties and dynamics directly and at very close range. In the 21st century, 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, which permeates interplanetary space. 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 about 4.2 light years. (A light year is the distance light travels in a year, at about 300,000 km per second.)

In Cosmic Perspective

Our whole solar system, together with all the local stars you can see on a clear dark night, orbits the center of our home galaxy. This spiral disk we call the Milky Way includes some 200 billion stars, thousands of gigantic clouds of gas and dust, and enormous quantities of mysterious dark matter.

Interstellar space is the term given to the space between stars in the galaxy. We are beginning to find that many stars besides the sun harbor their own planets, called extrasolar planets. As of January 2004 astronomers have detected about 100 planets orbiting other stars. They are all giant, Jupiter-like planets, made mostly of gas, since current detection methods cannot reveal smaller worlds. Their formation process is still unclear.

The image at right shows a galaxy similar to the Milky Way, known as M100 (click the image for details). 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 generally receding from each other, although those relatively close together may exhibit additional local motion toward or away from each other.

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.

Light Time Approximate Distance Example
3 seconds 900,000 km ~Earth-Moon Round Trip
3 minutes 54,000,000 km ~Sun to Mercury
8.3 minutes 149,600,000 km Sun to Earth (1 AU)
1 hour 1,000,000,000 km ~1.5 x Sun-Jupiter Distance
12.5 hours 90 AU Voyager-1 (January, 2004)
1 year 63,000 AU Light Year
4 years 252,000 AU ~Next closest star

Temperatures Within the Solar System

Select the above link for the Solar System Temperature Reference showing examples and comparing temperatures of objects and conditions from absolute zero through planet temperatures, to those of stars. The reference also includes temperature conversion factors and links to a conversion engine.

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.

Mass Distribution Within the Solar System

99.85% Sun
0.135% Planets
0.015% Comets
Minor Planets
Interplanetary Medium

Image of a solar prominence 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.

These and many other aspects of the sun are the subjects of ongoing research.

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.

Diagram of the heliosphere
Diagram courtesy Dr. Gary Zank, University of Delaware

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."


Quiz-1: The Sun and Interplanetary Space

1.01 The sun and all its neighboring stars orbit the center of:

      The universe.
      The solar system.
      A spiral galaxy.
      Intergalactic space.

1.02 True or false? All the sun's planets revolve around it in the same direction.


1.03 Which of the following describe the Astronomical unit (AU)?

      About a Light year.
      Based on sun-earth mean distance.
      Based on Earth-Moon mean distance.
      About 1.5x108Km.

1.04 The sun is...

      A red giant star.
      A yellow-white dwarf star.
      A supergiant star.
      Not a star.

1.05 Even though the planets make up a small fraction of the solar system's mass, they do retain the vast majority of the solar system's...

      Angular momentum.

1.06 The solar wind and solar magnetic field stop at the...

      Termination shock.

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Chapter 1. The Solar System     CONTINUED


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 Venus 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.

Earth 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 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.

Van Allen belts 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.


  Mercury Venus Earth Mars
Mean distance from sun (AU) 0.387 0.723 1 1.524
Light minutes from sun 3.2 6.0 8.3 12.7
Mass (x Earth) 0.0553 0.815 1 0.107
Equatorial radius (x Earth) 0.383 0.949 1 0.533
Rotation period
(Earth days)
175.942 − 116.75
1 1.027
Orbit period (Earth years) 0.241 0.615 1 1.881
Mean orbital velocity (km/s) 47.87 35.02 29.78 24.13
Natural satellites 0 0 1 2
Surface atmospheric pressure (bars) Near 0 92 1 .0069
to .009
Global Magnetic field Faint None Yes None

Mean Distances of the Terrestrial Planets from Sun

Orbits are drawn approximately to scale.

Terrestrial planet orbits


The Asteroids

Asteroids, or minor planets, orbit the Sun mostly between Mars and Jupiter. They are covered in the next section following the Planets.

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

Pluto and Charon Pluto is neither a rocky terrestrial planet nor a Jovian gas giant. It is a Kuiper Belt Object, composed of material left over after the formation of the other planets (see Comets in the next section). Kuiper Belt Objects were never exposed to the higher temperatures and solar radiation levels of the inner solar system. Pluto has large quantities of nitrogen ice, and simple molecules of carbon, hydrogen and oxygen. They remain on Pluto as a sample of the primordial material that set the stage for the evolution of the solar system as it exists today, including life. Its nitrogen atmosphere will precipitate out onto the surface as snow when its orbit takes it much farther from the sun than it is today. Since it has not yet been visited by a spacecraft, comparatively little is known about this small, distant body. Objects that orbit the Sun beyond Neptune's orbit are known as trans-Neptunian objects.

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


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.



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.


Jovian Planetary Data (Approximate)
  Jupiter Saturn Uranus Neptune
Mean distance from sun (AU) 5.20 AU 9.58 AU 19.20 AU 30.05 AU
Light hours from sun 0.72 1.3 2.7 4.2
Mass (x Earth) 317.8 95.2 14.5 17.1
Radius (x Earth) 11.21 9.45 4.01 3.88
Rotation period (hours) 9.9 10.7 17.2 16.1
Orbit period (Earth years) 11.9 29.4 83.7 163.7
Mean orbital velocity (km/s) 13.07 9.69 6.81 5.43
Known natural satellites (2004) 63 33 27 13
Rings Dust Extensive system Thin, dark Broken ring arcs

Mean Distances of the Jovian Planets from Sun

Orbits are drawn approximately to scale.
Pluto omitted to accommodate scale.



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.

Crescent of planet nearly in front of sun
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. Named positions of planets 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.




The Planets


1.07 Which of the following are jovian planets?


1.08 The environment of which of the following planets present(s) a serious trapped radiation hazard?


1.09 True or false? Jupiter's moon Europa has a thick hazy atmosphere.


1.10 Roughly how many light minutes (average) is Saturn from the sun?


1.11 A superior planet is...

      always seen as a crescent.
      closer than Earth to the sun.
      farther than Earth from the sun.
      larger than Earth.

1.12 When the Moon is full, it is also...

      in transit.
      at greatest western elongation.
      at inferior conjunction.
      at opposition.


Chapter 2. Reference Systems

Upon completion of this chapter you will be able to describe the system of terrestrial coordinates, the rotation of Earth, precession, nutation, and the revolution of Earth about the sun. You will be able to describe how the locations of celestial objects are stated in the coordinate systems of the celestial sphere. You will be able to describe the use of epochs and various conventions of timekeeping.


Spatial coordinates and timing conventions are adopted in order to consistently identify locations and motions of an observer, of natural objects in the solar system, and of spacecraft traversing interplanetary space or orbiting planets or other bodies. Without these conventions it would be impossible to navigate the solar system.


Terrestrial Coordinates

A great circle is an imaginary circle on the surface of a sphere whose center is the center of the sphere. Great circles that pass through both the north and south poles are called meridians, or lines of longitude. For any point on the surface of Earth a meridian can be defined.

The prime meridian, the starting point measuring the east-west locations of other meridians, marks the site of the old Royal Observatory in Greenwich, England. Longitude is expressed in degrees, minutes, and seconds of arc from 0 to 180 degrees eastward or westward from the prime meridian. For example, downtown Pasadena, California, is located at 118 degrees, 8 minutes, 41 seconds of arc west of the prime meridian: 118 8' 41" W.

ParallelsThe starting point for measuring north-south locations on Earth is the equator, a great circle which is everywhere equidistant from the poles. Circles in planes parallel to the equator define north-south measurements called parallels, or lines of latitude. Latitude is expressed as an arc subtended between the equator and the parallel, as seen from the center of the Earth. Downtown Pasadena is located at 34 degrees, 08 minutes, 44 seconds latitude north of the equator: 34 08' 44" N.

    Throughout the history of navigation, determining one's latitude on the Earth's surface has been relatively easy. In the northern hemisphere for example, simply measuring the height of the star Polaris above the horizon results in a fairly close approximation of one's latitude. Measurement of longitude, however, has been a historically siginificant endeavor, since its determination requires accurate timekeeping. John Harrison (1693-1776) eventually succeeded in developing a chronometer good enough to do the trick.


One degree of latitude equals approximately 111 km on the Earth's surface, and by definition exactly 60 nautical miles. Because meridians converge at the poles, the length of a degree of longitude varies from 111 km at the equator to 0 at the poles where longitude becomes a point.

Terrestrial Coordinates Grid

          Grid on small world map

Rotation and Revolution

"Rotation" refers to an object's spinning motion about its own axis. "Revolution" refers the object's orbital motion around another object. For example, Earth rotates on its own axis, producing the 24-hour day. Earth revolves about the Sun, producing the 365-day year. A satellite revolves around a planet.

Earth's Rotation

The Earth rotates on its axis relative to the sun every 24.0 hours mean solar time, with an inclination of 23.45 degrees from the plane of its orbit around the sun. Mean solar time represents an average of the variations caused by Earth's non-circular orbit. Its rotation relative to "fixed" stars (sidereal time) is 3 minutes 56.55 seconds shorter than the mean solar day, the equivalent of one solar day per year.

Precession of Earth's Axis

Forces associated with the rotation of Earth cause the planet to be slightly oblate, displaying a bulge at the equator. The moon's gravity primarily, and to a lesser degree the sun's gravity, act on Earth's oblateness to move the axis perpendicular to the plane of Earth's orbit. However, due to gyroscopic action, Earth's poles do not "right themselves" to a position perpendicular to the orbital plane. Instead, they precess at 90 degrees to the force applied. This precession causes the axis of Earth to describe a circle having a 23.4 degree radius relative to a fixed point in space over about 26,000 years, a slow wobble reminiscent of the axis of a spinning top swinging around before it falls over.

Precession of Earth's Axis Over 26,000 Years

Precession Diagram

Because of the precession of the poles over 26,000 years, all the stars, and other celestial objects, appear to shift west to east at the rate of .01 degree each year (360 degrees in 26,000 years). This apparent motion is the main reason for astronomers as well as spacecraft operators to refer to a common epoch such as J2000.0.

At the present time in Earth's 26,000 year precession cycle, a bright star happens to be very close, less than a degree, from the north celestial pole. This star is called Polaris, or the North Star.

Stars do have their own real motion, called proper motion. In our vicinity of the galaxy, only a few bright stars exhibit a large enough proper motion to measure over the course of a human lifetime, so their motion does not generally enter into spacecraft navigation. Because of their immense distance, stars can be treated as though they are references fixed in space. (Some stars at the center of our galaxy, though, display tremendous speeds as they orbit close to the massive black hole located there.)


Superimposed on the 26,000-year precession is a small nodding motion with a period of 18.6 years and an amplitude of 9.2 arc seconds. This nutation can trace its cause to the 5 degree difference between the plane of the Moon's orbit, the plane of the Earth's orbit, and the gravitational tug on one other.

Revolution of Earth

Earth revolves in orbit around the sun in 365 days, 6 hours, 9 minutes with reference to the stars, at a speed ranging from 29.29 to 30.29 km/s. The 6 hours, 9 minutes adds up to about an extra day every fourth year, which is designated a leap year, with the extra day added as February 29th. Earth's orbit is elliptical and reaches its closest approach to the sun, a perihelion of 147,090,000 km, on about January fourth of each year. Aphelion comes six months later at 152,100,000 km.


Because we make observations from Earth, knowledge of Earth's natural motions is essential. As described above, our planet rotates on its axis daily and revolves around the sun annually. Its axis precesses and nutates. Even the "fixed" stars move about on their own. Considering all these motions, a useful coordinate system for locating stars, planets, and spacecraft must be pinned to a single snapshot in time. This snapshot is called an epoch.

By convention, the epoch in use today is called J2000.0, which refers to the mean equator and equinox of year 2000, nominally January 1st 12:00 hours Universal Time (UT). The "J" means Julian year, which is 365.25 days long. Only the 26,000-year precession part of the whole precession/nutation effect is considered, defining the mean equator and equinox for the epoch.

The last epoch in use previously was B1950.0 - the mean equator and equinox of 1949 December 31st 22:09 UT, the "B" meaning Besselian year, the fictitious solar year introduced by F. W. Bessell in the nineteenth century. Equations are published for interpreting data based on past and present epochs.

Making Sense

Given an understanding of the Earth's suite of motions -- rotation on axis, precession, nutation, and revolution around the sun -- and given knowledge of an observer's location in latitude and longitude, meaningful observations can be made. For example, to measure the precise speed of a spacecraft flying to Saturn, you have to know exactly where you are on the Earth's surface as you make the measurement, and then subtract out the Earth's motions from that measurement to obtain the spacecraft's speed. The same applies if you are trying to measure the proper motion of a distant star -- or a star's subtle wobble, to reveal a family of planets.


1 The Solar System
2 Reference Systems
3 Gravity & Mechanics
4 Trajectories
5 Planetary Orbits
6 Electromagnetics


7 Mission Inception
8 Experiments
9 S/C Classification
10 Telecommunications
11 Onboard Systems
12 Science Instruments
13 Navigation


14 Launch
15 Cruise
16 Encounter
17 Extended Operations
18 Deep Space Network

Chapter 2. Reference Systems     CONTINUED


Celestial Sphere

The Celestial Sphere

A useful construct for describing locations of objects in the sky is the celestial sphere. The center of the earth is the center of the celestial sphere. The figure at right illustrates that the sphere's poles and equator are analogs of the corresponding constructs on the surface of the Earth. We can specify precise location of objects on the celestial sphere by giving the celestial equivalent of their latitudes and longitudes.

The point on the celestial sphere directly overhead for an observer is the zenith. An imaginary arc passing through the celestial poles and through the zenith is called the observer's meridian. The nadir is the direction opposite the zenith: for example, straight down from a spacecraft to the center of the planet.

Declination and Right Ascension

Declination (DEC) is the celestial sphere's equivalent of latitude and it is expressed in degrees, just like latitude. For DEC, + and - refer to north and south. The celestial equator is 0 DEC, and the poles are +90 and -90.

Right ascension (RA) is the celestial equivalent of longitude. RA can be expressed in degrees, but it is more common to specify it in hours, minutes, and seconds: the sky appears to turn 360 in 24 hours, and that's 15 in an hour. An hour of RA is 15 of sky rotation.

Celestial Sphere Another important feature intersecting the celestial sphere is the ecliptic plane. This is the plane in which the Earth orbits the sun, 23.4 from the celestial equator. Looking at the ecliptic, the great circle marking the intersection of the ecliptic plane on the celestial sphere, is where the sun and planets appear to travel, and it's where the Sun and Moon are during eclipses (that's where the plane and circle get their names).

The zero point for RA is one of the points where the ecliptic circle intersects the celestial equator circle. It's defined to be the point where the sun crosses into the northern hemisphere beginning spring: the vernal equinox, also known as the first point of Aries.


The equinoxes are times at which the center of the Sun is directly above the equator, marking the beginning of spring and autumn. The day and night would be of equal length at that time, if the Sun were a point and not a disc, and if there were no atmospheric refraction. Given the apparent disc of the Sun, and the Earth's atmospheric refraction, day and night actually become equal at a point within a few days of each equinox.


The RA and DEC of an object specify its position uniquely on the celestial sphere just as the latitude and longitude do for an object on the Earth's surface. For example, the very bright star Sirius has celestial coordinates 6 hr 45 min RA and -16 43' DEC.

HA-DEC versus AZ-EL Radio Telescopes

The discussion gets a little more involved here, but this section serves only to explain the old design for Deep Space Network antennas, as well as large optical and radio telescopes, and why it all changed not too long ago.

Before you can use RA and DEC to point to an object in the sky, you have to know where the RA is at present for your location, since the Earth's rotation continuously moves the fixed stars (and their RA) with respect to your horizon. If the RA of the object happens to place it overhead on your meridian, you're fine. But it probably isn't, so you determine the object's hour angle (HA), which is the distance in hours, minutes, and seconds westward along the celestial equator from the observer's meridian to the object's RA. In effect, HA represents the RA for a particular location and time of day. HA is zero when the object is on your meridian.

Older radio telescopes were designed with one mechanical axis parallel to Earth's axis. To track an interplanetary spacecraft, the telescope would point to the spacecraft's known HA and DEC, and then for the rest of the tracking period it would simply rotate in HA about the tilted axis (called its polar axis), as Australian HA-DEC DSN Antenna the Earth turns. This kind of mounting is traditionally called an equatorial mount when used for optical telescopes. It's a fine mount for a small instrument, but unsuited to very heavy structures because the tilted polar bearing has to sustain large asymmetric loads. These loads include not only the whole reflector dish, but also an HA counterweight heavy enough to balance the antenna, the DEC bearing, and its DEC counterweight! Also the structure has to be designed specifically for its location, since the polar bearing's angle depends on the station's latitude. This image shows the first Deep Space Network (DSN) antenna installed at the Canberra, Australia site, looking down along the polar bearing, which is the axis of the antenna's large central wheel. This HA-DEC antenna is no longer in service, nor is its sister at the DSN site at Madrid, Spain. Its counterpart at the Goldstone, California site has been converted to a radio telescope dedicated to educational use. Click the image for an enlarged and annotated view of its complex design.

A simpler system was needed for larger Deep Space Network antennas. The solution is an azimuth-elevation configuration. The design permits mechanical loads to be symmetric, resulting in less cumbersome, less expensive hardware that is easier to maintain. It locates a point in the sky by elevation (EL) in degrees above the horizon, and azimuth (AZ) in degrees clockwise (eastward) from true north. These coordinates are derived from published RA and DEC by computer programs. This computerization was the key that permitted the complex mechanical structures to be simplified.

In an AZ-EL system anywhere on Earth, east is 90 degrees AZ, and halfway up in EL or altitude (ALT) would be 45 degrees. AZ-EL and ALT-AZ are simply different names for the same Australian HA-DEC DSN Antenna reference system, ALTitude being the same measurement as ELevation.

The image at right shows a DSN antenna at Goldstone that has a 70-meter aperture, over twice that of the Australian HA-DEC antenna shown above. In the picture it is pointing to an EL around 10. The EL bearing is located at the apex of the triangular support visible near the middle right of the image. The whole structure rotates in AZ clockwise or counterclockwise atop the large cylindrical concrete pedestal. It is pointing generally east in the image (around 90 azimuth), probably beginning to track a distant spacecraft as it rises over the desert horizon. All newly designed radio telescopes use the AZ-EL system.

Then There's X-Y

To complete our survey of mounting schemes for DSN antennas (including steerable non-DSN radio telescopes as well), we need to describe the X-Y mount. The X-Y mount has two perpendicular axes. By examining the image of DSS16 here (click the image for a larger view) you can see that it cannot directly swivel in azimuth as can the AZ-EL (ALT-AZ) -mounted antenna. But the X-Y mount has an advantage over AZ-EL. Goldstone XY-mounted DSN Antenna DSS16

The advantage is a matter of keyholes. To compare, picture an AZ-EL antenna like the 70-m DSS in the section above. If a spacecraft were to pass directly overhead, the AZ-EL antenna would rise in elevation until it reached its straight-up maximum near 90. But then the antenna would have to whip around rapidly in azimuth as the spacecraft is first on the east side of the antenna, and then a moment later is on the west. The antennas' slew rate isn't fast enough to track that way, so there would be an interruption in tracking until acquiring on the other side. (AZ-EL antennas in the DSN aren't designed to bend over backwards, or "plunge" in elevation.)

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For an HA-DEC antenna, the keyhole is large and, in the northern hemisphere, is centered near the north star. To track through that area the antenna would have to whip around prohibitively fast in hour angle.

The X-Y antenna is mechanically similar to the old HA-DEC antenna, but with its "polar" axis laid down horizontally, and not necessarily aligned to a cardinal direction. The X-Y antenna is situated so that its keyholes (two of them) are at the eastern and western horizon. This leaves the whole sky open for tracking spacecraft without needing impossibly high angular rates around either axis... it can bend over backwards and every which way. The X-Y's were first built for tracking Earth-orbiting spacecraft that require high angular rates and overhead passes. Earth-orbiters usually have an inclination that avoids the east and west keyholes, as well. Interplanetary spacecraft typically do not pass overhead, but rather stay near the ecliptic plane in most cases. Of course X-Y's can be used with interplanetary spacecraft also, but in the DSN they are only equipped with a 26-m aperture, smaller than most other DSN stations, and thus not useful for most interplanetary craft.







Copyright 2005 -  S. B. EglI